Carbohydrate Polymers 140 (2016) 269–278

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Effect of dual modification with hydroxypropylation and cross-linking on physicochemical properties of taro starch Bidyut Jyoti Hazarika, Nandan Sit ∗ Department of Food Engineering and Technology, Tezpur University, Tezpur 784028, Assam, India

a r t i c l e

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Article history: Received 21 August 2015 Received in revised form 29 November 2015 Accepted 21 December 2015 Available online 23 December 2015 Keywords: Taro starch Hydroxypropylation Cross-linking Single modification Dual modification Physicochemical properties

a b s t r a c t Dual modification of taro starch by hydroxypropylation and cross-linking was carried out and the properties of the modified starches were investigated. Two different levels of hydroxypropylation (5 and 10%) and cross-linking (0.05 and 0.10%) were used in different sequences. The amylose contents of the starch decreased due to single and dual modification. For the dual-modified starches, the swelling, solubility and clarity was found to increase with level of hydroxypropylation and decrease with level of cross-linking. The freeze-thaw stability of the dual-modified starches was also affected by the sequence of modification. The viscosities of the cross-linked and dual-modified starches were more than native and hydroxypropylated starches. The firmness of the dual-modified starches was also higher than native and single modified starches. The dual-modified starches have benefits of both type of modifications and could be used for specific purposes e.g. food products requiring high viscosity as well as freeze-thaw stability. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Starch is the major carbohydrate in human diet and is the basic source of energy. Besides fulfilling the carbohydrate requirement of human diet, starch is widely used as an ingredient in food industry as thickening, gelling, binding and stabilizing agent and as a replacement or extender for more costly ingredients. Starch is also widely used in textiles, cosmetics, pharmaceuticals, paper, plastics and adhesive industries due to its diverse functionalities (Wurzburg, 1986). Starch is a natural, versatile, cheap, readily available, renewable and biodegradable polymer and has unique properties which extended its use in wide range of industrial applications. Native starches have limited use in food application as it has many weaknesses such as narrow peak viscosity range, poor process tolerance, low shear stress resistance, thermal decomposition, high retrogradation, and syneresis and cooked starches will formed a weak, cohesive and rubbery paste. These facts motivated the employment of modified starches as important functional ingredients in processed foods in recent years because of their improved functional properties over unmodified starches (Fleche, 1985).

∗ Corresponding author. Tel.: +91 3712275704; fax: +91 3712267005. E-mail address: [email protected] (N. Sit). http://dx.doi.org/10.1016/j.carbpol.2015.12.055 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

Starch modification can be physical, chemical or enzymatic (Miyazaki, Van Hung, Maeda, & Morita, 2006). Starch modification, which involves the alteration of the physical and chemical characteristics of the native starch to improve its functional characteristics, can be used to tailor starch to specific food applications (Hermansson & Svegmark, 1996). Numerous chemical modifications may be applied to starch to impart properties that are useful for particular applications. Chemical modification involves the introduction of functional groups into the starch molecule, resulting in markedly altered physico-chemical properties. Such chemical modification of native granular starches profoundly alters their physicochemical and functional properties such as gelatinization, pasting and retrogradation behaviour etc (Liu, Ramsden, & Corke, 1999). Chemical modification is generally achieved through oxidation, etherification, esterification, cross-linking, dual modification etc. Cross-linking alters, not only the physical properties, but also the thermal transition characteristics of starch, although the effect of cross-linking depends on the botanical source of the starch and the cross-linking agent. Decrease in retrogradation rate and increase in gelatinization temperature has been observed with cross-linked starch, and these phenomena are related to the reduced mobility of amorphous chains in the starch granule as a result of intermolecular bridges. Food grade hydroxypropylated starches are generally prepared by etherification of native starch with propylene oxide in the presence of an alkaline catalyst. The hydroxypropyl groups

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introduced into the starch chains are capable of disrupting the inter- and intra-molecular hydrogen bonds, thereby weakening the granular structure of starch, leading to an increase in motional freedom of starch chains in amorphous regions (Seow & Thevamalar, 1993). Hydroxypropyl starches are important in food applications due to their relatively low pasting temperature, high paste clarity, and desirable low-temperature storage stability (Karim, Sufha, & Zaidul, 2008). Although, numerous researches were carried out on chemical modification of starch from various sources, very few investigations were reported on dual chemical modification of starch. Liu et al. (2014) studied the properties of cross-linked oxidized maize starch. Xiao, Lin, Liu, and Yu (2012) also studied the characteristics of cross-linked oxidized dual-modified rice starches. Effect of cross-linking and acetylation on properties of rice starches were reported by Raina, Singh, Bawa, and Saxena (2007). Investigations on dual modification by cross-linking and hydoxypropylation have been reported by Wattanachant, Muhammad, Hashim, and Rahman (2003) for sago starch and Van Hung and Morita (2005) for wheat starch. Most of the works on chemical modification were carried out on starches from conventional sources like wheat, maize, potato, sago, rice and tapioca (Xiao et al., 2012), there are very few studies on chemical modification of taro starch (Karmakar, Ban, & Ghosh, 2014; Alam & Hasnain, 2009). Taro as a source of starch has great potential and could be used in place of other commercial starches in many applications (Sit, Misra, & Deka, 2013). Although, few reports are available on dual chemical modification of taro starch (Lee, Hong, Lee, Chung, & Lim, 2015; Xiao et al., 2012), there are no reports on dual chemical modification of taro starch. Thus, the objectives of this study was to prepare dual-modified taro starches with two different levels, each of hydroxypropylation and cross-linking applied in different sequences, and to compare the physicochemical properties of native, hydroxypropylated, crosslinked, and dual-modified starches. 2. Materials and methods 2.1. Sample collection Taro tubers (Colocasia esculenta var. antiquorum) locally known as “Panchamukhi” was purchased from a local agricultural farm near Tezpur University, Napaam, Tezpur, Sonitpur District, Assam, India. 2.2. Starch isolation Starch from taro tubers was extracted as per the method of Sit et al. (2013). Tubers were washed under tap water, peeled and cut into cubes of approximately 1 cm. The cubes were ground using a high speed laboratory blender (Philips HL 1632, India) for 2 min. The slurry was mixed with 10 times its volumes of distilled water. The suspension was filtered through double fold cheese cloth (approximately 50 mesh) and the filtrate was kept for sedimentation for 6 h. The supernatant was discarded and the sediment thus obtained was washed with distilled water for two times. The final sediment was dried at 45 ◦ C for 24 h in drying oven. The dried starch was ground and passed through 100 mesh sieve and kept in air tight plastic containers for further analysis. 2.3. Modification of taro starch by cross-linking Cross-linking of Taro starch was carried out according to Kaur, Singh, and Singh (2006), using phosphoryl chloride (POCl3) as crosslinking agent. Starch (30 g, dry basis) was suspended in distilled water (48 mL) containing 0.6 g of Na2 SO4 with mild stirring, and then pH was adjusted to 11.5 with 0.5 M sodium hydroxide. The

