International Journal of Biological Macromolecules 80 (2015) 557–565
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Effect of citric acid concentration and hydrolysis time on physicochemical properties of sweet potato starches Ayenampudi Surendra Babu a , Ramanathan Parimalavalli a,∗ , Shalini Gaur Rudra b a b
Department of Food Science and Nutrition, School of Professional Studies, Periyar University, Salem 636011, Tamil Nadu, India Department of Food Science and Post Harvest Technology, Indian Agriculture Research Institute, New Delhi 110012, India
a r t i c l e
i n f o
Article history: Received 29 April 2015 Received in revised form 25 June 2015 Accepted 12 July 2015 Available online 15 July 2015 Keywords: DSC Fat replacer SEM Sweet potato starch XRD
a b s t r a c t Physicochemical properties of citric acid treated sweet potato starches were investigated in the present study. Sweet potato starch was hydrolyzed using citric acid with different concentrations (1 and 5%) and time periods (1 and 11 h) at 45 ◦ C and was denoted as citric acid treated starch (CTS1 to CTS4) based on their experimental conditions. The recovery yield of acid treated starches was above 85%. The CTS4 sample displayed the highest amylose (around 31%) and water holding capacity its melting temperature was 47.66 ◦ C. The digestibility rate was slightly increased for 78.58% for the CTS3 and CTS4. The gel strength of acid modified starches ranged from 0.27 kg to 1.11 kg. RVA results of acid thinned starches confirmed a low viscosity profile. CTS3 starch illustrated lower enthalpy compared to all other modified starches. All starch samples exhibited a shear-thinning behavior. SEM analysis revealed that the extent of visible degradation was increased at higher hydrolysis time and acid concentration. The CTS3 satisfied the criteria required for starch to act as a fat mimetic. Overall results conveyed that the citric acid treatment of sweet potato starch with 5% acid concentration and 11 h period was an ideal condition for the preparation of a fat replacer. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Today’s dietary concern is the consumption of huge quantity of fat and sugar. With the mounting incidence of diabetes and obesity, low calorie foods have acquired the huge esteem. In general, the best suitable approach in terms of fat reduction diets involves either the use of low-fat foods or fat substitutes or modifications such as trimming of fat from foods [1,2]. Fat Replacers consist of mixtures of lipid-originated fat substitutes, protein- or carbohydrate-originated fat mimetic, or their combinations [3]. Carbohydrate-based Replacers incorporate water into a gel type structure, resulting in a lubricant or flow properties similar to those of fats in food systems [2]. Even though a variety of fat replacers have been developed, there are unfortunately no ideal fat replacers which completely function like conventional fat [4]. Native starch can sometimes be used to replace fat [1]; however starch modified by acid or enzymatic hydrolysis, oxidation,
Abbreviations: NS, native sweet potato starch; CTS, citric acid treated starch; HT, hydrolysis time; AC, acid concentration; DE, dextrose equivalent; WHC, water holding capacity; PV, peak viscosity; BD, break down; TV, trough viscosity; SB, setback; FV, final viscosity; Pt, pasting time; PT, pasting temperature; To, onset temperature; Tp, peak temperature; Tc, final temperatures; H, gelatinization enthalpy. ∗ Corresponding author. E-mail address: [email protected] (R. Parimalavalli). http://dx.doi.org/10.1016/j.ijbiomac.2015.07.020 0141-8130/© 2015 Elsevier B.V. All rights reserved.
dextrinization, cross linking, or mono-substitution is more commonly used to achieve desired functional and sensory properties [1]. Generally, acid hydrolysis occurs more rapidly in amorphous regions than in crystalline region and the residue after prolonged acid hydrolysis consists of acid-resistant crystalline parts of amylopectin [5]. Thys et al. [6] investigated the functional properties of acid-thinned pinhao starch and it showed low syneresis, high solubility, thermo reversibility and melting point similar to fat. They concluded that the acid treatment was efficient in producing a potential fat substitute from pinhao starch. Amaya-Llano et al. [7] produced acid hydrolyzed jicama starch and used as a fat substitute in yoghurt. The addition of hydrolyzed jicama starch (2.03 g/100 g) as a fat substitute in the preparation of stirred yoghurt had good functional and sensorial properties. Ma et al. [8] reported that enzymatic hydrolyzed corn starch could be used as fat replacers. The hydrolyzed starch with fine particles was used to produce low fat mayonnaise and the result indicated that the 60% fat-reduced mayonnaise with fat replacers had similar sensory quality as compared with the high fat one. The following are the criteria for a starch based fat mimetic – (a) Starch should contain an amylose content of ∼20% [9]. (b) Starch ought to require a granule size of 2 m or in similar size to liquid micelle to act as fat mimetic [10]. (c) According to FDA [11] a starch-based fat mimetic is supposed to be partially or completely digestible. (d) Starch must possess a DE (dextrose equivalent) of
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≤5.0 [12]. (e) Starch gel with a melting point close to that of the fats (37–45 ◦ C) could be used as a fat substitute [13]. (f) Starch must possess high water-holding capacity [9] and better emulsifying properties [4]. (g) Starch should display shear thinning characteristic [14]. Sweet potato (Ipomoea batatas (L.)) belongs to the Convolvulaceae family [15]. Sweet potato is considered as the world’s most important and under-exploited crop [16]. Industrial application of sweet potato flour and starch is insignificantly causing a negative growth in its production. The intended use of sweet potato starch for industrial purpose depends on advanced processing technologies to prepare sweet potato starch with desirable functional properties, and on thorough indulgent of the effect of processing conditions on their properties. Basing on this background the research was aimed to study the influence of hydrolysis time and acid concentration on the sweet potato starch properties and evaluate the distinctive properties of citric acid treated sweet potato starch as fat mimetic.
2. Materials and methods 2.1. Materials The sweet potato was purchased from a local supermarket (Palzhamuthir sollai), Salem, Tamil Nadu, India. Glucose Oxidase–Peroxidase (GOD–POD) kit was obtained from Beacon Diagnostics, Navasari, India. Amyloglucosidase from Aspergillus niger (≥300 U/mL) (E.C-3.2.1.3), citric acid and all other chemicals and reagents were analytical grade and purchased from SigmaAldrich, Steinheim, Germany. 2.2. Starch isolation and preparation of acid-thinned sweet potato starch Starch was isolated from sweet potato by the method of Wickramasinghe et al. [17]. An edible portion of sweet potato was cut into small pieces and homogenized with distilled water. The slurry was then passed through the double-layered cheesecloth and the filtrate was allowed to settle for a minimum of 3 h at room temperature. The precipitated starch was washed three times with distilled water, dried at room temperature (20–25 ◦ C) for 48 h and then the dried starch was kept in an oven at 50 ◦ C for three hours and ground into fine powder and named as Native sweet potato starch (NS). Citric acid treated starch (CTS) was prepared by the method of Zambrano and Camargo [18]. Starch slurry was prepared by dispersing NS (40 g dry basis) in 1% or 5% citric acid solution kept in a water bath at 45 ◦ C for 1 h or 11 h respectively with constant stirring. After each assay of hydrolysis, the pH was adjusted to 5.5 ± 0.2 by slowly adding aqueous sodium hydroxide (5 g/100 ml). The starch was washed three times with two fold volume of deionised water prior to filtration and dried in a convection oven at 45 ◦ C for 48 h. The dried starch was made into powder and packed in airtight containers for further use and recovery yield was calculated by the following formula. Recovery yield (%) =
dry weight of starch after hydrolysis × 100 dry weight of starch before hydrolysis
2.3. Physiochemical properties of native and modified starches 2.3.1. Analysis of ash, protein, fat and total fiber Estimation of ash, protein, fat and total fiber in the isolated starch material was carried out following the AOAC protocol [19].
2.3.2. Dextrose equivalent (DE) The reducing sugar value was measured using the dinitrosalicylic acid method of Miller [20] to determine its dextrose equivalent (DE). Different concentrations of dextrose standard solution were taken in test tubes and dinitrosalicylic acid reagent was added in each of the test tubes. The test tubes were heated in a boiling water bath for 5 min. The Rochelle salt solution was added to each of the test tubes while the contents were still warm. The test tubes were cooled and the absorbance at 560 nm was noted and percentage of reducing sugar was determined. DE was calculated by the formula given by Miller [20]: DE =
g reducing sugar × 100 g dry weight of starch
2.3.3. Apparent amylose Apparent amylose content determination was carried out using a colorimetric iodine affinity procedure [21] briefly a mixture of 0.1 g of the starch sample, 1 ml of ethanol and 9 ml 1 N sodium hydroxide was boiled for 10 min in a boiling water bath and allowed to cool. To a portion (5 ml) of the mixture, 1 ml of 1 N acetic acid and 2 ml of iodine solution were added and Absorbance (A) was read using a Spectrophotometer at 620 nm. The apparent amylose content was calculated as follows: Apparent amylose content (%) = 3.06 × A × 20; where A = absorbance value
2.3.4. Moisture and dry matter Moisture content and dry matter were determined by the method of Adebayo, Lateef, and Elizabeth [22]. Two milligrams (2 mg) of starch sample was measured into a previously weighed crucible. The crucible plus sample was then transferred into the oven set at 100 ◦ C, for 24 h. At the end of 24 h, the crucible plus sample was removed from the oven and transfer to desiccator cooled for ten minutes and weighed. The Moisture content and dry matter were determined and expressed in percentage. 2.3.5. Melting point, clear point and thermo-reversibility of the starch gel Starch gel was prepared by the method described by the National Starch and Chemical Corporation [12] with modification. Sweet potato starch suspension (5%) was prepared with 0.02% (w/v) sodium metabisulfite in a beaker at 80 ◦ C for 10 min and then autoclaved at 121 ◦ C for 15 min. Subsequently beaker was cooled, hermetically sealed and stored at 4 ◦ C for 24 h. The obtained gel was melted in a water bath at 80 ◦ C under agitation. The change in consistency was visually observed, and melting point was considered as the temperature at which the liquid phase was formed and mixing of the gel was possible. The clear point was considered as the temperature at which the sol appeared optically clear. For the gel thermo-reversibility, the gel was melted in a water bath with constant stirring and allowed to cool down to room temperature, followed by refrigeration at 4 ◦ C for 18 h and the gel formation was observed [7]. 2.3.6. In vitro digestibility of starch samples In vitro digestibility of starch was analyzed according to the method of Noda et al. [23] with some modifications. A mixture consisting of 4% (w/v) starch suspension in tubes was placed in a water bath at 100 ◦ C for 10 min to obtain the starch suspension. A 0.5 ml of starch suspension and 0.25 ml of 100 mM acetate buffer (pH 5.0) and 0.25 ml of glucoamylase solution was incubated at 40 ◦ C for 2 h
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with stirring. After digestion, surplus starch was removed by centrifuge (4350 tour/min during 10 min), and glucose in the filtrate was analyzed using the glucose oxidase–peroxidase reagent. 2.4. Functional properties 2.4.1. Water holding capacity (WHC) Water holding capacity was determined by the method described by Niba et al. [24]. Starch sample (1 g) was suspended in 5 ml water in a centrifuge tube. The slurry was shaken on a wristaction shaker for 1 min at ambient temperature and centrifuged at 3000 × g for 10 min. The supernatant was poured carefully into a tared evaporating dish. Water holding capacity was calculated as follows (g/g): Mass of water added to sample − Mass of water removed from the sample/mass of starch sample. 2.4.2. Emulsifying properties Emulsifying activity and stability of starch samples were determined by the method of Neto, Narain, Silvia, and Bora [25]. Starch dispersion was prepared (10 mg/ml) and homogenized for 1 min with 5 ml of refined sunflower oil. The emulsion was centrifuged (1100 × g, 5 min) and the emulsifying activity was calculated as follows. Emulsifying activity (%) = (height of the emulsified layer/height of the total content) × 100. Emulsion stability was determined by heating the emulsion at 80 ◦ C for 30 min before centrifuging (1100 × g, 5 min). Emulsion stability (%) = (height of the emulsified layer after heating/height of emulsified layer before heating) × 100. 2.5. Textural properties of the starch gel Gel textural properties were determined using a texture analyzer (Texture analyzer HD plus, Stable Microsystems, Godalming, UK). A starch suspension of 10% in a 50 ml beaker was heated to 95 ◦ C for 15 min and cooled to ambient temperature, then stored at 4 ◦ C for 24 h. The gel formed (33 mm in height and 21 mm in diameter) in the beaker was used directly for the texture analysis, and each gel was penetrated 4 mm by a P25/L (25 mm diameter) cylindrical probe. Two strength-time curves were obtained with a 1.0 mm/s speed during the penetration cycles. The texture profile curves were used to calculate gel strength, cohesiveness, adhesiveness, springiness, gumminess and chewiness. 2.6. Pasting properties Pasting profile of starch was recorded using a Rapid Visco Analyser (RVA) (RVA Tech Master, Perten Instruments, and Japan). The viscosity profiles were recorded using starch suspensions (12%, w/v). The Standard profile 1 of the Perten Instruments was used, where the samples were held at 50 ◦ C for 1 min, heated from 50 ◦ C to 95 ◦ C at 12.16 ◦ C/min, held at 95 ◦ C for 2.30 min, cooled from 95 ◦ C to 50 ◦ C at 11.84 ◦ C/min, and held at 50 ◦ C for 2 min. The peak viscosity (PV), breakdown (BD) trough viscosity (TV), setback (SB), final viscosity (FV), pasting time (Pt) and pasting temperature (PT) were recorded. 2.7. Thermal analysis The thermal properties of starches were measured using a DSC (TA Instrument, Q2000, New Castle, NJ, USA). A 4.5 mg sample (dry basis) was weighed in an aluminum pan and 10 L of deionised water was added. The pan was sealed tightly and then it was allowed to stand for 1 h before carrying out the analysis. An empty