temperature of the slurry was maintained at 25 ◦ C. Phosphoryl chloride was added at 0.5 or 1.0 g kg−1 levels (0.05 or 0.10% based on dry weight of starch) using a micropipette and the reaction vessel was sealed. Starch was reacted with POCl3 for 1 h with stirring. After the designated cross-linking time elapsed, the starch slurry was neutralised to pH 5.5 with diluted HCl (0.1 M). The starch was recovered by centrifuging (5000 rpm, 10 min); washed three times with distilled water, and oven-drying at 45 ◦ C for 24 h. 2.4. Modification of taro starch by hydroxypropylation (etherification) Hydroxypropylation of Taro starches was performed according to the method of Lawal, Ogundiran, Adesogan, Ogunsanwo, and Sosanwo (2008). Taro starch (100 g, dry basis) was weighed inside 500 mL screw cap jars and distilled water (200 mL) was added, followed by the addition of 20 g of (anhydrous sodium sulfate) Na2 SO4 . The slurry was mixed for 30 min and the pH was adjusted to 11.15 with 1 M NaOH. Propylene oxide 5 mL or 10 mL per 100 g starch (5 or 10% based on dry weight of starch) was added and the suspension was mixed thoroughly with jars closed. The reaction was maintained at 40 ◦ C for 24 h. The reaction was terminated by adjusting pH 6.0–6.5 with dilute HCl (0.1 M). The slurry was centrifuged for 15 min at 10,000 rpm. The starch cakes obtained was washed with distilled water until its sulfate content showed negative test with BaCl2 . The hydroxypropylated starch was dried in the oven at 45 ◦ C for 24 h. 2.5. Single and dual modification of taro starch Single and dual modification of taro starch was carried out using two levels of each of the above two methods applied singly and in combination in different sequences. A total of twelve treatments (modifications) were carried out as follows: 1. Hydroxypropylation level 1 (E1): Hydroxypropylation was carried out using 5% Propylene oxide based on dry weight of starch as described earlier. 2. Hydroxypropylation level 2 (E2): Hydroxypropylation was carried out using 10% Propylene oxide based on dry weight of starch as described earlier. 3. Cross-linking level 1 (C1): Cross-linking was carried out using 0.05% POCl3 based on dry weight of starch as described earlier. 4. Cross-linking level 2 (C2): Cross-linking was carried out using 0.10% POCl3 based on dry weight of starch as described earlier. 5. Dual-modification 1 (E1C1): E1 followed by C1. 6. Dual-modification 2 (E1C2): E1 followed by C2. 7. Dual-modification 3 (E2C1): E2 followed by C1. 8. Dual-modification 4 (E2C2): E2 followed by C2. 9. Dual-modification 5 (C1E1): C1 followed by E1. 10. Dual-modification 6 (C1E2): C1 followed by E2. 11. Dual-modification 7 (C2E1): C2 followed by E1. 12. Dual-modification 8 (C2E2): C2 followed by E2. 2.6. Determination of chemical composition of native and modified taro starches The moisture, fat, ash and crude fibre content of the isolated native starch was determined by AOAC methods (AOAC, 1990). Protein content (N × 6.5) was determined by Kjeldahl method (AACC, 1990). The amylose content was determined by colorimetric method (McGrance, Cornell, & Rix, 1998). The standard curve was prepared using pure potato amylose type III (HiMedia, India). Estimation of starch content was carried out by Anthrone method (Sadasivam & Manickam, 2008).

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2.7. Granule morphology Shape and size of starch granules were evaluated using scanning electron microscope (JEOL JSM 6390 LV, Singapore). A thin layer of starch granule was mounted on the aluminium specimen holder by double-sided tape. The samples were coated with platinum and examined under the microscope at an accelerating voltage of 15 kV with magnification of 4000×. 2.8. Swelling power and solubility Swelling power and solubility of the starches were determined by modified method of Torruco-Uco and Betancur-Ancona (2007). Starch (0.5 g) was dispersed in 20 mL distilled water in a preweighed 50 mL centrifuge tubes and kept in shaking water bath at 60, 70, 80 and 90 ◦ C for 30 min. The suspension was then centrifuged at 12,000 × g for 10 min. The supernatant was carefully decanted in a Petri dish and dried at 103 ◦ C for 12 h. After decantation the weight swollen granules were taken. The swelling power and percentage solubility were calculated using the following formulas: Swelling Power Weight of swollen granules × 100 = (Weight of sample–Weight of dissolved starch)

% Solubility =

Weight of dried starch in Petri dish × 100 Sample weight

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viscosity (BV) and setback viscosity (SV) were recorded from the graph. 2.12. Texture analysis Textural properties such as firmness, consistency and cohesiveness of starch pastes were determined by back extrusion method in Texture Analyzer, TA.HDplus (Stable Micro Systems, UK) using a cylindrical probe (P-35). Starch pastes were prepared by heating a 2% aqueous suspension of starch (1 g starch in 50 mL distilled water) in a shaking water bath at 100 ◦ C for 30 min. The starch pastes were cooled to 25 ◦ C for determining the textural properties. The probe was allowed to penetrate 20 mm from the surface of the sample at a speed of 1 mm s−1 . Firmness, cohesiveness, consistency and index of viscosity were calculated from the graphs using the software Exponent Lite 32 provided with the instrument. The textural properties of the soup samples were analysed by the same method. 2.13. Colour analysis (starch pastes and dry powders) Colour parameters of the starch pastes containing 2% starch and that of the dry powders were measured using colorimeter (Ultrascan VIS, Hunterlab, USA). Results were obtained in terms of L* (lightness), ranging from 0 (black) to 100 (white), a* (redness), ranging from +60 (red) to −60 (green) and b* (yellowness), ranging from +60 (yellow) to −60 (blue) values. 2.14. Statistical analysis

2.9. Determination of paste clarity Clarity and stability of the starches was measured following the method described by Sandhu and Singh (2007). Aqueous starch suspension containing 1% starch was prepared by heating 0.2 g starch in 20 mL water in shaking water bath at 90 ◦ C for 1 h. The starch paste was cooled to room temperature. The starch pastes were stored at 4 ◦ C in refrigerator and the absorbance was measured at 640 nm in spectrophotometer (Spectrascan UV-2600, Thermo Fisher Scientific, India). The absorbance was measured after every 24 h for 5 days to determine the stability of the pastes. The light transmittance was determined with distilled water used as the blank. 2.10. Freeze-thaw stability The method of Singhal and Kulkarni (1990) was followed in determining the freeze-thaw stability of starch. Five percent (w/v) starch (db) in distilled water was heated at 95 ◦ C for 30 min. with constant stirring. Ten millilitre of paste was transferred to weighed centrifuge tubes. The weight of the paste was then determined. This was subjected to alternate freezing and thawing cycles (22 h freezing at −20 ◦ C followed by 2 h thawing at 30 ◦ C) for 5 days, centrifuged at 8000 × g for 10 min after each cycle and the percentage syneresis was determined as weight of exudates to the weight of paste. 2.11. Pasting properties Pasting properties of the starches were evaluated using Rapid Visco-Analyzer (RVA), model StarchMaster2 from Newport Scientific (Australia). Viscosity profiles were recorded using 12.5% starch slurry in distilled water (total weight 28 g). A heating and cooling cycle of 13 min was used where the samples were heated from 50 to 95 ◦ C in 5 min, held at 95 ◦ C for 2 min, cooled from 95 to 50 ◦ C in 4 min and held at 50 ◦ C for 2 min. Pasting temperature (PT), peak viscosity (PV), hold viscosity (HV), final viscosity (FV), breakdown

The data were subjected to single factor analysis of variance (ANOVA) using ‘Data Analysis Tool’ of ‘Microsoft Excel’. Fisher’s ‘Least Significant Difference (LSD)’ method was used to determine the statistical difference between the results obtained. 3. Results and discussions 3.1. Chemical composition and amylose content The moisture content of native starch was found to be 11.79 ± 1.61% (wet basis, n = 3). Nand, Charan, Rohindra, and Khurma (2008) found that the moisture contents of starch obtained from the cassava and taro samples ranged between 9–14%. Mweta, Labuschagne, Koen, Benesi, and Saka (2008) found that the moisture content of the cocoyam and cassava starches ranged from 10.42 to 11.13%. The ash content was 0.14 ± 0.09% (dry basis, n = 3) which was in accordance to the findings of Nand et al., 2008 and Mweta et al. (2008). The starch content was 96.68 ± 2.21% (dry basis, n = 3). Thus, as the purity of extracted starch is high, it implies that taro tubers can be used as a potential source of starch in food and other industrial applications. The protein content of the extracted starch was 0.65 ± 0.11% (dry basis, n = 3) which is very low. Alvani, Qi, Tester, and Snape (2011) found that the protein content in different extracted tuber starches to be very low. The fat content was found to be 0.08 ± 0.01% (dry basis, n = 3). Jane et al. (1992) reported that the lipid content in taro starches ranged between 0.08 to 0.12% (dry basis) which is in accordance with the present findings. The crude fibre content was 0.25% (dry basis) which is very low. The amylose content of the extracted starch was found to be 21.65%. This value is comparable to those reported for other tropical tubers such as cassava (13.6–23.8%), sweet potatoes (8.5–38%), Xanthosoma (15–25%), Dioscorea (12.5–29.7%), Colocasia (13–43%) (Moorthy, 2002). The amylose contents in the modified starches (Table 1) were found to be lower than the native starch which may be due to the leaching of some of the amylose from the starch granules

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Table 1 Solubility, swelling and amylose contents of native and modified starches. Starch Samples Native E1 E2 C1 C2 E1C1 E1C2 E2C1 E2C2 C1E1 C1E2 C2E1 C2E2

Solubility at different temperatures 60 ◦ C 6.33 19.21 22.33 5.50 4.64 7.53 6.42 9.53 8.43 6.45 7.21 8.39 8.86

70 ◦ C ± ± ± ± ± ± ± ± ± ± ± ± ±

0.18f 0.17b 0.20a 0.25g 0.33h 0.34e 0.32f 0.38c 0.32d 0.23f 0.19e 0.28d 0.67d

9.37 30.44 33.45 7.48 6.49 10.40 11.91 20.46 15.45 10.77 11.39 10.38 11.23

Swelling at different temperatures 80 ◦ C

± ± ± ± ± ± ± ± ± ± ± ± ±

0.19h 0.48b 0.34a 0.47i 0.43j 0.33g 0.58e 0.40c 0.40d 0.55f,g 0.28e,f 0.24g 0.35f