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aluminum pan was used as reference. The sample was subjected to a heating program over a range of temperature from 10 to 125 ◦ C and a heating rate of 5 ◦ C/min. The onset, peak, and final temperatures (To, Tp, and Tc, respectively) and gelatinization enthalpy (H) were determined. H =
KA m
where H – enthalpy change of reaction, m – mass of the sample at the beginning of the experiment, K – calibration coefficient, and A – area under the peak [26]. 2.8. Rheological properties Dynamic rheological measurements of native and acid modified sweet potato starch-water suspensions (10%, dry sample to distilled water) were performed at 85 ◦ C with a CVOR Rheometer (Bohlin, Malvern, Worcestershire, United Kingdom), using parallel plate geometry (20 mm diameter, 1 mm gap). For each measurement, 1 ml of sample was carefully deposited over the plateau of the rheometer. After the plateau comes in contact with the plate, the exposed surface of the sample was covered with a thin layer of low-density silicone oil to prevent evaporation during the measurement. In order to describe the variation in the rheological properties of samples under steady shear, the data were fitted to the wellknown power law model (Eq. (1)), which was used extensively to describe the flow properties of non-Newtonian liquid engineering applications. A linear regression of shear rate versus shear stress was plotted to obtain statistically best values of R2 , K and n. = K n
(1)
where , shear stress (Pa); , shear rate (s−1 ); K, consistency index (Pa sn ); n, the flow behavior index (dimensionless). 2.9. Structural properties 2.9.1. Powder X-ray diffraction (XRD) X-ray diffraction pattern of starch samples was obtained using a Powder X-ray diffractometer (Rigaku Mini Hex-II, Japan). Since X-ray intensity of starch was affected by moisture content samples were conditioned to 75% relative humidity before taking the X-ray diffraction pattern. The graphs were plotted between the 2 angles of 10 and 60 and smoothed with the software PowderX (Chinese Academy of Sciences, Beijing). The degree of crystallinity of samples was quantitatively estimated following the method of Nara and Komiya [27] with the Origin – version 6.0 software (Microcal Software Inc., Northampton, MA, USA). 2.9.2. Scanning electron microscope (SEM) Starch granules were observed using a Scanning Electron Microscope (SEM) (JEOL-Model 6390, Japan). Granule size was determined by using ImageJ 1.46r (National Institute of Health, USA) software. 3. Experimental design and statistical analysis Zambrano and Camargo [18] evaluated the effect of acid hydrolysis time (3,6,9,11 h), concentration (1.5, 2.0, 3.0, 4.0, 4.5 g/100 g) and temperature (40, 43, 45, 47, 51, 54 ◦ C) on the hydrolysis of native cassava starch. They revealed that the treatment, 3.0 g/100 g HCl, 40 ◦ C/6 h showed acceptable values of gel formation and thermo reversibility with a low hydrolysis degree, which were the essential characteristics required for a fat replacer. Hence, these conditions were adopted as reference for the present study. A 2 × 2 factorial experimental design was used in the present study for
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evaluating the effect of citric acid concentration (AC) and hydrolysis time (HT) on the response variables (Table 1). The data of physiochemical and functional properties of the citric acid treated starches were six replications. All data obtained were subjected to one way Analysis of Variance (ANOVA) or student t-test using SPSS program (Statistical Package for Social Science) version 14.0 (SPSS Inc., Illinois, USA). Comparison of means was performed using Tukey–Kramer HSD at P < 0.05. 4. Results and discussion 4.1. Physiochemical properties 4.1.1. Yield of isolated and acid treated sweet potato starches The yield of isolated sweet potato starch was 10.20%. The recovery yield of citric acid modified sweet potato starches was above 85% (Table 2) and it was in the range as reported by Dutta et al. [28] and Babu et al. [29]. With the increase in acid concentration, yield was reduced as starches might be hydrolyzed more rapidly at higher acid concentration. 4.1.2. Analysis of ash, protein, fat and total fiber The isolated native sweet potato starch had 0.26 ± 0.11% ash, 0.25 ± 0.14% protein, 0.07 ± 0.02% fat and 0.57 ± 0.10 total fiber. These values were consistent with the earlier report [30]. The isolated starch had minor protein and fat contents. 4.1.3. Dextrose equivalent (DE) DE value is an indication of extent of acid hydrolysis. The degree of citric acid hydrolysis of sweet potato starch was not severe as revealed by dextrose equivalent (DE) value. The DE value of acid-thinned sweet potato starches (Table 2) was ranged between 1.90 and 2.34% and it showed that the DE value was increased with increased acid concentration and hydrolysis time. Since CTS4 starch was treated for a greater hydrolysis conditions, it exhibited a higher DE value of 2.34% compared to their counterparts. Thys, Aires, Marczak, and Norena [6] reported a DE value of 6.5 for pinhao starch treated with HCl. Nevertheless DE values obtained in the present study was much lower as citric acid used for acid treatment was a weak organic acid, hence the degree of hydrolysis of these starches seems to be lower. Since the DE value of acid treated starches were within the range referred by National Starch Table 1 Hydrolysis conditions. Treatments
HT (h)
AC (%)
NS CTS1 CTS2 CTS3 CTS4
– 1 1 11 11
– 1 5 1 5
NS, native starch; CTS1.CTS4, citric acid treated starches; HT, hydrolysis time; AC, acid concentration.
and Chemical Corporation [12], indicates a potential applicability of the citric acid thinned sweet potato starch as a fat mimetic. 4.1.4. Apparent amylose The apparent amylose content of NS was 18.56%, which was on par with Tsakama, Mwangwela, Manani, and Mahungu [31]. Acid treated starches found to display a higher fraction of apparent amylose which was significant with NS. The highest apparent amylose (around 31%) content was noticed for CTS4 sample. This increased trend was in linear fashion with acid concentration and hydrolysis period. The removal of lipids from the starch samples by acid may result in higher value for amylose content. Increase in amylose might also be due to the formation of intramolecular and intermolecular linkages between residues of amylose, which increases the length of these chains and their capacity to form complexes with iodine, increasing the apparent amylose values. Another possible reason might be due to the de-polymerization of amylopectin fractions on continuous acid hydrolysis. High degree of acid hydrolysis led increased apparent amylose content of starch [32]. Starches with a higher linear fraction (amylose content) are able to bind strongly and orient water to endow with a sensation comparable to the rheology of fat in the oral cavity [33] hence CTS4 starch could mimic the functionality of fat when used as a fat replacer. Vanderveen and Glinsmann [9] suggested that starch should possess a 20% amylose to act as a fat replacer. 4.1.5. Moisture and dry matter Moisture content and dry matter of NS was 14.11% and 85.89%, respectively, whereas all acid thinned starches showed low moisture content and high dry matter. Hydrolysis time showed a noticeable effect on the moisture content. Increase in hydrolysis time would provide ample time for the starch to react with citric acid which results in increase in moisture content of starch. This pattern might be related to the reaction between the OH groups of glucose units of starch and the functional groups (OH) of citric acid used in this chemical modification, decreasing the possibility of reaction between OH of starch chains and the water molecules. Consequently, the probability of joining of water to this polymer would be reduced causing decrease in moisture content of modified starch thereby increase in dry matter [34]. A similar trend was observed by Omojola, Manu, and Thomas [35] during acid hydrolysis of cola starch. 4.1.6. Melting point, clear point and gel thermo-reversibility Melting point, clear point and thermo-reversibility of native and acid-thinned sweet potato starches are shown in Table 2. All the starches illustrated a perfect gel formation when gelatinized and stored under refrigeration at 4 ◦ C. Melting point of NS was observed at 69.33 ◦ C whilst the acid treated starches melted at a temperature lower than the native starch. The CTS4 starch had shown its melting temperature at 47.66 ◦ C, similar to the melting point of fats, which indicated that it would be used as a fat substitute. Amylopectin plays a major role in starch granule crystallinity and the presence of amylose indirectly lowers the
Table 2 Physicochemical properties of native and acid-thinned sweet potato starches. S. No.
Starch recovery yield (%)
DE (%)
NS CTS1 CTS2 CTS3 CTS4
100 92.93 89.84 91.33 85.05
± ± ± ± ±
– 1.90 ± 2.21 ± 2.04 ± 2.34 ±
0.00aA 1.10bB 1.24cB 0.80bB 2.56cC
Apparent amylose (%) 0.08aA 0.05bA 0.03aA 0.05bB
18.56 24.78 29.59 30.27 31.04
± ± ± ± ±
1.06aA 1.36bB 1.32cB 1.88bC 1.38cC
Moisture (%) 14.11 6.62 11.00 11.30 11.83
± ± ± ± ±
2.17aA 1.45bB 1.00aA 0.60aA 1.04aA
Dry matter (%) 85.89 93.38 89.00 88.70 88.17
± ± ± ± ±
0.80aA 3.07bB 1.00abB 1.47bAB 0.76abB
Melting point (◦ C) 67.00 56.33 50.33 55.66 47.66
± ± ± ± ±
2.64aA 2.04bB 2.65cB 1.86bB 1.94cC
Clear point (◦ C)
GTR
In vitro digestibility (%)
– 68.33 ± 61.00 ± 66.33 ± 58.33 ±
No Yes Yes Yes Yes
63.27 72.59 70.59 76.22 78.54
0.57aA 1.52bA 1.52aB 2.88bB
± ± ± ± ±
4.27aA 11.67aA 5.78aA 7.18abA 7.40bB
Mean values followed by different letters in the same column indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations. DE, dextrose equivalent; GTR, gel thermo reversibility.
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Table 3 Crystallinity, granule size and functional properties of native and acid-thinned sweet potato starches. Samples
Crystallinity (%)
NS CTS1 CTS2 CTS3 CTS4
35.33 34.26 38.80 40.50 35.55
± ± ± ± ±
2.62aA 1.63aA 1.31aA 0.50bB 0.50aA
Granule size (m) 8.61 8.08 8.67 8.00 8.66
± ± ± ± ±
5.32aA 2.77aA 2.39aA 3.01aA 4.12aA
Water holding capacity (%) 34.90 36.21 46.13 38.80 56.15
± ± ± ± ±
Emulsion activity (%)
3.31aA 1.17aA 3.12bB 3.48bB 4.20aA
54.44 66.45 65.71 66.30 68.62
± ± ± ± ±
1.92aA 0.79bB 1.37cbB 1.72cbC 1.53bB
Emulsion stability (%) 41.60 42.89 42.72 41.78 42.99
± ± ± ± ±
1.75aA 2.06aA 1.62aA 1.28aA 4.05aA
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentration.
Table 4 Textural profile of native and acid thinned sweet potato starch. Samples
Hardness (kg)
NS CTS1 CTS2 CTS3 CTS4
1.64 1.11 0.56 0.28 0.27
± ± ± ± ±
0.07aA 0.10bB 0.32cB 0.02bC 0.00bB
Adhesiveness (kg/s) 0.05 0.01 0.00 0.08 0.02
± ± ± ± ±
0.01aA 0.00bB 0.00bB 0.01bA 0.00aB
Springiness 0.74 0.77 0.65 0.76 0.82
± ± ± ± ±
Cohesiveness
0.01aAB 0.11aA 0.04aB 0.02aA 0.07aA
0.49 0.71 0.49 0.49 0.61
± ± ± ± ±
0.05aA 0.13aB 0.11aA 0.00aA 0.00bA
Chewiness 0.40 0.39 0.16 0.10 0.13
± ± ± ± ±
0.11aA 0.00aA 0.07bB 0.00bB 0.01bB
Gumminess 0.54 0.63 0.23 0.14 0.16
± ± ± ± ±
0.14aA 0.04aA 0.09bB 0.01bB 0.00bB
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
melting point of the starch granule [36]. Acid treatment of sweet potato starch might result in the formation of shorter amylopectin chains with less stable crystalline structure consequently facilitating a lower melting point and clear point [37]. Thys, Aires, Marczak, and Norena [6] noticed a melting point of 46 ◦ C for acid treated pinhao starch. All the treated starches had shown a clear point around 60 ◦ C however the clear point was not displayed by native starch even at 80 ◦ C and it might be above 80 ◦ C. Native starch resisted the gel thermo-reversibility. Conversely, acid-thinned sweet potato starches displayed gel thermo-reversibility which implies acid treatment of starch caused partial hydrolysis of starch chains, resulting in lower paste viscosity. However, when the paste cools down, acid-thinned starch chains tend to associate with each other more easily, forming a thermo-reversible gel [38]. Similar fashion of gel thermo-reversibility was registered in the previous study [6] for pinhao and corn starches.