14.36 38.51 47.52 8.56 7.39 18.62 19.57 27.43 23.53 17.45 16.34 15.53 16.48

90 ◦ C ± ± ± ± ± ± ± ± ± ± ± ± ±

0.26j 0.32b 0.31a 0.28k 0.27l 0.31f 0.15e 0.26c 0.32d 0.37g 0.20h 0.27i 0.29h

20.91 37.15 45.72 13.69 10.53 24.51 22.73 31.95 27.33 19.56 22.74 17.27 20.82

60 ◦ C ± ± ± ± ± ± ± ± ± ± ± ± ±

0.79g 0.25b 0.43a 0.23j 0.41k 0.32e 0.43f 0.54c 0.45d 0.29h 0.38f 0.40i 0.37g

2.72 7.21 8.37 2.28 1.97 5.29 4.65 7.40 6.59 6.29 7.37 5.29 6.30

70 ◦ C ± ± ± ± ± ± ± ± ± ± ± ± ±

0.23f 0.21b 0.34a 0.33f,g 0.34g 0.18d 0.21e 0.35b 0.25c 0.21c 0.26b 0.18d 0.29c

5.45 9.49 16.29 4.60 4.54 8.38 6.24 8.5 8.57 7.58 10.32 6.75 8.39

80 ◦ C ± ± ± ± ± ± ± ± ± ± ± ± ±

0.24h 0.41c 0.17a 0.38i 0.32i 0.22d 0.33g 0.26d 0.29d 0.36e 0.19b 0.23f 0.24d

10.64 16.74 20.15 8.87 7.15 10.50 7.91 11.57 10.24 9.70 12.34 8.53 11.26

Amylose content (%)

90 ◦ C ± ± ± ± ± ± ± ± ± ± ± ± ±

0.48e 0.38b 0.40a 0.34g 0.16i 0.33e 0.29h 0.31d 0.35e,f 0.40f 0.39c 0.37g 0.36d

13.79 15.75 19.63 10.57 8.43 11.64 9.75 13.40 11.77 11.63 15.88 10.65 12.19

± ± ± ± ± ± ± ± ± ± ± ± ±

0.17c 0.14b 0.54a 0.55e 0.32g 0.43d 0.45f 0.37c 0.51d 0.41d 0.36b 0.58e 0.69d

21.65 20.47 20.26 19.89 19.64 18.53 18.26 18.15 18.49 18.32 17.96 18.19 18.33

± ± ± ± ± ± ± ± ± ± ± ± ±

1.05a 2.13ab 0.68ab 0.52ab 0.29ab 1.03b 1.52b 0.61b 2.12b 0.36b 0.56b 0.49b 0.96b

Values are reported as mean ± standard deviation of three replications. Means followed by the same superscript small letters within a column are not significantly different (p > 0.05).

during the modification process in high alkaline pH (Karmakar et al., 2014). For the hydroxypropylated starches another reason for lowering of amylose content might be the introduction of the hydroxypropyl groups in the amorphous regions which mainly contains amylose, thereby lowering the amylose content reading. It has been reported that during hydroxypropylation, the hydroxypropyl groups in the starch chains are mainly introduced in the amorphous regions which composed primarily of amylose (Lawal et al., 2008; Blanshard, 1987). Similarly, the decrease in amylose content of cross-linked starch could be attributed to inter-molecular bonding between the amylose and amylopectin molecules or between two amylose molecules. The amylose content of the native starch was 21.65%, which was decreased to 20.26% in E2 and to19.64% in C2, but the differences were not statistically significant. In case of the dual modified starches the amylose content decreased further and lowest amylose content of 17.96% was observed for C1E2.

3.2. Granule morphology The scanning electron micrographs of the native and modified taro starch granules (Fig. 1) were polygonal, dome shaped, split and irregular in shape and had very small size of less than 5 ␮m. Taro starches showed small and medium polyhedral (polygonal) and truncated granules and exhibited smooth surfaces with some portions of the surface being irregular. Modification did not carry out a huge change but the smoothness was degraded to some extent due to erosion and the grooves are more pronounced in the modified starches. The erosion of the starch granules can be attributed to the effect of the sodium hydroxide treatment during the process of hydroxypropylation and cross-linking. Higher erosion was observed in case of etherified starches because of the prolonged period of 24 h during the preparation of hydroxypropylated starches. Starch granules were found to be aggregated due to chemical modifications. Some fusion of the starch granules were also observed for the modified starches, particularly dualmodified starches. Due to the smaller size of the starch granules, no prominent damage or rupture was observed due to modification. Mirmoghtadaie, Kadivar, and Shahedi (2009) and Xiao et al. (2012) also could not observe any noticeable differences in the morphological structure of the starch granules of cross-linked and acetylated oat starch and cross-linked oxidized rice starch respectively due to the small size of the granules. Although, Van Hung and Morita (2005) reported greater damage to cross-linked acetylated wheat starch granules due to the larger size of the granules.

3.3. Swelling power and solubility Solubility and swelling patterns of all starches at different temperatures are shown in Table 1. The solubility and swelling power of all the starch samples increased with the increase of temperature. The increase in swelling power and solubility of hydroxyproylated starches might be due to the incorporation of a hydroxypropyl group that is capable of disrupting inter- and intra-molecular hydrogen bonds in the starch chains, thereby weakening the granular structure of starch and increasing the accessibility of the starch granules to water. Solubility and swelling power also increased to a greater extent with increase in level of hydroxypropylation. Solubility at 90 ◦ C was 37.15% and 45.72% in E1 and E2, respectively, while it was 20.91% in case of unmodified native taro starch. Similarly, swelling power at 90 ◦ C of the modified starches E1 and E2 were 15.75% and 19.63% respectively as compared to native taro starch which was 13.79%. A similar trend has been reported by several authors for hydroxypropylated starches from various sources (Choi & Kerr, 2004; Chun & Yoo, 2007; Kaur, Singh, & Singh, 2004; Lawal et al., 2008; Senanayake, Gunaratne, Ranaweera, & Bamunuarachchi, 2014). Solubility and swelling power increased with increase in level of hydroxypropylation due to increased molecular substitution by hydoxypropyl groups (Senanayake et al., 2014) which disrupted the inter- and intra-molecular hydrogen bonds in the starch chains and weakened the granular structure of starch, thereby increasing the accessibility of the starch granules to water (Lee & Yoo, 2011). In the amorphous region of the starch where most of the swelling takes place, introduction of hydrophilic hydroxypropyl groups enhanced water percolation into the granules thereby causing expansion and increase in swelling. Increases observed as the temperature increased is because water penetrated into the more amorphous region of the starch granule and as the temperature increases the swelling of the amorphous phase accelerates the disruption of the crystalline region leading to enhanced swelling. This is why in starch applications such as food thickening and hydrogels, hydroxypropyl starches would be relevant. Cross-linking with POCl3 led to reduction in swelling power and solubility. Reduced solubility and swelling was observed in the cross-linked starches with the increased concentrations of the modifying agent. This is a typical property of the cross-linked starch. At 90 ◦ C the solubility and swelling of C2 was found to be 10.53% and 8.43 (g/g) which was quite lower than the native and hydroxypropylated starches. Cross-linking reinforces the inter- and intra-molecular bonding within the granule with covalent bonds that act as a bridge between the starch molecules. The bonding between the starch chains are strengthened due to cross-linking, thereby increasing the resistance of the granules to swelling.

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Fig. 1. Scanning electron micrograph of native and modified starch granules.

Increase in level of added POCl3 resulted in lowering of swelling of the cross-linked starch due to the formation of a hard outer crust that restricted granule swelling as cross-linking takes place mostly on the outer portion of the granules (Kaur et al., 2006; Singh, Kaur, & McCarthy, 2007; Mirmoghtadaie et al., 2009). For the dual-modified starches, the swelling and solubility was found to increase with increase in level of hydroxypropylation and decrease with increase in level of cross-linking. It was further noticed that the sequence in which the modifications were carried out played an important role

in the swelling and solubility of the dual-modified starches. The solubility of the dual-modified starches in which hydroxypropylation was carried out first were found to be higher from the starches in which cross-linking was carried out first for same levels of both the treatments, e.g. the solubility of the sample E1C1 was higher than C1E1 and solubility of E1C2 was higher than C2E1, etc. at 90 ◦ C. For the swelling of dual-modified starches the trend was found to be opposite. Cross-linking of hydroxypropylated starches might have involved the hydroxypropyl groups as well which would have

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Table 2 Paste clarity of native and modified starches. Starch samples

% Transmitance Day 1

Native E1 E2 C1 C2 E1C1 E1C2 E2C1 E2C2 CIE1 C1E2 C2E1 C2E2

22.57 34.23 41.2 15.17 11.33 17.53 15.37 16.33 14.2 17.17 19.43 13.73 15.3

Day 2 ± ± ± ± ± ± ± ± ± ± ± ± ±

0.41c 0.15b 0.46a 0.60g 0.12i 0.25e 0.25g 0.32f 0.2h 0.21e 0.41d 0.38h 0.6g

21.37 33.53 40.4 13.86 10.7 16.2 14.43 15.56 14.33 16.4 18.8 13.17 14.47

Day 3 ± ± ± ± ± ± ± ± ± ± ± ± ±

0.15c 0.47b 0.1a 0.35g 0.1i 0.26e 0.41g 0.31f 0.42g 0.26e 0.46d 0.31h 0.47g

20.53 33.07 39.43 12.33 9.93 15.97 13.8 14.7 13.07 15.37 17.53 11.56 13.3

Day 4 ± ± ± ± ± ± ± ± ± ± ± ± ±

0.38c 0.15b 0.15a 0.49i 0.15k 0.49e 0.36g 0.2f 0.35h 0.31e 0.45d 0.35j 0.65g,h

19.33 31.37 38.63 11.3 9.43 14.67 12.36 14.27 12.26 14.6 16.63 10.63 11.73

Day 5 ± ± ± ± ± ± ± ± ± ± ± ± ±

0.15c 0.15b 0.57a 0.46h 0.06j 0.51e 0.31f 0.15e 0.25f,g 0.26e 0.35d 0.41i 0.67g,h

18.26 30.67 37.67 9.53 8.37 13.36 11.8 13.33 11.53 12.43 15.3 10.13 10.77

± ± ± ± ± ± ± ± ± ± ± ± ±

0.31c 0.15b 0.15a 0.59i 0.31j 0.21e 0.26f,g 0.35e 0.35g 0.31f 0.4d 0.71h,i 0.51h

Values are reported as mean ± standard deviation of three replications. Means followed by the same superscript small letters within a column are not significantly different (p > 0.05).

restricted swelling. Also, the lowering of swelling might be due to depolymerization of amylopectin chains in cross-linked starches similar to oxidized starches which resulted in a sponge like granule structure that is able to absorb water during heating, but cannot retain the absorbed water under centrifugation (Vanier et al., 2012). Therefore, the swelling of hydoxypropylated-cross-linked starches were lower than the cross-linked-hydroxypropylated starches. 3.4. Paste clarity Improved paste clarity is a useful property in the manufacture of some foods like jellies, sausages and fruit pastes, which require transparency. The clarity of the starch pastes were found to increase with level of hydroxypropylation and decrease with increase in cross-linking level for both single and dual-modified starches (Table 2). The hydroxypropylated starch develops highest paste clarity than native starches in contrary to the cross linked starches. Percent Transmittance was found highest in E2 which was 41.2% in day 1 and decreased to 37.67% in day 5. Substitution of the hydroxyl groups with hydroxypropyl groups on the starch molecules prevented formation of inter and intramolecular bonds which decreases turbidity and increased clarity. The higher paste clarity of hydroxypropylated can be explained by the greater swelling caused by the incorporation of hydrophilic hydroxypropyl groups, which dissociate the starch molecules and ultimately inhibit the association of starch chains after pasting, resulting in more transmittance of light, as described by Craig,