4.1.7. In vitro digestibility Table 2 displays the in vitro digestibility of NS and CTS which was measured by glucoamylase. The in vitro digestibility of NS was about 63.27% and it was in the range with the previous report on sweet potato starch (28.3–67.2%) [38]. The digestibility rate of the CTS4 sample was significantly increased up to 78.54%. In view of the fact that acid hydrolysis preferentially attacks the amorphous area in the starch granule, the crystallites were decoupled from the amorphous parts consequently, unlocked amorphous regions would be more sensitive to the enzyme attack and prone to rapid hydrolysis on the external glucose residues of amylose or amylopectin. In the report of Shi et al. [39] HylonV starch, normal maize starch and waxy maize starch samples when subjected to acid treatment resulted in less resistant to ␣-amylase digestion. After acid hydrolysis of the starches, the amorphous structure of the starches was hydrolyzed, so that the density of the residual amorphous structure of the starches decreased. However, as the specific surface area increased, starches were easily reacted with the enzymes and as a result the hydrolysis rate of starch samples was greater than native starch. This higher digestibility of citric acid treated starches would be beneficial for its role as a fat replacer since FDA [11] recommended that a fat replacer to be partially or completely digestible.
5. Functional properties 5.1. Water holding capacity (WHC) WHC of NS was 34.90%. A significant difference was observed in the WHC among the starches (Table 3). The highest and lowest water holding capacity was detected in CTS4 starch and CTS1 starches respectively. WHC was directly proportional to the acid concentration and as well as hydrolysis time. High acid concentration probably increased the low molecular weight starch fraction with hydroxyl groups which may hold water molecules forming hydrogen bonds consequently increasing the WHC. This high water holding capacity of citric acid treated starch (CTS4) may find a significant role as a fat replacer. 5.2. Emulsion activity and emulsion stability Acid thinned starch, CTS4 exhibited a higher emulsion activity and emulsion stability compared to native starch (Table 3). Starch (CTS4) with higher amylose found to exhibit superior emulsion properties. The present study revealed that higher amount of linear amylose fraction would contribute to emulsion activity. The high amylose starch might function as the interface between oil and water during which linear amylose chains of starch granules were more favored in stabilizing the emulsion system than branched amylopectin chains. The linear amylose fractions could be capable of film formation that enhances the emulsion capacity and stability of the starch [40]. No significant difference in emulsion stability was noticed among the starches. 6. Texture analysis The texture profile analysis of NS and CTS samples is shown in Table 4. The gel strength of NS was 1.644 kg and acid modified starches ranged from 0.27 kg to 1.11 kg. The gel formed from NS was harder than CTS samples due to the degree of long chains in sweet potato native starch which contributed to its firmer gel. The lesser gel strength of the acid-thinned sweet potato starch might be attributed to a higher degree of short chains due to acid hydrolysis [41]. Wang and Wang [41] reported that gel strength (GS) of 0.30, 0.48 and 0.09 kg for acid thinned corn, potato and rice starches respectively.
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Table 5 Pasting properties of native and acid thinned sweet potato starch. Sample
Peak viscosity (cP)
NS CTS1 CTS2 CTS3 CTS4
6338.00 6097.00 4812.00 4736.33 4655.33
± ± ± ± ±
340.54 aA 128.31 aA 79.30bB 113.95bB 190.11bB
Trough viscosity (cP) 3288.00 3027.00 1971.66 2248.66 1738.33
± ± ± ± ±
189.59aA 62.69aA 55.94bB 84.31bB 144.36cB
Break down (cP) 3050.00 3070.00 2840.33 2487.66 2917.00
± ± ± ± ±
186.34aA 77.11 aA 83.18aA 34.50aA 53.45 aA
Final viscosity (cP) 4290.66 4087.33 3028.66 3190.66 2683.66
± ± ± ± ±
168.35aA 75.79 aA 83.57 bB 120.35 bB 189.95cB
Set back (cP) 1002.66 1060.33 1057.00 942.00 945.33
± ± ± ± ±
Peak time (min)
24.68abA 48.67aA 29.05 bA 41.60aA 45.65aA
4.00 3.93 3.82 3.91 3.80
± ± ± ± ±
0.07 aA 0.00 aA 0.04bB 0.03aA 0.07aAB
Peak temperature (◦ C) 70.81 71.00 71.51 70.76 70.73
± ± ± ± ±
0.49aA 0.82aA 0.49 aA 0.40aA 0.37aA
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
A similar significant decrease in the hardness of chick pea starch gel upon acid treatment was reported by Sodhi, Chang, Midha, and Kohyama [42]. Springiness represents the ability of a gel to recover its original shape/height after a deforming force is removed [43]. No significant change in the springiness was noticed due to hydrolysis time. Adhesiveness is the ability of the gel sample to become sticky [44]. It is a surface characteristic which depends on a combined effect of adhesive and cohesive forces, viscosity, and viscoelasticity of the sample [45]. Adhesiveness of NS and CTS was ranged between 0.00 g/s and 0.08 g/s. Starch with high amylose content was observed to have lower adhesiveness [46]. Cohesiveness is how well a sample withstands a second deformation related to how it behaved under the first deformation [47]. Cohesiveness indicates how good the sample retains its structure after the first compression. Cohesiveness of starch samples was ranged from 0.49 to 0.71. Gumminess is the product of hardness and cohesiveness, a characteristic of semi-solid foods which have a low degree of hardness and a high degree of cohesiveness [47]. NS displayed a higher chewiness compared to the CTS samples while CTS1 showed a higher gumminess than NS. The difference in textural properties of all sample gels was influenced by rigidity in gelatinized starch, amylose content as well as interaction between the dispensed and continuous phase of the gel, which in turn was dependent on the amylose and amylopectin structure [48]. 7. Pasting properties Table 5 reveals a significant influence of acid concentration and reaction time on RVA of the acid thinned sweet potato starch. RVA results of the acid thinned starches confirmed a low PV ranging from 4655.33 cP to 6097.00 cP compared to native starch (6338.00 cP). Results showed that the PV decreased with increase in acid concentration and reaction time. Similar fashion of change was reported in the literature [28,49]. CTS1 presented a viscosity profile higher than their counterparts, although substantially lower than the native starch. The lower PV of acid modified starches could be due to considerable breakdown of amorphous regions and the production of low molecular weight dextrins [50]. TV and BD values of CTS samples also displayed the same decreasing trend. The increased degree of amylose recrystallization by acid thinning might be due to the change in BD [51]. Acid thinned sweet potato starches displayed a lower FV ranging between 3028.66 and 4087.33 cP against 4290.66 cP for sweet potato native starch. Han, Campanella, Mix, and Hamaker [52] reported that acid hydrolysis resulted a considerable lyses of glycoside linkages of the long amylopectin chains, which apparently caused the fall in FV. SB is a measure of recrystallization of gelatinized starch. CTS1 and CTS2 registered a higher SB than NS indicating that these starches got a higher retrogradation tendency than NS. The low SB of the rest of the acid-thinned starches was likely due to in sufficient time for the starch molecules to rearrange themselves during the stipulated period [53]. Native starch took more time (4 min) to reach its PV than acid thinned starches. Hydrolysis time and acid concentration basically did not affect the PT of sweet potato starch in native form and acid modified form, however with the increase
Table 6 Thermal properties of native and acid-treated sweet potato starch. Gelatinization temperature (◦ C)
Sample To NS CTS1 CTS2 CTS3 CTS4
42.31 35.81 35.35 35.97 39.44
Tp ± ± ± ± ±
2.12aA 1.61bB 0.83bB 1.00bB 1.50aA
81.25 83.58 81.56 83.75 86.75
H (J/g)
Tc ± ± ± ± ±
0.66 aA 2.50aA 1.77aA 0.03cA 0.25bB
116.12 120.10 119.43 116.77 122.69
± ± ± ± ±
2.01 aA 2.85aA 2.06aAB 2.04aA 2.33bB
12.96 12.74 12.13 12.69 11.95
± ± ± ± ±
0.05aA 0.03aA 0.80aA 1.29aA 1.31aA
To, Tp and Tc stand for onset, peak, and conclusion temperatures respectively. H (J/g) indicates enthalpy. Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
in acid concentration and hydrolysis time, mild changes in pasting parameters were noticed. A similar decrease in pasting profile was observed upon acid treatment of corn by Singh, Sodhi, and Singh [50] in acid thinned sorghum starch. 8. Thermal analysis Thermal properties of starches determined by the DSC are represented in Table 6. Results showed variations in To, Tp, Tc temperatures and H among NS and CTS samples. NS had higher To of 42.31 ◦ C while citric acid treated starches showed a decreased trend. This was in agreement with the result reported for acid modified maize starch which showed a decreased To value [54]. The Tp and Tc of native starch were 81.25 ◦ C and 116.12 ◦ C respectively, conversely the acid modified starches displayed a higher Tp and Tc temperatures. However, this trend was more pronounced in case of CTS4. This specifies that a higher degree of hydrolysis might occur in CTS4 at amorphous region, thus resulting in an increase in relative crystallinity and subsequently an increase in the gelatinization temperature. Similar results were reported for acid modified potato starch [40] and sweet potato starch [55]. The H of NS was 12.96 J/g, on the other hand H of acid modified starches was lower than their counterpart. Enthalpy of the citric acid treated starches ranged between 11.95 and 12.74 J/g. CTS4 starch showed a lower enthalpy compared to all other modified starches. The decrease in H was the result of citrate substitution that altered the chain packing and generated more amorphous region [56]. H gives an overall measure of crystallinity and is an indicator of the loss of molecular order within the granule during gelatinization [57]. The increase in citric acid concentration and treatment time decreased the enthalpy that might be due to greater loss of ordered structure of starch
9. Rheological analysis Shear rate versus shear stress plot at 85 ◦ C was well fitted to the power law model (Eq. (1)) with determination coefficients (R2 ) ranging from 0.97 to 0.99, represented in Table 7. Native and acid modified starches displayed the values of flow behavior indexes (n) less than 1 indicating a shear-thinning behavior. NS showed a
A
200 180 160 140 120 100 80 60 40 20 0
563
B
70 60 NS CTS1 CTS2 CTS3 CTS4
Viscosity (Pa.S)
Shear Stress (Pa)
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50
NS CTS1
40
CTS2
30
CTS3 20
CTS4
10 0 0
50 Shear Rate (s-1)
100
0
10 20 30 40 50 60 70 80 90 100 Shear Rate (1/s)
Fig. 1. (A) Plot of shear rate (Pa) versus shear stress (s−1 ), (B) plot of shear rate (Pa) versus viscosity (Pa s) for a 10% NS and CTS dispersion heated at 85 ◦ C.
Table 7 Rheological properties of acid treated sweet potato starches (10%, w/v) at 85 ◦ C. Samples
n
NS CTS1 CTS2 CTS3 CTS4
0.46 0.42 0.40 0.35 0.22
K (Pa sn ) ± ± ± ± ±
0.03aA 0.02abA 0.01bB 0.01dD 0.01cC
157.94 87.92 101.12 102.81 55.63
± ± ± ± ±
R2 1.93aA 1.73bB 1.20cB 0.92bC 1.09cC
0.97 0.98 0.99 0.97 0.98
± ± ± ± ±
0.01aA 0.01bB 0.00cA 0.01bA 0.01cA
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
higher flow behavior index while CTS4 samples exhibited a lower flow behavior index suggesting that it possess a greater shear thinning behavior than its counter NS studied. This shear-thinning behavior might be attributed to the higher amount of breakage of the intra-and inter-molecular associative bonding system in the starch network micelles due to shearing at higher shear rates, as noted by Bhandari et al. [58]. It was evident that n was higher for NS which contains lower amylose content. The decrease in the power law index (n) with increasing amylose content (CTS4) was generally attributed to an increase in the entanglements between amylose chains, since the highly branched amylopectin was not expected to form effective entanglements [59]. Same was the case with the consistency index where NS exhibited a higher K value while CTS4 showed a lower K value. All starches showed a nonNewtonian behavior, where the viscosity decreased when shear rate was increased (Fig. 1). This pattern is defined as shear thinning and is produced when the stress disorganized the arrangement of the macromolecules inside of the matrix [60]. When shear force is applied, the particles have rearranged themselves into a parallel direction with shear force and big particles are broken into small particles. The particles can flow easily because of resistance arising from particle–particle interaction which results in decreasing of viscosity [61].
10. Structural properties 10.1. X-ray diffraction (XRD) X-ray diffractogram of starch samples are presented in Fig. 2. The native and modified sweet potato starches exhibited a C-type X-ray diffractogram pattern. All the starches displayed peaks at the 2 diffraction angles around 10, 11, 15, 17, 20, 23 and 26. The degree of crystallinity was higher in CTS3 (40.50%) followed by CTS2 (38.80%). The results of the present study showed that the crystallinity of sweet potato starch was increased with increased
time of hydrolysis (Table 3) as suggested by Atichokudomchai, Shobsngob, and Varavinit [62] nevertheless increased reaction time and acid concentration does not alter the diffraction pattern. Similar observations were reported by Babu et al. [29]. This was attributed to preferential hydrolysis of the amorphous regions of the starch granule [63].
10.2. Scanning electron micrograph (SEM) Scanning electron micrograph (SEM) of the sweet potato native starch is illustrated in Fig. 2. Illustration of starch sample showed the presence of starch granules from small to large sizes. The granule surface of starches appeared to be smooth with no sign of any fissure. Zhu et al. [64] also observed that the sweet potato starches with smooth granule surface without cracks. Most of the sweet potato starch granules were polygonal in shape, however round and irregular shapes were also noted. Surface of CTS1 starch granule was found to have a crack (depicted by arrows) as acid might have attacked the surface of the granule. While there was a loss of smooth surfaces of the starch granules was noticed in the CTS2 and this was more pronounced in CTS4. Starch granules were found to conglutinate in CTS2 and CTS4 which was due to the acid erosion on the core part of the granules. Similar result was noticed in the study conducted by Babu et al. [65]. The extent of visible degradation was increased when the granules were subjected to a higher hydrolysis time and acid concentration. An acid acts on the surface of the starch granule first before it gradually enters the inner region. The acid to diffuse into the internal part of the granule it must pass through some alternating amorphous and crystalline regions [66]. The amorphous areas of the starch granules had a looser structure than the crystalline regions which was easier to attack with the hydrogen ions. Furthermore, native starches had some flaws on the surface of the granules which could provide channels for the infiltration of hydrogen ions. These hydrogen ions primarily got to the amorphous areas and attacked them [41]. Hence acid cause surface alterations and degrade the external part of the granule by exo-corrosion. When exo-corrosion occurs, the internal part of the granule was corroded through small cracks through which acids could penetrate the granule [67]. Erosion and roughness on the surface of sorghum starch granules were recorded after acid treatment by Ali and Hasnain [68]. They also stated that acid treatments did not significantly change the size of sorghum starch granules. Granule size of sweet potato starches was not affected by acid treatment. Granule size of native and acid modified starches (Table 3) varied from 8.00 to 8.67 m respectively. Granule size of CTS was greater than the size required for a fat replacer which was mentioned in the literature.
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Fig. 2. Scanning electron micrograph (SEM) of (A) NS, (B) CTS1, (C) CTS2, (D) CTS3, (E) CTS4 and (F) X-ray diffractogram of NS and CTS samples.
11. Conclusion The recovery of starch after acid treatment at various concentrations and hydrolysis times was more than 85%. The degree of citric acid hydrolysis of sweet potato starch was quiet low as revealed by the dextrose equivalent (DE) value. The citric acid treatments lead to an increase in apparent amylose content. Acid hydrolysis might facilitate to lower the melting point of sweet potato starch. Functional properties were significantly affected upon treatment with acid. CTS demonstrated a reduced gel strength that attributed
to a higher degree of short chains. A marked decline in viscosity was observed for acid treated starch compared to NS as proved by RVA results. Higher gelatinization temperature (Tp and Tc) and crystallinity displayed by CTS that specifies acid hydrolysis might occur at amorphous region. The power-law model was found to best describe the shear stress versus shear rate data with R2 values ranged between 0.97 and 0.99 for NS and CTS samples with n value less than 1. A visible degradation was observed when the granules were subjected to citric acid treatment with no significant change in granule size. Our results suggest that the citric acid
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treatment of sweet potato starch with 5% acid concentration and 11 h period was an ideal condition for the preparation of an acid thinned starch which would find a potential role as a fat mimetic in real food systems. Conflict of interest The authors have declared no conflicts of interest. References [1] C.C. Akoh, Fat Replace. 52 (1998) 47–53. [2] C.A. Hassell, Cereal Food World 38 (1993) 142–144. [3] M. Güven, K. Yas¸ar, O.B. Karaca, A.A. Hayaloglu, Int. J. Dairy Technol. 58 (2005) 180–184. [4] J. Lim, G.E. Inglett, S. Lee, Jpn. J. Food Eng. 11 (2010) 147–152. [5] J.P. Robin, C. Mercier, F. Duprat, R. Charbonniere, A. Guilbot, Starch/Stärke 27 (1975) 36–45. [6] R.C.S. Thys, A.G. Aires, L.D.F. Marczak, C.P.Z. Norena, Ciênc. Tecnol. Aliment. Camp. 33 (2013) 89–94. [7] S.L. Amaya-Llano, A.L. Martínez-Alegría, J.J. Zazueta-Morales, F. Martínez-Bustos, LWT-Food Sci. Technol. 41 (2008) 1274–1281. [8] Y. Ma, C. Cai, J. Wang, D. Sun, J. Food Eng. 73 (2006) 297–303. [9] J.E. Vanderveen, W.H. Glinsmann, Annu. Rev. Nutr. 12 (1992) 473–487. [10] J.R. Daniel, R.L. Whistler, Cereal Food World 35 (1990) 825. [11] Food and Drug Administation, Toxicological Principles for the Safety Assessment of Direct Food Additives and Colour Additives Used in Food, Bureau of Foods, Washington, DC, 1982. [12] National Starch and Chemical Corporation. US Patent 4,510,166, 1985. [13] J.A. Radley, Industrial Uses of Starch and its Derivates, Applied Science Publishers Ltd., London, 1976, pp. 58. [14] D. Clark, Food Technol. 48 (1994) 86–87. [15] Sweet Potato Starch Production-TM22-3e, International Starch Institute, Science Park Aarhus, Denmark, 1999–2006, www.Starch.Dk [16] S.K. Sathe, D.K. Salunkhe, J. Food Sci. 46 (1999) 71–76. [17] H.A.M. Wickramasinghe, S. Takigawa, G. Matsura-Endo, H. Yamauchi, T. Noda, Food Chem. 112 (2009) 98–103. [18] F. Zambrano, C.R.O. Camargo, Braz. J. Food Technol. 4 (2002) 147–154. [19] Association of Official Chemists (AOAC), Official Methods of Analysis, 15th ed., Association of Official Analytical Chemists, Arlington, VA, USA, 2000. [20] G.L. Miller, Anal. Chem. 31 (1959) 426–428. [21] V.R. Williams, W. Wu, H.Y. Tsai, H.G. Bates, J. Agric. Food Chem. 6 (1958) 47–48. [22] A.O. Adebayo, S.O. Lateef, A.B. Elizabeth, J. Am. Sci. 6 (2010) 65–72. [23] T. Noda, S. Takigawa, C. Matsuura-Endo, K. Saito, K. Takata, T. Tabiki, H.A.M. Wickramasinghe, H. Yamauchi, Carbohydr. Polym. 56 (2004) 271–277. [24] L.L. Niba, M.M. Bokanga, F.L. Jackson, D.S. Schlimme, B.W. Li, J. Food Sci. 67 (2001) 1701–1705. [25] V.Q. Neto, N. Narain, J.B. Silvia, P.S. Bora, Nahrung 45 (2001) 258–262. [26] R. Artiaga, S. Nayab, A. Garcia, F. Barbadilloa, L. Garcia, Thermochim. Acta 428 (2005) 137–139. [27] S. Nara, T. Komiya, Starch/Starke 35 (1983) 407–410. [28] H. Dutta, S.K. Paul, D. Kalita, C.L. Mahanta, Food Chem. 128 (2011) 284–291. [29] A.S. Babu, R. Parimalavalli, K. Jagannadham, J.S. Rao, J. Food Sci. Technol. (2014), http://dx.doi.org/10.1007/s13197-014-1650-x
565
[30] S.N. Moorthy, Starch/Stärke 54 (2002) 559–592. [31] M. Tsakama, A.M. Mwangwela, T.A. Manani, N.M. Mahungu, Int. Res. J. Agric. Sci. Soil Sci. 1 (2011) 254–260. [32] A.D. Betancourt, G. Chel, J. Agric. Food Chem. 45 (1997) 4237–4241. [33] W.C. Yackel, C. Cox, Food Technol. 46 (1992) 146–148. [34] R. Carmona-Garcia, M.M. Sanchez-Rivera, G. Méndez-Montealvo, B. Garza-Montoya, L.A. Bello-Pérez, Carbohydr. Polym. 76 (2009) 117–122. [35] M.O. Omojola, N. Manu, S.A. Thomas, Afr. J. Pure Appl. Chem. 5 (2011) 307–315. [36] J.J.G. van Soest, R.C. Bezemer, D. de Wit, J.F.G. Vliegenthart, Ind. Crops Prod. 5 (1996) 1–9. [37] R.C. Hoseney, Principles of Cereal Science and Technology, American Association of Cereal Chemists, Inc., St. Paul, 1994, pp. 52–54. [38] Z. Zhang, Nutritional Quality and Starch Physicochemical Properties in Sweetpotato, Ph.D. thesis, The University of Hong Kong, Hong Kong, 2001 http://hdl.handle.net/10722/36214 [39] M. Shi, W. Lu, S. Yu, R. Ward, Q. Gao, Starch/Stärke 66 (2014) 429–435. [40] J.A. Han, S.T. Lim, Carbohydr. Polym. 90 (2012) 1480–1485. [41] L. Wang, Y.J. Wang, Starch/Stärke 53 (2001) 570–576. [42] N.S. Sodhi, Y.-H. Chang, S. Midha, K. Kohyama, Int. J. Food Prop. 16 (2013) 125–138. [43] E.Y. Park, B. Baik, S. Lim, J. Cereal Sci. 50 (2009) 43–48. [44] C. Saric¸oban, M.T. Yilmaz, M. Karakaya, Meat Sci. 83 (2009) 610–619. [45] M. Huang, J.F. Kennedy, B. Li, X. Xu, B.J. Xie, Carbohydr. Polym. 69 (2007) 411–418. [46] S. Yu, M. Ying, S. Da-Wen, J. Cereal Sci. 50 (2009) 139–144. [47] M.C. Bourne, Food texture and viscosity concept and measurement, in: Food Science and Technology International Series, 2nd ed., Academic Press, San Diego, 2002, pp. 427. [48] F.F. Yamin, M. Lee, L.M. Pollak, P.J. White, Cereal Chem. 76 (1999) 175–181. [49] K.S. Sandhu, N. Singh, S.T. Lim, LWT: Food Sci. Technol. 40 (2007) 1527–1536. [50] H. Singh, N.S. Sodhi, N. Singh, Int. J. Food Prop. 12 (2009) 713–725. [51] H.-J. Chung, H.-Y. Jeong, S.-T. Lim, Carbohydr. Polym. 54 (2003) 449–455. [52] X.Z. Han, O.H. Campanella, N.C. Mix, B.R. Hamaker, Cereal Chem. 79 (2002) 897–901. [53] D. Thirathumthavorn, S. Charoenrein, Starch/Stärke 57 (2005) 217–222. [54] J.H. Lin, S.Y. Lee, Y.H. Chang, Carbohydr. Polym. 53 (2003) 475–482. [55] S. Singh, C.S. Raina, A.S. Bawa, D.C. Saxena, J. Food Sci. 70 (2005) E373–E378. [56] H. Liu, R. Liang, J. Antoniou, F. Liu, C.F. Shoemaker, Y. Li, F. Zhong, Food Chem. 159 (2014) 222–229. [57] R.A. Hoover, T. Vasanthan, Carbohydr. Res. 252 (1994) 33–53. [58] P.N. Bhandari, R.S. Singhal, D.D. Kale, Carbohydr. Polym. 47 (2002) 365–371. [59] L. Yu, G. Christie, J. Mater. Sci. 40 (2005) 111–116. [60] D. Guerra-DellaValle, M.M. Sánchez-Rivera, P.B. Zamudio-Flores, G. Méndez-Montealvo, L.A. Bello-Pérez, Rev. Mex. Ing. Quím. 8 (2009) 197–203. [61] S.E. Charm, Adv. Food Res. 11 (1962) 356–435. [62] N. Atichokudomchai, S. Shobsngob, S. Varavinit, Starch/Stärke 52 (2000) 281–283. [63] R. Hoover, Food Rev. Int. 16 (2000) 369–392. [64] F. Zhu, X. Yang, Y. Cai, E. Bertoft, H. Corke, Starch/Starke 63 (2011) 249–259. [65] A.S. Babu, R. Parimalavalli, K. Jagannadham, J. Rao, R.S. Gaur, Int. J. Food Prop. (2015), http://dx.doi.org/10.1080/10942912.2015.1020390 [66] N.K. Genkina, J. Wikman, E. Bertoft, V.P. Yuryev, Biomacromolecules 8 (2007) 2329–2335. [67] O.O. Olayinka, K.O. Adebowale, I.B. Olu-Owolabi, J. Food Sci. Technol. 50 (2013) 70–77. [68] T.M. Ali, A. Hasnain, Int. J. Food Prop. 17 (2014) 523–535.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Effect of citric acid concentration and hydrolysis time on physicochemical properties of sweet potato starches Ayenampudi Surendra Babu a , Ramanathan Parimalavalli a,∗ , Shalini Gaur Rudra b a b
Department of Food Science and Nutrition, School of Professional Studies, Periyar University, Salem 636011, Tamil Nadu, India Department of Food Science and Post Harvest Technology, Indian Agriculture Research Institute, New Delhi 110012, India
a r t i c l e
i n f o
Article history: Received 29 April 2015 Received in revised form 25 June 2015 Accepted 12 July 2015 Available online 15 July 2015 Keywords: DSC Fat replacer SEM Sweet potato starch XRD
a b s t r a c t Physicochemical properties of citric acid treated sweet potato starches were investigated in the present study. Sweet potato starch was hydrolyzed using citric acid with different concentrations (1 and 5%) and time periods (1 and 11 h) at 45 ◦ C and was denoted as citric acid treated starch (CTS1 to CTS4) based on their experimental conditions. The recovery yield of acid treated starches was above 85%. The CTS4 sample displayed the highest amylose (around 31%) and water holding capacity its melting temperature was 47.66 ◦ C. The digestibility rate was slightly increased for 78.58% for the CTS3 and CTS4. The gel strength of acid modified starches ranged from 0.27 kg to 1.11 kg. RVA results of acid thinned starches confirmed a low viscosity profile. CTS3 starch illustrated lower enthalpy compared to all other modified starches. All starch samples exhibited a shear-thinning behavior. SEM analysis revealed that the extent of visible degradation was increased at higher hydrolysis time and acid concentration. The CTS3 satisfied the criteria required for starch to act as a fat mimetic. Overall results conveyed that the citric acid treatment of sweet potato starch with 5% acid concentration and 11 h period was an ideal condition for the preparation of a fat replacer. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Today’s dietary concern is the consumption of huge quantity of fat and sugar. With the mounting incidence of diabetes and obesity, low calorie foods have acquired the huge esteem. In general, the best suitable approach in terms of fat reduction diets involves either the use of low-fat foods or fat substitutes or modifications such as trimming of fat from foods [1,2]. Fat Replacers consist of mixtures of lipid-originated fat substitutes, protein- or carbohydrate-originated fat mimetic, or their combinations [3]. Carbohydrate-based Replacers incorporate water into a gel type structure, resulting in a lubricant or flow properties similar to those of fats in food systems [2]. Even though a variety of fat replacers have been developed, there are unfortunately no ideal fat replacers which completely function like conventional fat [4]. Native starch can sometimes be used to replace fat [1]; however starch modified by acid or enzymatic hydrolysis, oxidation,
Abbreviations: NS, native sweet potato starch; CTS, citric acid treated starch; HT, hydrolysis time; AC, acid concentration; DE, dextrose equivalent; WHC, water holding capacity; PV, peak viscosity; BD, break down; TV, trough viscosity; SB, setback; FV, final viscosity; Pt, pasting time; PT, pasting temperature; To, onset temperature; Tp, peak temperature; Tc, final temperatures; H, gelatinization enthalpy. ∗ Corresponding author. E-mail address: [email protected] (R. Parimalavalli). http://dx.doi.org/10.1016/j.ijbiomac.2015.07.020 0141-8130/© 2015 Elsevier B.V. All rights reserved.