Maningat, Seib, and Hoseney (1989). Paste clarity of maize starches also increased after hydroxypropylation (Liu et al., 1999). The crosslinking of starches led to decrease the paste clarity. C2 showed the lowest percent transmittance among all the combinations which was 11.33% in day 1 and decreased to 8.37% in day 5. All the dual modified taro starches involving cross-linking with etherification showed decrease in paste clarity and stability than the native taro, but were higher than only cross-linked starches. This was due to the association of the cross-linked starch in water which forms more hydrogen bonding with the soluble starch and this network of bonding in aggregate forms prevent the transmittance and the paste clarity was found to be decreased. Similarly, Van Hung and Morita (2005) reported decrease of light transmittance in crosslinked starches. The results of paste clarity was found to be closely related to the data obtained for swelling power of the native, single and dual modified starches at 90 ◦ C, where higher % transmittance was observed for starches with higher swelling power. The stability of all the starch was found to be similar, and a decrease in transmittance of 4–5% was observed for all the starch pastes during the 5 days. 3.5. Freeze-thaw stability The freeze-thaw stability of the native and modified taro starches are presented in Table 3. In native starch, percentage syneresis increased progressively as the number of free thaw cycles increased. Native starch showed 26.66% syneresis in

Table 3 Freeze-thaw stability of native and modified starches. Starch samples

% Syneresis Cycle 1

Native E1 E2 C1 C2 E1C1 E1C2 E2C1 E2C2 CIE1 C1E2 C2E1 C2E2

26.66 18.78 12.27 39.75 48.21 19.13 21.1 15.77 17.28 25.75 22.13 31.39 35.6

± ± ± ± ± ± ± ± ± ± ± ± ±

Cycle 2 0.48e 0.46i 0.59l 0.56b 0.62a 0.65i 0.49h 0.39k 0.52j 0.40f 0.47g 0.36d 0.51c

30.63 20.4 15.18 41.19 51.31 21.58 21.41 17.18 17.36 27.21 23.25 33.25 36.3

± ± ± ± ± ± ± ± ± ± ± ± ±

Cycle 3 0.27e 0.55i 0.44k 0.37b 0.56a 0.36h 0.58h 0.36j 0.47j 0.55f 0.67g 0.66d 0.45c

31.18 20.53 15.71 43.87 51.48 22.33 23.06 17.62 17.79 27.57 25.12 33.68 36.85

± ± ± ± ± ± ± ± ± ± ± ± ±

Cycle 4 0.37e 0.39i 0.5k 0.54b 0.34a 0.44h 0.51h 0.36j 0.42j 0.52f 0.57g 0.54d 0.38c

32.63 21.51 16.55 44.47 52.56 23.43 23.49 18.18 18.54 28.84 25.78 35.09 37.89

± ± ± ± ± ± ± ± ± ± ± ± ±

Cycle 5 0.39e 0.3i 0.31k 0.35b 0.4a 0.40h 0.35h 0.32j 0.37j 0.31f 0.31g 0.79d 0.57c

34.45 21.93 17.4 45.25 53.55 24.55 24.28 19.46 20.49 29.69 26.44 35.8 38.31

± ± ± ± ± ± ± ± ± ± ± ± ±

0.38e 0.47i 0.46l 0.35b 0.34a 0.32h 0.36h 0.4k 0.41j 0.42f 0.51g 0.48d 0.39c

Values are reported as mean ± standard deviation of three replications. Means followed by the same superscript small letters within a column are not significantly different (p > 0.05).

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Table 4 Pasting properties of native and modified starches. Starch sample

Pasting temperature (◦ C)

Native E1 E2 C1 C2 E1C1 E1C2 E2C1 E2C2 C1E1 C1E2 C2E1 C2E2

81.6 77.7 73.1 80.1 83.1 78.9 80.5 76.9 68.5 74.8 69.3 71.0 69.4

± ± ± ± ± ± ± ± ± ± ± ± ±

0.5b 0.2de 1.0g 0.5c 0.6a 0.3d 0.4c 0.1e 0.5j 0.4f 0.2i 0.4h 0.8i

Peak viscosity (cP) 5120 2519 2814 5368 6109 4972 3179 5472 3386 6797 6116 5425 5491

± ± ± ± ± ± ± ± ± ± ± ± ±

250de 223h 125gh 102cd 99b 154e 214fg 125c 179f 162a 258b 153c 64c

Hold viscosity (cP) 2337 1657 1740 4217 4831 3874 2512 4258 2495 5081 4510 4064 4070

± ± ± ± ± ± ± ± ± ± ± ± ±

123g 115h 63h 125de 143b 157f 112g 98d 78g 145a 126c 134ef 64ef

Final viscosity (cP) 3282 2598 2662 6036 6685 5416 3545 6639 4227 7219 6598 5798 6140

± ± ± ± ± ± ± ± ± ± ± ± ±

152f 158g 147g 136c 195b 253d 315f 258b 124e 249a 169b 237cd 112c

Breakdown (cP)

Set Back (cP)

2783 ± 99a 862 ± 56f 1074 ± 23ef 1151 ± 121de 1278 ± 110cd 1098 ± 25e 667 ± 26g 1214 ± 45d 891 ± 87f 1716 ± 113b 1606 ± 69b 1361 ± 74c 1421 ± 106c

945 941 922 1819 1854 1542 1033 2381 1732 2138 2088 1734 2070

± ± ± ± ± ± ± ± ± ± ± ± ±

56g 25g 36g 87cd 42c 59e 15f 98a 106 115b 65b 68d 94b

Values are reported as mean ± standard deviation of three replications. Means followed by the same superscript small letters within a column are not significantly different (p > 0.05).