dextrinization, cross linking, or mono-substitution is more commonly used to achieve desired functional and sensory properties [1]. Generally, acid hydrolysis occurs more rapidly in amorphous regions than in crystalline region and the residue after prolonged acid hydrolysis consists of acid-resistant crystalline parts of amylopectin [5]. Thys et al. [6] investigated the functional properties of acid-thinned pinhao starch and it showed low syneresis, high solubility, thermo reversibility and melting point similar to fat. They concluded that the acid treatment was efficient in producing a potential fat substitute from pinhao starch. Amaya-Llano et al. [7] produced acid hydrolyzed jicama starch and used as a fat substitute in yoghurt. The addition of hydrolyzed jicama starch (2.03 g/100 g) as a fat substitute in the preparation of stirred yoghurt had good functional and sensorial properties. Ma et al. [8] reported that enzymatic hydrolyzed corn starch could be used as fat replacers. The hydrolyzed starch with fine particles was used to produce low fat mayonnaise and the result indicated that the 60% fat-reduced mayonnaise with fat replacers had similar sensory quality as compared with the high fat one. The following are the criteria for a starch based fat mimetic – (a) Starch should contain an amylose content of ∼20% [9]. (b) Starch ought to require a granule size of 2 m or in similar size to liquid micelle to act as fat mimetic [10]. (c) According to FDA [11] a starch-based fat mimetic is supposed to be partially or completely digestible. (d) Starch must possess a DE (dextrose equivalent) of
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≤5.0 [12]. (e) Starch gel with a melting point close to that of the fats (37–45 ◦ C) could be used as a fat substitute [13]. (f) Starch must possess high water-holding capacity [9] and better emulsifying properties [4]. (g) Starch should display shear thinning characteristic [14]. Sweet potato (Ipomoea batatas (L.)) belongs to the Convolvulaceae family [15]. Sweet potato is considered as the world’s most important and under-exploited crop [16]. Industrial application of sweet potato flour and starch is insignificantly causing a negative growth in its production. The intended use of sweet potato starch for industrial purpose depends on advanced processing technologies to prepare sweet potato starch with desirable functional properties, and on thorough indulgent of the effect of processing conditions on their properties. Basing on this background the research was aimed to study the influence of hydrolysis time and acid concentration on the sweet potato starch properties and evaluate the distinctive properties of citric acid treated sweet potato starch as fat mimetic.
2. Materials and methods 2.1. Materials The sweet potato was purchased from a local supermarket (Palzhamuthir sollai), Salem, Tamil Nadu, India. Glucose Oxidase–Peroxidase (GOD–POD) kit was obtained from Beacon Diagnostics, Navasari, India. Amyloglucosidase from Aspergillus niger (≥300 U/mL) (E.C-3.2.1.3), citric acid and all other chemicals and reagents were analytical grade and purchased from SigmaAldrich, Steinheim, Germany. 2.2. Starch isolation and preparation of acid-thinned sweet potato starch Starch was isolated from sweet potato by the method of Wickramasinghe et al. [17]. An edible portion of sweet potato was cut into small pieces and homogenized with distilled water. The slurry was then passed through the double-layered cheesecloth and the filtrate was allowed to settle for a minimum of 3 h at room temperature. The precipitated starch was washed three times with distilled water, dried at room temperature (20–25 ◦ C) for 48 h and then the dried starch was kept in an oven at 50 ◦ C for three hours and ground into fine powder and named as Native sweet potato starch (NS). Citric acid treated starch (CTS) was prepared by the method of Zambrano and Camargo [18]. Starch slurry was prepared by dispersing NS (40 g dry basis) in 1% or 5% citric acid solution kept in a water bath at 45 ◦ C for 1 h or 11 h respectively with constant stirring. After each assay of hydrolysis, the pH was adjusted to 5.5 ± 0.2 by slowly adding aqueous sodium hydroxide (5 g/100 ml). The starch was washed three times with two fold volume of deionised water prior to filtration and dried in a convection oven at 45 ◦ C for 48 h. The dried starch was made into powder and packed in airtight containers for further use and recovery yield was calculated by the following formula. Recovery yield (%) =
dry weight of starch after hydrolysis × 100 dry weight of starch before hydrolysis
2.3. Physiochemical properties of native and modified starches 2.3.1. Analysis of ash, protein, fat and total fiber Estimation of ash, protein, fat and total fiber in the isolated starch material was carried out following the AOAC protocol [19].
2.3.2. Dextrose equivalent (DE) The reducing sugar value was measured using the dinitrosalicylic acid method of Miller [20] to determine its dextrose equivalent (DE). Different concentrations of dextrose standard solution were taken in test tubes and dinitrosalicylic acid reagent was added in each of the test tubes. The test tubes were heated in a boiling water bath for 5 min. The Rochelle salt solution was added to each of the test tubes while the contents were still warm. The test tubes were cooled and the absorbance at 560 nm was noted and percentage of reducing sugar was determined. DE was calculated by the formula given by Miller [20]: DE =
g reducing sugar × 100 g dry weight of starch
2.3.3. Apparent amylose Apparent amylose content determination was carried out using a colorimetric iodine affinity procedure [21] briefly a mixture of 0.1 g of the starch sample, 1 ml of ethanol and 9 ml 1 N sodium hydroxide was boiled for 10 min in a boiling water bath and allowed to cool. To a portion (5 ml) of the mixture, 1 ml of 1 N acetic acid and 2 ml of iodine solution were added and Absorbance (A) was read using a Spectrophotometer at 620 nm. The apparent amylose content was calculated as follows: Apparent amylose content (%) = 3.06 × A × 20; where A = absorbance value
2.3.4. Moisture and dry matter Moisture content and dry matter were determined by the method of Adebayo, Lateef, and Elizabeth [22]. Two milligrams (2 mg) of starch sample was measured into a previously weighed crucible. The crucible plus sample was then transferred into the oven set at 100 ◦ C, for 24 h. At the end of 24 h, the crucible plus sample was removed from the oven and transfer to desiccator cooled for ten minutes and weighed. The Moisture content and dry matter were determined and expressed in percentage. 2.3.5. Melting point, clear point and thermo-reversibility of the starch gel Starch gel was prepared by the method described by the National Starch and Chemical Corporation [12] with modification. Sweet potato starch suspension (5%) was prepared with 0.02% (w/v) sodium metabisulfite in a beaker at 80 ◦ C for 10 min and then autoclaved at 121 ◦ C for 15 min. Subsequently beaker was cooled, hermetically sealed and stored at 4 ◦ C for 24 h. The obtained gel was melted in a water bath at 80 ◦ C under agitation. The change in consistency was visually observed, and melting point was considered as the temperature at which the liquid phase was formed and mixing of the gel was possible. The clear point was considered as the temperature at which the sol appeared optically clear. For the gel thermo-reversibility, the gel was melted in a water bath with constant stirring and allowed to cool down to room temperature, followed by refrigeration at 4 ◦ C for 18 h and the gel formation was observed [7]. 2.3.6. In vitro digestibility of starch samples In vitro digestibility of starch was analyzed according to the method of Noda et al. [23] with some modifications. A mixture consisting of 4% (w/v) starch suspension in tubes was placed in a water bath at 100 ◦ C for 10 min to obtain the starch suspension. A 0.5 ml of starch suspension and 0.25 ml of 100 mM acetate buffer (pH 5.0) and 0.25 ml of glucoamylase solution was incubated at 40 ◦ C for 2 h
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with stirring. After digestion, surplus starch was removed by centrifuge (4350 tour/min during 10 min), and glucose in the filtrate was analyzed using the glucose oxidase–peroxidase reagent. 2.4. Functional properties 2.4.1. Water holding capacity (WHC) Water holding capacity was determined by the method described by Niba et al. [24]. Starch sample (1 g) was suspended in 5 ml water in a centrifuge tube. The slurry was shaken on a wristaction shaker for 1 min at ambient temperature and centrifuged at 3000 × g for 10 min. The supernatant was poured carefully into a tared evaporating dish. Water holding capacity was calculated as follows (g/g): Mass of water added to sample − Mass of water removed from the sample/mass of starch sample. 2.4.2. Emulsifying properties Emulsifying activity and stability of starch samples were determined by the method of Neto, Narain, Silvia, and Bora [25]. Starch dispersion was prepared (10 mg/ml) and homogenized for 1 min with 5 ml of refined sunflower oil. The emulsion was centrifuged (1100 × g, 5 min) and the emulsifying activity was calculated as follows. Emulsifying activity (%) = (height of the emulsified layer/height of the total content) × 100. Emulsion stability was determined by heating the emulsion at 80 ◦ C for 30 min before centrifuging (1100 × g, 5 min). Emulsion stability (%) = (height of the emulsified layer after heating/height of emulsified layer before heating) × 100. 2.5. Textural properties of the starch gel Gel textural properties were determined using a texture analyzer (Texture analyzer HD plus, Stable Microsystems, Godalming, UK). A starch suspension of 10% in a 50 ml beaker was heated to 95 ◦ C for 15 min and cooled to ambient temperature, then stored at 4 ◦ C for 24 h. The gel formed (33 mm in height and 21 mm in diameter) in the beaker was used directly for the texture analysis, and each gel was penetrated 4 mm by a P25/L (25 mm diameter) cylindrical probe. Two strength-time curves were obtained with a 1.0 mm/s speed during the penetration cycles. The texture profile curves were used to calculate gel strength, cohesiveness, adhesiveness, springiness, gumminess and chewiness. 2.6. Pasting properties Pasting profile of starch was recorded using a Rapid Visco Analyser (RVA) (RVA Tech Master, Perten Instruments, and Japan). The viscosity profiles were recorded using starch suspensions (12%, w/v). The Standard profile 1 of the Perten Instruments was used, where the samples were held at 50 ◦ C for 1 min, heated from 50 ◦ C to 95 ◦ C at 12.16 ◦ C/min, held at 95 ◦ C for 2.30 min, cooled from 95 ◦ C to 50 ◦ C at 11.84 ◦ C/min, and held at 50 ◦ C for 2 min. The peak viscosity (PV), breakdown (BD) trough viscosity (TV), setback (SB), final viscosity (FV), pasting time (Pt) and pasting temperature (PT) were recorded. 2.7. Thermal analysis The thermal properties of starches were measured using a DSC (TA Instrument, Q2000, New Castle, NJ, USA). A 4.5 mg sample (dry basis) was weighed in an aluminum pan and 10 L of deionised water was added. The pan was sealed tightly and then it was allowed to stand for 1 h before carrying out the analysis. An empty
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aluminum pan was used as reference. The sample was subjected to a heating program over a range of temperature from 10 to 125 ◦ C and a heating rate of 5 ◦ C/min. The onset, peak, and final temperatures (To, Tp, and Tc, respectively) and gelatinization enthalpy (H) were determined. H =
KA m