cycle 1 and it was increased to 34.45% in the cycle 5. Hydroxypropylation reduced percentage syneresis significantly. Syneresis reduced with the increased level of hydroxypropylation. Syneresis in freeze-thawed gels is attributed to the increase of molecular association between starch chains at reduced temperature, exuding water from the gel structure (Chotipratoom, Choi, Bae, Kim, & Baik, 2015). The hydrophilic nature of the hydroxypropyl groups enhanced water-holding ability of the starch pastes, thereby limiting amount of water exuded. Similar observations have been reported for hydroxypropylated potato starches (Kaur et al., 2004), for hydroxypropylated plantain starches (Lawal et al., 2008) and for hydroxypropylated corn and amaranth starches (Pal, Singhal, & Kulkarni, 2002). The % syneresis in cross-linked starch pastes was found to be significantly higher compared to hydroxypropylated starches and native starch paste. Syneresis in the cross-linked starches increased with increase in level of cross-linking. This could be attributed to increased molecular association between the starch chains due to cross-linking, which increased retrogradation during refrigerated storage, thereby excluding more water from the gel structure (Mirmoghtadaie et al., 2009). The freeze-thaw stability of the dual-modified starches where hydroxypropylation was carried prior to cross-linking was found to be better than those modified starch pastes where cross-linking was carried out first i.e. the hydroxypropylated-cross-linked starches were able to hold more water during the freeze-thaw cycles than cross-linked-hydroxypropylated starches. This implies the suitability of the hydroxypropylated-cross-linked starches to use in the frozen food preparations where viscosity is also desired. The cross-linked-hydroxypropylated starches were found to exude more during the free-thaw cycles due to the retrogradation of the starch granules during the low temperature cycles in which the network formation and reassociation expels the water out due to which the syneresis was found to be increased. The difference in freeze-thaw stability of dual-modified starches due to the sequence of modification for same levels of both the treatments could be attributed the dominating nature of the modification which was carried out first (Xiao et al., 2012). The hydroxypropylatedcross-linked starches might have developed more numbers of hydroxypropyl groups and fewer numbers of intra and intermolecular linkages than cross-linked-hydroxypropylated starches. The increase in syneresis of all the starch pastes were found to be more between 1st and 2nd cycles and was less for the subsequent cycles. Van Hung and Morita (2005) also reported larger syneresis of cross-linked starches during first two freeze-thaw cycles compared to native counterparts and smaller syneresis during second two freeze-thaw cycles.

3.6. Pasting properties The pasting profile of the native and modified taro starches is presented in Table 4. The viscosity profile gives the idea of the viscous force experienced during a gelatinization and retrogradation process. The pasting properties of starch are used in assessing the suitability of its application as functional ingredient in food and other industrial products. The pasting temperatures of the hydroxypropylated starches decreased significantly which might be attributed to the hydrophilic nature of the hydroxypropyl groups, which facilitate water penetration into the starch granules and weaken the granular structure (Han, 2010). For cross-linked starches pasting temperatures increased or were found to be closer to native starch. The increase in pasting temperature of cross-linked starches might be due to increase in crystallinity, as a result of reorientation of the amylose and amylopectin molecules of the starch granule (Olu-Owolabi, Olayinka, Adegbemile, & Adebowale, 2014). For dual-modified starches the pasting temperature was found to decrease with increase in level of hydroxypropylation. The cross-linked starch showed a higher peak viscosity than the native and hydroxypropylated starches. High peak viscosity suggests suitability as thickening agent in food (Rapaille & Vanhemelrijck, 1999). This high peak viscosity of cross-linked starches can be attributed to the high integration of the granules. Cross-linking provides higher resistance against breakage and disintegration of granules due to the increased intra- and inter-bonding between the starch molecules resulting in high viscosity. Higher force was required to break these bonding which experienced by the rotating paddle and showed a high viscosity. The peak viscosities of cross-linked starches were negatively related to swelling and solubility values as observed in Table 1. Similar observations were reported by Jyothi, Moorthy, and Rajasekharan (2006) at low concentrations of epichlorohydrin. The starches modified by hydroxypropylation only showed lower values of peak viscosity and pasting temperature than the native starch and were positively related to swelling power and solubility (Ai & Jane, 2015). This implies that the pastes of hydroxypropylated starches are stable at lower temperature and are suitable for lower temperature processing. However, a hydroxypropylated starch that is not cross-linked swells excessively during cooking to produce a stringy paste that is unstable to shear and acid. That is why hydroxypropylated starches were cross-linked to use in the applications where extended cooking is required at higher temperature. The final viscosities of the crosslinked and dual modified starches were found to be more than the peak viscosities of these starches, which implies their application in which no stirring is required during cooling. The high final viscosities of the cross-linked and dual modified starches

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Table 5 Texture properties of native and modified taro starch pastes. Starch samples

Firmness (g)

Native E1 E2 C1 C2 E1C1 E1C2 E2C1 E2C2 C1E1 C1E2 C2E1 C2E2

11.762 12.648 13.848 14.558 15.377 16.576 17.538 16.847 19.454 15.341 15.918 16.202 17.328

± ± ± ± ± ± ± ± ± ± ± ± ±

0.07l 0.09k 0.12j 0.08i 0.12h 0.17e 0.13b 0.13d 0.10a 0.11h 0.09g 0.16f 0.09c

Consistency (g s) 193.737 207.435 222.873 228.408 233.953 231.022 239.319 230.369 243.427 229.287 231.383 232.324 235.784

± ± ± ± ± ± ± ± ± ± ± ± ±

0.65j 1.57i 1.39h 1.00g 2.51c,d 0.90e,f 1.07b 0.71e,f,g 0.99a 1.03f,g 1.45e,f 1.08d,e 1.09c

Cohesiveness (g) 8.627 9.239 9.861 10.002 11.129 10.429 11.937 10.923 13.123 10.352 10.839 11.748 12.458

± ± ± ± ± ± ± ± ± ± ± ± ±

0.09j 0.09i 0.14h 0.10h 0.09e 0.19g 0.09c 0.08f 0.10a 0.09g 0.11g 0.11d 0.07b

Index of viscosity (g s) 1.515 1.746 2.054 1.329 1.447 1.262 1.171 1.245 1.404 1.247 1.267 1.314 1.477

± ± ± ± ± ± ± ± ± ± ± ± ±

0.09c 0.087b 0.06a 0.06e,f 0.05c,d 0.046f,g 0.059g 0.102f,g 0.02d,e 0.03f,g 0.031f,g 0.04e,f 0.06c