where H – enthalpy change of reaction, m – mass of the sample at the beginning of the experiment, K – calibration coefficient, and A – area under the peak [26]. 2.8. Rheological properties Dynamic rheological measurements of native and acid modified sweet potato starch-water suspensions (10%, dry sample to distilled water) were performed at 85 ◦ C with a CVOR Rheometer (Bohlin, Malvern, Worcestershire, United Kingdom), using parallel plate geometry (20 mm diameter, 1 mm gap). For each measurement, 1 ml of sample was carefully deposited over the plateau of the rheometer. After the plateau comes in contact with the plate, the exposed surface of the sample was covered with a thin layer of low-density silicone oil to prevent evaporation during the measurement. In order to describe the variation in the rheological properties of samples under steady shear, the data were fitted to the wellknown power law model (Eq. (1)), which was used extensively to describe the flow properties of non-Newtonian liquid engineering applications. A linear regression of shear rate versus shear stress was plotted to obtain statistically best values of R2 , K and n. = K n
(1)
where , shear stress (Pa); , shear rate (s−1 ); K, consistency index (Pa sn ); n, the flow behavior index (dimensionless). 2.9. Structural properties 2.9.1. Powder X-ray diffraction (XRD) X-ray diffraction pattern of starch samples was obtained using a Powder X-ray diffractometer (Rigaku Mini Hex-II, Japan). Since X-ray intensity of starch was affected by moisture content samples were conditioned to 75% relative humidity before taking the X-ray diffraction pattern. The graphs were plotted between the 2 angles of 10 and 60 and smoothed with the software PowderX (Chinese Academy of Sciences, Beijing). The degree of crystallinity of samples was quantitatively estimated following the method of Nara and Komiya [27] with the Origin – version 6.0 software (Microcal Software Inc., Northampton, MA, USA). 2.9.2. Scanning electron microscope (SEM) Starch granules were observed using a Scanning Electron Microscope (SEM) (JEOL-Model 6390, Japan). Granule size was determined by using ImageJ 1.46r (National Institute of Health, USA) software. 3. Experimental design and statistical analysis Zambrano and Camargo [18] evaluated the effect of acid hydrolysis time (3,6,9,11 h), concentration (1.5, 2.0, 3.0, 4.0, 4.5 g/100 g) and temperature (40, 43, 45, 47, 51, 54 ◦ C) on the hydrolysis of native cassava starch. They revealed that the treatment, 3.0 g/100 g HCl, 40 ◦ C/6 h showed acceptable values of gel formation and thermo reversibility with a low hydrolysis degree, which were the essential characteristics required for a fat replacer. Hence, these conditions were adopted as reference for the present study. A 2 × 2 factorial experimental design was used in the present study for
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evaluating the effect of citric acid concentration (AC) and hydrolysis time (HT) on the response variables (Table 1). The data of physiochemical and functional properties of the citric acid treated starches were six replications. All data obtained were subjected to one way Analysis of Variance (ANOVA) or student t-test using SPSS program (Statistical Package for Social Science) version 14.0 (SPSS Inc., Illinois, USA). Comparison of means was performed using Tukey–Kramer HSD at P < 0.05. 4. Results and discussion 4.1. Physiochemical properties 4.1.1. Yield of isolated and acid treated sweet potato starches The yield of isolated sweet potato starch was 10.20%. The recovery yield of citric acid modified sweet potato starches was above 85% (Table 2) and it was in the range as reported by Dutta et al. [28] and Babu et al. [29]. With the increase in acid concentration, yield was reduced as starches might be hydrolyzed more rapidly at higher acid concentration. 4.1.2. Analysis of ash, protein, fat and total fiber The isolated native sweet potato starch had 0.26 ± 0.11% ash, 0.25 ± 0.14% protein, 0.07 ± 0.02% fat and 0.57 ± 0.10 total fiber. These values were consistent with the earlier report [30]. The isolated starch had minor protein and fat contents. 4.1.3. Dextrose equivalent (DE) DE value is an indication of extent of acid hydrolysis. The degree of citric acid hydrolysis of sweet potato starch was not severe as revealed by dextrose equivalent (DE) value. The DE value of acid-thinned sweet potato starches (Table 2) was ranged between 1.90 and 2.34% and it showed that the DE value was increased with increased acid concentration and hydrolysis time. Since CTS4 starch was treated for a greater hydrolysis conditions, it exhibited a higher DE value of 2.34% compared to their counterparts. Thys, Aires, Marczak, and Norena [6] reported a DE value of 6.5 for pinhao starch treated with HCl. Nevertheless DE values obtained in the present study was much lower as citric acid used for acid treatment was a weak organic acid, hence the degree of hydrolysis of these starches seems to be lower. Since the DE value of acid treated starches were within the range referred by National Starch Table 1 Hydrolysis conditions. Treatments
HT (h)
AC (%)
NS CTS1 CTS2 CTS3 CTS4
– 1 1 11 11
– 1 5 1 5
NS, native starch; CTS1.CTS4, citric acid treated starches; HT, hydrolysis time; AC, acid concentration.
and Chemical Corporation [12], indicates a potential applicability of the citric acid thinned sweet potato starch as a fat mimetic. 4.1.4. Apparent amylose The apparent amylose content of NS was 18.56%, which was on par with Tsakama, Mwangwela, Manani, and Mahungu [31]. Acid treated starches found to display a higher fraction of apparent amylose which was significant with NS. The highest apparent amylose (around 31%) content was noticed for CTS4 sample. This increased trend was in linear fashion with acid concentration and hydrolysis period. The removal of lipids from the starch samples by acid may result in higher value for amylose content. Increase in amylose might also be due to the formation of intramolecular and intermolecular linkages between residues of amylose, which increases the length of these chains and their capacity to form complexes with iodine, increasing the apparent amylose values. Another possible reason might be due to the de-polymerization of amylopectin fractions on continuous acid hydrolysis. High degree of acid hydrolysis led increased apparent amylose content of starch [32]. Starches with a higher linear fraction (amylose content) are able to bind strongly and orient water to endow with a sensation comparable to the rheology of fat in the oral cavity [33] hence CTS4 starch could mimic the functionality of fat when used as a fat replacer. Vanderveen and Glinsmann [9] suggested that starch should possess a 20% amylose to act as a fat replacer. 4.1.5. Moisture and dry matter Moisture content and dry matter of NS was 14.11% and 85.89%, respectively, whereas all acid thinned starches showed low moisture content and high dry matter. Hydrolysis time showed a noticeable effect on the moisture content. Increase in hydrolysis time would provide ample time for the starch to react with citric acid which results in increase in moisture content of starch. This pattern might be related to the reaction between the OH groups of glucose units of starch and the functional groups (OH) of citric acid used in this chemical modification, decreasing the possibility of reaction between OH of starch chains and the water molecules. Consequently, the probability of joining of water to this polymer would be reduced causing decrease in moisture content of modified starch thereby increase in dry matter [34]. A similar trend was observed by Omojola, Manu, and Thomas [35] during acid hydrolysis of cola starch. 4.1.6. Melting point, clear point and gel thermo-reversibility Melting point, clear point and thermo-reversibility of native and acid-thinned sweet potato starches are shown in Table 2. All the starches illustrated a perfect gel formation when gelatinized and stored under refrigeration at 4 ◦ C. Melting point of NS was observed at 69.33 ◦ C whilst the acid treated starches melted at a temperature lower than the native starch. The CTS4 starch had shown its melting temperature at 47.66 ◦ C, similar to the melting point of fats, which indicated that it would be used as a fat substitute. Amylopectin plays a major role in starch granule crystallinity and the presence of amylose indirectly lowers the
Table 2 Physicochemical properties of native and acid-thinned sweet potato starches. S. No.
Starch recovery yield (%)
DE (%)
NS CTS1 CTS2 CTS3 CTS4
100 92.93 89.84 91.33 85.05
± ± ± ± ±
– 1.90 ± 2.21 ± 2.04 ± 2.34 ±
0.00aA 1.10bB 1.24cB 0.80bB 2.56cC
Apparent amylose (%) 0.08aA 0.05bA 0.03aA 0.05bB
18.56 24.78 29.59 30.27 31.04
± ± ± ± ±
1.06aA 1.36bB 1.32cB 1.88bC 1.38cC
Moisture (%) 14.11 6.62 11.00 11.30 11.83
± ± ± ± ±
2.17aA 1.45bB 1.00aA 0.60aA 1.04aA
Dry matter (%) 85.89 93.38 89.00 88.70 88.17
± ± ± ± ±
0.80aA 3.07bB 1.00abB 1.47bAB 0.76abB
Melting point (◦ C) 67.00 56.33 50.33 55.66 47.66
± ± ± ± ±
2.64aA 2.04bB 2.65cB 1.86bB 1.94cC
Clear point (◦ C)
GTR
In vitro digestibility (%)
– 68.33 ± 61.00 ± 66.33 ± 58.33 ±
No Yes Yes Yes Yes
63.27 72.59 70.59 76.22 78.54
0.57aA 1.52bA 1.52aB 2.88bB
± ± ± ± ±
4.27aA 11.67aA 5.78aA 7.18abA 7.40bB
Mean values followed by different letters in the same column indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations. DE, dextrose equivalent; GTR, gel thermo reversibility.
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Table 3 Crystallinity, granule size and functional properties of native and acid-thinned sweet potato starches. Samples
Crystallinity (%)
NS CTS1 CTS2 CTS3 CTS4
35.33 34.26 38.80 40.50 35.55
± ± ± ± ±
2.62aA 1.63aA 1.31aA 0.50bB 0.50aA
Granule size (m) 8.61 8.08 8.67 8.00 8.66
± ± ± ± ±
5.32aA 2.77aA 2.39aA 3.01aA 4.12aA
Water holding capacity (%) 34.90 36.21 46.13 38.80 56.15
± ± ± ± ±
Emulsion activity (%)
3.31aA 1.17aA 3.12bB 3.48bB 4.20aA
54.44 66.45 65.71 66.30 68.62
± ± ± ± ±
1.92aA 0.79bB 1.37cbB 1.72cbC 1.53bB
Emulsion stability (%) 41.60 42.89 42.72 41.78 42.99
± ± ± ± ±
1.75aA 2.06aA 1.62aA 1.28aA 4.05aA
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentration.
Table 4 Textural profile of native and acid thinned sweet potato starch. Samples
Hardness (kg)
NS CTS1 CTS2 CTS3 CTS4
1.64 1.11 0.56 0.28 0.27
± ± ± ± ±
0.07aA 0.10bB 0.32cB 0.02bC 0.00bB
Adhesiveness (kg/s) 0.05 0.01 0.00 0.08 0.02
± ± ± ± ±
0.01aA 0.00bB 0.00bB 0.01bA 0.00aB
Springiness 0.74 0.77 0.65 0.76 0.82
± ± ± ± ±
Cohesiveness
0.01aAB 0.11aA 0.04aB 0.02aA 0.07aA
0.49 0.71 0.49 0.49 0.61
± ± ± ± ±
0.05aA 0.13aB 0.11aA 0.00aA 0.00bA
Chewiness 0.40 0.39 0.16 0.10 0.13
± ± ± ± ±
0.11aA 0.00aA 0.07bB 0.00bB 0.01bB
Gumminess 0.54 0.63 0.23 0.14 0.16
± ± ± ± ±
0.14aA 0.04aA 0.09bB 0.01bB 0.00bB
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
melting point of the starch granule [36]. Acid treatment of sweet potato starch might result in the formation of shorter amylopectin chains with less stable crystalline structure consequently facilitating a lower melting point and clear point [37]. Thys, Aires, Marczak, and Norena [6] noticed a melting point of 46 ◦ C for acid treated pinhao starch. All the treated starches had shown a clear point around 60 ◦ C however the clear point was not displayed by native starch even at 80 ◦ C and it might be above 80 ◦ C. Native starch resisted the gel thermo-reversibility. Conversely, acid-thinned sweet potato starches displayed gel thermo-reversibility which implies acid treatment of starch caused partial hydrolysis of starch chains, resulting in lower paste viscosity. However, when the paste cools down, acid-thinned starch chains tend to associate with each other more easily, forming a thermo-reversible gel [38]. Similar fashion of gel thermo-reversibility was registered in the previous study [6] for pinhao and corn starches.
4.1.7. In vitro digestibility Table 2 displays the in vitro digestibility of NS and CTS which was measured by glucoamylase. The in vitro digestibility of NS was about 63.27% and it was in the range with the previous report on sweet potato starch (28.3–67.2%) [38]. The digestibility rate of the CTS4 sample was significantly increased up to 78.54%. In view of the fact that acid hydrolysis preferentially attacks the amorphous area in the starch granule, the crystallites were decoupled from the amorphous parts consequently, unlocked amorphous regions would be more sensitive to the enzyme attack and prone to rapid hydrolysis on the external glucose residues of amylose or amylopectin. In the report of Shi et al. [39] HylonV starch, normal maize starch and waxy maize starch samples when subjected to acid treatment resulted in less resistant to ␣-amylase digestion. After acid hydrolysis of the starches, the amorphous structure of the starches was hydrolyzed, so that the density of the residual amorphous structure of the starches decreased. However, as the specific surface area increased, starches were easily reacted with the enzymes and as a result the hydrolysis rate of starch samples was greater than native starch. This higher digestibility of citric acid treated starches would be beneficial for its role as a fat replacer since FDA [11] recommended that a fat replacer to be partially or completely digestible.
5. Functional properties 5.1. Water holding capacity (WHC) WHC of NS was 34.90%. A significant difference was observed in the WHC among the starches (Table 3). The highest and lowest water holding capacity was detected in CTS4 starch and CTS1 starches respectively. WHC was directly proportional to the acid concentration and as well as hydrolysis time. High acid concentration probably increased the low molecular weight starch fraction with hydroxyl groups which may hold water molecules forming hydrogen bonds consequently increasing the WHC. This high water holding capacity of citric acid treated starch (CTS4) may find a significant role as a fat replacer. 5.2. Emulsion activity and emulsion stability Acid thinned starch, CTS4 exhibited a higher emulsion activity and emulsion stability compared to native starch (Table 3). Starch (CTS4) with higher amylose found to exhibit superior emulsion properties. The present study revealed that higher amount of linear amylose fraction would contribute to emulsion activity. The high amylose starch might function as the interface between oil and water during which linear amylose chains of starch granules were more favored in stabilizing the emulsion system than branched amylopectin chains. The linear amylose fractions could be capable of film formation that enhances the emulsion capacity and stability of the starch [40]. No significant difference in emulsion stability was noticed among the starches. 6. Texture analysis The texture profile analysis of NS and CTS samples is shown in Table 4. The gel strength of NS was 1.644 kg and acid modified starches ranged from 0.27 kg to 1.11 kg. The gel formed from NS was harder than CTS samples due to the degree of long chains in sweet potato native starch which contributed to its firmer gel. The lesser gel strength of the acid-thinned sweet potato starch might be attributed to a higher degree of short chains due to acid hydrolysis [41]. Wang and Wang [41] reported that gel strength (GS) of 0.30, 0.48 and 0.09 kg for acid thinned corn, potato and rice starches respectively.
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Table 5 Pasting properties of native and acid thinned sweet potato starch. Sample
Peak viscosity (cP)
NS CTS1 CTS2 CTS3 CTS4
6338.00 6097.00 4812.00 4736.33 4655.33
± ± ± ± ±
340.54 aA 128.31 aA 79.30bB 113.95bB 190.11bB
Trough viscosity (cP) 3288.00 3027.00 1971.66 2248.66 1738.33
± ± ± ± ±
189.59aA 62.69aA 55.94bB 84.31bB 144.36cB
Break down (cP) 3050.00 3070.00 2840.33 2487.66 2917.00
± ± ± ± ±
186.34aA 77.11 aA 83.18aA 34.50aA 53.45 aA
Final viscosity (cP) 4290.66 4087.33 3028.66 3190.66 2683.66
± ± ± ± ±
168.35aA 75.79 aA 83.57 bB 120.35 bB 189.95cB
Set back (cP) 1002.66 1060.33 1057.00 942.00 945.33
± ± ± ± ±
Peak time (min)
24.68abA 48.67aA 29.05 bA 41.60aA 45.65aA
4.00 3.93 3.82 3.91 3.80
± ± ± ± ±
0.07 aA 0.00 aA 0.04bB 0.03aA 0.07aAB
Peak temperature (◦ C) 70.81 71.00 71.51 70.76 70.73
± ± ± ± ±
0.49aA 0.82aA 0.49 aA 0.40aA 0.37aA
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
A similar significant decrease in the hardness of chick pea starch gel upon acid treatment was reported by Sodhi, Chang, Midha, and Kohyama [42]. Springiness represents the ability of a gel to recover its original shape/height after a deforming force is removed [43]. No significant change in the springiness was noticed due to hydrolysis time. Adhesiveness is the ability of the gel sample to become sticky [44]. It is a surface characteristic which depends on a combined effect of adhesive and cohesive forces, viscosity, and viscoelasticity of the sample [45]. Adhesiveness of NS and CTS was ranged between 0.00 g/s and 0.08 g/s. Starch with high amylose content was observed to have lower adhesiveness [46]. Cohesiveness is how well a sample withstands a second deformation related to how it behaved under the first deformation [47]. Cohesiveness indicates how good the sample retains its structure after the first compression. Cohesiveness of starch samples was ranged from 0.49 to 0.71. Gumminess is the product of hardness and cohesiveness, a characteristic of semi-solid foods which have a low degree of hardness and a high degree of cohesiveness [47]. NS displayed a higher chewiness compared to the CTS samples while CTS1 showed a higher gumminess than NS. The difference in textural properties of all sample gels was influenced by rigidity in gelatinized starch, amylose content as well as interaction between the dispensed and continuous phase of the gel, which in turn was dependent on the amylose and amylopectin structure [48]. 7. Pasting properties Table 5 reveals a significant influence of acid concentration and reaction time on RVA of the acid thinned sweet potato starch. RVA results of the acid thinned starches confirmed a low PV ranging from 4655.33 cP to 6097.00 cP compared to native starch (6338.00 cP). Results showed that the PV decreased with increase in acid concentration and reaction time. Similar fashion of change was reported in the literature [28,49]. CTS1 presented a viscosity profile higher than their counterparts, although substantially lower than the native starch. The lower PV of acid modified starches could be due to considerable breakdown of amorphous regions and the production of low molecular weight dextrins [50]. TV and BD values of CTS samples also displayed the same decreasing trend. The increased degree of amylose recrystallization by acid thinning might be due to the change in BD [51]. Acid thinned sweet potato starches displayed a lower FV ranging between 3028.66 and 4087.33 cP against 4290.66 cP for sweet potato native starch. Han, Campanella, Mix, and Hamaker [52] reported that acid hydrolysis resulted a considerable lyses of glycoside linkages of the long amylopectin chains, which apparently caused the fall in FV. SB is a measure of recrystallization of gelatinized starch. CTS1 and CTS2 registered a higher SB than NS indicating that these starches got a higher retrogradation tendency than NS. The low SB of the rest of the acid-thinned starches was likely due to in sufficient time for the starch molecules to rearrange themselves during the stipulated period [53]. Native starch took more time (4 min) to reach its PV than acid thinned starches. Hydrolysis time and acid concentration basically did not affect the PT of sweet potato starch in native form and acid modified form, however with the increase
Table 6 Thermal properties of native and acid-treated sweet potato starch. Gelatinization temperature (◦ C)
Sample To NS CTS1 CTS2 CTS3 CTS4
42.31 35.81 35.35 35.97 39.44
Tp ± ± ± ± ±
2.12aA 1.61bB 0.83bB 1.00bB 1.50aA
81.25 83.58 81.56 83.75 86.75
H (J/g)
Tc ± ± ± ± ±
0.66 aA 2.50aA 1.77aA 0.03cA 0.25bB
116.12 120.10 119.43 116.77 122.69
± ± ± ± ±
2.01 aA 2.85aA 2.06aAB 2.04aA 2.33bB
12.96 12.74 12.13 12.69 11.95
± ± ± ± ±
0.05aA 0.03aA 0.80aA 1.29aA 1.31aA
To, Tp and Tc stand for onset, peak, and conclusion temperatures respectively. H (J/g) indicates enthalpy. Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
in acid concentration and hydrolysis time, mild changes in pasting parameters were noticed. A similar decrease in pasting profile was observed upon acid treatment of corn by Singh, Sodhi, and Singh [50] in acid thinned sorghum starch. 8. Thermal analysis Thermal properties of starches determined by the DSC are represented in Table 6. Results showed variations in To, Tp, Tc temperatures and H among NS and CTS samples. NS had higher To of 42.31 ◦ C while citric acid treated starches showed a decreased trend. This was in agreement with the result reported for acid modified maize starch which showed a decreased To value [54]. The Tp and Tc of native starch were 81.25 ◦ C and 116.12 ◦ C respectively, conversely the acid modified starches displayed a higher Tp and Tc temperatures. However, this trend was more pronounced in case of CTS4. This specifies that a higher degree of hydrolysis might occur in CTS4 at amorphous region, thus resulting in an increase in relative crystallinity and subsequently an increase in the gelatinization temperature. Similar results were reported for acid modified potato starch [40] and sweet potato starch [55]. The H of NS was 12.96 J/g, on the other hand H of acid modified starches was lower than their counterpart. Enthalpy of the citric acid treated starches ranged between 11.95 and 12.74 J/g. CTS4 starch showed a lower enthalpy compared to all other modified starches. The decrease in H was the result of citrate substitution that altered the chain packing and generated more amorphous region [56]. H gives an overall measure of crystallinity and is an indicator of the loss of molecular order within the granule during gelatinization [57]. The increase in citric acid concentration and treatment time decreased the enthalpy that might be due to greater loss of ordered structure of starch
9. Rheological analysis Shear rate versus shear stress plot at 85 ◦ C was well fitted to the power law model (Eq. (1)) with determination coefficients (R2 ) ranging from 0.97 to 0.99, represented in Table 7. Native and acid modified starches displayed the values of flow behavior indexes (n) less than 1 indicating a shear-thinning behavior. NS showed a
A
200 180 160 140 120 100 80 60 40 20 0
563
B
70 60 NS CTS1 CTS2 CTS3 CTS4
Viscosity (Pa.S)
Shear Stress (Pa)
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50
NS CTS1
40
CTS2
30
CTS3 20
CTS4
10 0 0
50 Shear Rate (s-1)
100
0
10 20 30 40 50 60 70 80 90 100 Shear Rate (1/s)
Fig. 1. (A) Plot of shear rate (Pa) versus shear stress (s−1 ), (B) plot of shear rate (Pa) versus viscosity (Pa s) for a 10% NS and CTS dispersion heated at 85 ◦ C.
Table 7 Rheological properties of acid treated sweet potato starches (10%, w/v) at 85 ◦ C. Samples
n
NS CTS1 CTS2 CTS3 CTS4
0.46 0.42 0.40 0.35 0.22
K (Pa sn ) ± ± ± ± ±
0.03aA 0.02abA 0.01bB 0.01dD 0.01cC
157.94 87.92 101.12 102.81 55.63
± ± ± ± ±
R2 1.93aA 1.73bB 1.20cB 0.92bC 1.09cC
0.97 0.98 0.99 0.97 0.98
± ± ± ± ±
0.01aA 0.01bB 0.00cA 0.01bA 0.01cA
Mean values followed by different letters in the same row indicate significant difference (p < 0.05). Lowercase letters indicate significant difference in hydrolysis times while uppercase letters indicate significant difference in acid concentrations.
higher flow behavior index while CTS4 samples exhibited a lower flow behavior index suggesting that it possess a greater shear thinning behavior than its counter NS studied. This shear-thinning behavior might be attributed to the higher amount of breakage of the intra-and inter-molecular associative bonding system in the starch network micelles due to shearing at higher shear rates, as noted by Bhandari et al. [58]. It was evident that n was higher for NS which contains lower amylose content. The decrease in the power law index (n) with increasing amylose content (CTS4) was generally attributed to an increase in the entanglements between amylose chains, since the highly branched amylopectin was not expected to form effective entanglements [59]. Same was the case with the consistency index where NS exhibited a higher K value while CTS4 showed a lower K value. All starches showed a nonNewtonian behavior, where the viscosity decreased when shear rate was increased (Fig. 1). This pattern is defined as shear thinning and is produced when the stress disorganized the arrangement of the macromolecules inside of the matrix [60]. When shear force is applied, the particles have rearranged themselves into a parallel direction with shear force and big particles are broken into small particles. The particles can flow easily because of resistance arising from particle–particle interaction which results in decreasing of viscosity [61].
10. Structural properties 10.1. X-ray diffraction (XRD) X-ray diffractogram of starch samples are presented in Fig. 2. The native and modified sweet potato starches exhibited a C-type X-ray diffractogram pattern. All the starches displayed peaks at the 2 diffraction angles around 10, 11, 15, 17, 20, 23 and 26. The degree of crystallinity was higher in CTS3 (40.50%) followed by CTS2 (38.80%). The results of the present study showed that the crystallinity of sweet potato starch was increased with increased
time of hydrolysis (Table 3) as suggested by Atichokudomchai, Shobsngob, and Varavinit [62] nevertheless increased reaction time and acid concentration does not alter the diffraction pattern. Similar observations were reported by Babu et al. [29]. This was attributed to preferential hydrolysis of the amorphous regions of the starch granule [63].
10.2. Scanning electron micrograph (SEM) Scanning electron micrograph (SEM) of the sweet potato native starch is illustrated in Fig. 2. Illustration of starch sample showed the presence of starch granules from small to large sizes. The granule surface of starches appeared to be smooth with no sign of any fissure. Zhu et al. [64] also observed that the sweet potato starches with smooth granule surface without cracks. Most of the sweet potato starch granules were polygonal in shape, however round and irregular shapes were also noted. Surface of CTS1 starch granule was found to have a crack (depicted by arrows) as acid might have attacked the surface of the granule. While there was a loss of smooth surfaces of the starch granules was noticed in the CTS2 and this was more pronounced in CTS4. Starch granules were found to conglutinate in CTS2 and CTS4 which was due to the acid erosion on the core part of the granules. Similar result was noticed in the study conducted by Babu et al. [65]. The extent of visible degradation was increased when the granules were subjected to a higher hydrolysis time and acid concentration. An acid acts on the surface of the starch granule first before it gradually enters the inner region. The acid to diffuse into the internal part of the granule it must pass through some alternating amorphous and crystalline regions [66]. The amorphous areas of the starch granules had a looser structure than the crystalline regions which was easier to attack with the hydrogen ions. Furthermore, native starches had some flaws on the surface of the granules which could provide channels for the infiltration of hydrogen ions. These hydrogen ions primarily got to the amorphous areas and attacked them [41]. Hence acid cause surface alterations and degrade the external part of the granule by exo-corrosion. When exo-corrosion occurs, the internal part of the granule was corroded through small cracks through which acids could penetrate the granule [67]. Erosion and roughness on the surface of sorghum starch granules were recorded after acid treatment by Ali and Hasnain [68]. They also stated that acid treatments did not significantly change the size of sorghum starch granules. Granule size of sweet potato starches was not affected by acid treatment. Granule size of native and acid modified starches (Table 3) varied from 8.00 to 8.67 m respectively. Granule size of CTS was greater than the size required for a fat replacer which was mentioned in the literature.
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Fig. 2. Scanning electron micrograph (SEM) of (A) NS, (B) CTS1, (C) CTS2, (D) CTS3, (E) CTS4 and (F) X-ray diffractogram of NS and CTS samples.
11. Conclusion The recovery of starch after acid treatment at various concentrations and hydrolysis times was more than 85%. The degree of citric acid hydrolysis of sweet potato starch was quiet low as revealed by the dextrose equivalent (DE) value. The citric acid treatments lead to an increase in apparent amylose content. Acid hydrolysis might facilitate to lower the melting point of sweet potato starch. Functional properties were significantly affected upon treatment with acid. CTS demonstrated a reduced gel strength that attributed
to a higher degree of short chains. A marked decline in viscosity was observed for acid treated starch compared to NS as proved by RVA results. Higher gelatinization temperature (Tp and Tc) and crystallinity displayed by CTS that specifies acid hydrolysis might occur at amorphous region. The power-law model was found to best describe the shear stress versus shear rate data with R2 values ranged between 0.97 and 0.99 for NS and CTS samples with n value less than 1. A visible degradation was observed when the granules were subjected to citric acid treatment with no significant change in granule size. Our results suggest that the citric acid
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treatment of sweet potato starch with 5% acid concentration and 11 h period was an ideal condition for the preparation of an acid thinned starch which would find a potential role as a fat mimetic in real food systems. Conflict of interest The authors have declared no conflicts of interest. References [1] C.C. Akoh, Fat Replace. 52 (1998) 47–53. [2] C.A. Hassell, Cereal Food World 38 (1993) 142–144. [3] M. Güven, K. Yas¸ar, O.B. Karaca, A.A. Hayaloglu, Int. J. Dairy Technol. 58 (2005) 180–184. [4] J. Lim, G.E. Inglett, S. Lee, Jpn. J. Food Eng. 11 (2010) 147–152. [5] J.P. Robin, C. Mercier, F. Duprat, R. Charbonniere, A. Guilbot, Starch/Stärke 27 (1975) 36–45. [6] R.C.S. Thys, A.G. Aires, L.D.F. Marczak, C.P.Z. Norena, Ciênc. Tecnol. Aliment. Camp. 33 (2013) 89–94. [7] S.L. Amaya-Llano, A.L. Martínez-Alegría, J.J. Zazueta-Morales, F. Martínez-Bustos, LWT-Food Sci. Technol. 41 (2008) 1274–1281. [8] Y. Ma, C. Cai, J. Wang, D. Sun, J. Food Eng. 73 (2006) 297–303. [9] J.E. Vanderveen, W.H. Glinsmann, Annu. Rev. Nutr. 12 (1992) 473–487. [10] J.R. Daniel, R.L. Whistler, Cereal Food World 35 (1990) 825. [11] Food and Drug Administation, Toxicological Principles for the Safety Assessment of Direct Food Additives and Colour Additives Used in Food, Bureau of Foods, Washington, DC, 1982. [12] National Starch and Chemical Corporation. US Patent 4,510,166, 1985. [13] J.A. Radley, Industrial Uses of Starch and its Derivates, Applied Science Publishers Ltd., London, 1976, pp. 58. [14] D. Clark, Food Technol. 48 (1994) 86–87. [15] Sweet Potato Starch Production-TM22-3e, International Starch Institute, Science Park Aarhus, Denmark, 1999–2006, www.Starch.Dk [16] S.K. Sathe, D.K. Salunkhe, J. Food Sci. 46 (1999) 71–76. [17] H.A.M. Wickramasinghe, S. Takigawa, G. Matsura-Endo, H. Yamauchi, T. Noda, Food Chem. 112 (2009) 98–103. [18] F. Zambrano, C.R.O. Camargo, Braz. J. Food Technol. 4 (2002) 147–154. [19] Association of Official Chemists (AOAC), Official Methods of Analysis, 15th ed., Association of Official Analytical Chemists, Arlington, VA, USA, 2000. [20] G.L. Miller, Anal. Chem. 31 (1959) 426–428. [21] V.R. Williams, W. Wu, H.Y. Tsai, H.G. Bates, J. Agric. Food Chem. 6 (1958) 47–48. [22] A.O. Adebayo, S.O. Lateef, A.B. Elizabeth, J. Am. Sci. 6 (2010) 65–72. [23] T. Noda, S. Takigawa, C. Matsuura-Endo, K. Saito, K. Takata, T. Tabiki, H.A.M. Wickramasinghe, H. Yamauchi, Carbohydr. Polym. 56 (2004) 271–277. [24] L.L. Niba, M.M. Bokanga, F.L. Jackson, D.S. Schlimme, B.W. Li, J. Food Sci. 67 (2001) 1701–1705. [25] V.Q. Neto, N. Narain, J.B. Silvia, P.S. Bora, Nahrung 45 (2001) 258–262. [26] R. Artiaga, S. Nayab, A. Garcia, F. Barbadilloa, L. Garcia, Thermochim. Acta 428 (2005) 137–139. [27] S. Nara, T. Komiya, Starch/Starke 35 (1983) 407–410. [28] H. Dutta, S.K. Paul, D. Kalita, C.L. Mahanta, Food Chem. 128 (2011) 284–291. [29] A.S. Babu, R. Parimalavalli, K. Jagannadham, J.S. Rao, J. Food Sci. Technol. (2014), http://dx.doi.org/10.1007/s13197-014-1650-x
565
[30] S.N. Moorthy, Starch/Stärke 54 (2002) 559–592. [31] M. Tsakama, A.M. Mwangwela, T.A. Manani, N.M. Mahungu, Int. Res. J. Agric. Sci. Soil Sci. 1 (2011) 254–260. [32] A.D. Betancourt, G. Chel, J. Agric. Food Chem. 45 (1997) 4237–4241. [33] W.C. Yackel, C. Cox, Food Technol. 46 (1992) 146–148. [34] R. Carmona-Garcia, M.M. Sanchez-Rivera, G. Méndez-Montealvo, B. Garza-Montoya, L.A. Bello-Pérez, Carbohydr. Polym. 76 (2009) 117–122. [35] M.O. Omojola, N. Manu, S.A. Thomas, Afr. J. Pure Appl. Chem. 5 (2011) 307–315. [36] J.J.G. van Soest, R.C. Bezemer, D. de Wit, J.F.G. Vliegenthart, Ind. Crops Prod. 5 (1996) 1–9. [37] R.C. Hoseney, Principles of Cereal Science and Technology, American Association of Cereal Chemists, Inc., St. Paul, 1994, pp. 52–54. [38] Z. Zhang, Nutritional Quality and Starch Physicochemical Properties in Sweetpotato, Ph.D. thesis, The University of Hong Kong, Hong Kong, 2001 http://hdl.handle.net/10722/36214 [39] M. Shi, W. Lu, S. Yu, R. Ward, Q. Gao, Starch/Stärke 66 (2014) 429–435. [40] J.A. Han, S.T. Lim, Carbohydr. Polym. 90 (2012) 1480–1485. [41] L. Wang, Y.J. Wang, Starch/Stärke 53 (2001) 570–576. [42] N.S. Sodhi, Y.-H. Chang, S. Midha, K. Kohyama, Int. J. Food Prop. 16 (2013) 125–138. [43] E.Y. Park, B. Baik, S. Lim, J. Cereal Sci. 50 (2009) 43–48. [44] C. Saric¸oban, M.T. Yilmaz, M. Karakaya, Meat Sci. 83 (2009) 610–619. [45] M. Huang, J.F. Kennedy, B. Li, X. Xu, B.J. Xie, Carbohydr. Polym. 69 (2007) 411–418. [46] S. Yu, M. Ying, S. Da-Wen, J. Cereal Sci. 50 (2009) 139–144. [47] M.C. Bourne, Food texture and viscosity concept and measurement, in: Food Science and Technology International Series, 2nd ed., Academic Press, San Diego, 2002, pp. 427. [48] F.F. Yamin, M. Lee, L.M. Pollak, P.J. White, Cereal Chem. 76 (1999) 175–181. [49] K.S. Sandhu, N. Singh, S.T. Lim, LWT: Food Sci. Technol. 40 (2007) 1527–1536. [50] H. Singh, N.S. Sodhi, N. Singh, Int. J. Food Prop. 12 (2009) 713–725. [51] H.-J. Chung, H.-Y. Jeong, S.-T. Lim, Carbohydr. Polym. 54 (2003) 449–455. [52] X.Z. Han, O.H. Campanella, N.C. Mix, B.R. Hamaker, Cereal Chem. 79 (2002) 897–901. [53] D. Thirathumthavorn, S. Charoenrein, Starch/Stärke 57 (2005) 217–222. [54] J.H. Lin, S.Y. Lee, Y.H. Chang, Carbohydr. Polym. 53 (2003) 475–482. [55] S. Singh, C.S. Raina, A.S. Bawa, D.C. Saxena, J. Food Sci. 70 (2005) E373–E378. [56] H. Liu, R. Liang, J. Antoniou, F. Liu, C.F. Shoemaker, Y. Li, F. Zhong, Food Chem. 159 (2014) 222–229. [57] R.A. Hoover, T. Vasanthan, Carbohydr. Res. 252 (1994) 33–53. [58] P.N. Bhandari, R.S. Singhal, D.D. Kale, Carbohydr. Polym. 47 (2002) 365–371. [59] L. Yu, G. Christie, J. Mater. Sci. 40 (2005) 111–116. [60] D. Guerra-DellaValle, M.M. Sánchez-Rivera, P.B. Zamudio-Flores, G. Méndez-Montealvo, L.A. Bello-Pérez, Rev. Mex. Ing. Quím. 8 (2009) 197–203. [61] S.E. Charm, Adv. Food Res. 11 (1962) 356–435. [62] N. Atichokudomchai, S. Shobsngob, S. Varavinit, Starch/Stärke 52 (2000) 281–283. [63] R. Hoover, Food Rev. Int. 16 (2000) 369–392. [64] F. Zhu, X. Yang, Y. Cai, E. Bertoft, H. Corke, Starch/Starke 63 (2011) 249–259. [65] A.S. Babu, R. Parimalavalli, K. Jagannadham, J. Rao, R.S. Gaur, Int. J. Food Prop. (2015), http://dx.doi.org/10.1080/10942912.2015.1020390 [66] N.K. Genkina, J. Wikman, E. Bertoft, V.P. Yuryev, Biomacromolecules 8 (2007) 2329–2335. [67] O.O. Olayinka, K.O. Adebowale, I.B. Olu-Owolabi, J. Food Sci. Technol. 50 (2013) 70–77. [68] T.M. Ali, A. Hasnain, Int. J. Food Prop. 17 (2014) 523–535.