Values are reported as mean ± standard deviation of three replications. Means followed by the same superscript small letters within a column are not significantly different (p > 0.05).

represented their stability in high temperatures which were suitable for the application like high temperature processing and require high viscosity in the final product as the viscosity is not lost even after cooling. Breakdown viscosity is used in assessing the ability of a starch paste used as thickener to withstand severe processing conditions. Significant lowering of breakdown values were shown by the modified starches, which indicated high stability of modified starch pastes during shearing at high temperatures. Similar results for breakdown viscosity were reported by Liu et al. (1999) for cross-linked normal and waxy rice starch, Xiao et al. (2011) for cross-linked rice and maize starches. For crosslinked starches, hydrogen bonds become weak when continuously heated in water and the granules gradually swells. However, when the granules rupture, the covalent cross-links within the starch molecules provide sufficient granule integrity which keeps the swollen granules intact and minimize or prevent loss in viscosity (Detduangchan, Sridach, & Wittaya, 2014; Jyothi et al., 2006; Lim & Seib, 1993). Setback viscosity is a measure of retrogradation that is the extent to which dissolved starch macromolecules ‘amylose’ are able to reassociate with themselves and granule fragments. During setback, the solubilized amylose molecules reassociate through the formation of a three dimensional network which results in a gel, thereby increasing final viscosity. The setback viscosities of the cross-linked and dual modified starches were found to be more compared to the native and hydroxypropylated starches which might be attributed to higher re-association of starch molecules due to cross-linking during setback. The sequence of modification was also found to affect the pasting properties of the dual-modified starches. The dual-modified starches which were cross-linked prior to hydroxypropylation were found to have higher viscosity values compared to the starches which were hydroxypropylated before cross-linking. This might be due to the reason that when cross-linking was carried out first, the intra and intermolecular bonding present were more compared to the starches which were hydroxypropylated in the first step. Further, it was observed that for the dual-modified starches viscosity decreased with increase in level of cross-linking, although, for single modification it was found to increase with increase in level of cross-linking. This might be attributed to the fact that higher level of cross-linking prevented extensive hydroxypropylation which restricted swelling, thereby decreasing viscosity. Highest peak, hold and final viscosities were observed for the dual-modified starch C1E1, which implies that maximum viscosity can be obtained when cross-linking and hydroxypropylation will be carried out in such a way which allows maximum swelling and provide enough strength to hold the granules intact during heating and shearing.

3.7. Texture of starch pastes The texture parameters of the native and modified taro starch pastes are presented in Table 5. All the modified starch shows increased firmness than the native taro starch. The native starch showed minimum firmness that is 11.762 g. The hydroxyporpylated starches showed a small increase in firmness compared to cross-linked and dual-modified starches. The higher firmness in hydroxypropylated starches was likely due to the higher solubility and swelling due to leaching of amylose in the etherified starches. The increase in firmness of cross-linked starches could be attributed to the intra and inter-molecular bonding due to cross-linking which strengthened the gel structure and prevented rupture during compression. The dual-modified starches showed higher firmness than the native and single modified starches, which might be due to the combined effect of both the treatments. The consistency of the starch pastes also showed a similar trend. The index of viscosity of the hydroxypropylated samples also increased, implying that the treated starch paste samples are more susceptible to viscosity change with temperature compared to the control, and was confirmed from the values of breakdown viscosities and setback viscosities in pasting profile. The index of viscosity of cross-linked and the dual modified starches were found to be closer to that of the native starch paste. Highest value of firmness, consistency and cohesiveness was observed for the dual-modified starch E2C2. All the dual-modified samples showed higher values in terms of firmness and consistency representing a good texture of the starch pastes. Higher values of firmness, consistency and cohesiveness, and lower values of index of viscosity of the cross-linked and dual-modified starch pastes might be attributed to the increase in granular integrity of the modified starches due to cross-linking. Cross-linked starches might be used to control the texture of products as it is able to tolerate heat, shear and acid during processing (Alam & Hasnain, 2009). The increase in firmness and consistency of starch pastes may be desirable for certain products and is important criteria in determining the application of a starch. 3.8. Colour of taro starch powder The colour of the native and modified taro starch powders are shown in Table 6. It is clearly observed that in all the modifications L* value increased significantly compared to the L* value of native starch and it certainly proved that modification by etherification and cross-linking and their combinations improves the colour of the starch powders and enhances its possibility for use in different food applications as starches are supposed to be white (Ali, Falade, & Akingbala, 2012). Dual modification showed a

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Table 6 Colour values of native and modified starch powders and pastes. Starch samples

Starch powders L*

Native E1 E2 C1 C2 E1C1 E1C2 E2C1 E2C2 C1E1 C1E2 C2E1 C2E2

91.31 94.7 95.47 95.87 96.27 95.79 97.82 98.84 98.07 97.81 96.73 97.31 97.36

Starch pastes a*

± ± ± ± ± ± ± ± ± ± ± ± ±

0.04j 0.22i 0.13h 0.09g 0.08f 0.19g 0.09c 0.06a 0.04b 0.09c 0.07e 0.08d 0.11d

1.28 0.49 0.55 0.72 0.98 0.72 0.9 0.44 0.67 0.65 0.48 0.52 0.28

b* ± ± ± ± ± ± ± ± ± ± ± ± ±

0.03a 0.03e,g 0.04e 0.02d 0.02b 0.02d 0.01c 0.05g 0.07d 0.04d 0.07f,g 0.05e 0.04h

2.47 1.8 1.91 1.58 2.18 2.24 1.77 1.34 1.56 2.38 2.57 1.82 1.37

L* ± ± ± ± ± ± ± ± ± ± ± ± ±

0.07a,b 0.04e 0.06d 0.06f 0.08c 0.06c 0.07e 0.08g 0.05f 0.06b 0.05a 0.06d,e 0.07g

16.52 18.93 22.37 9.0 9.94 8.43 8.21 8.45 8.33 9.82 10.50 10.33 10.09

a* ± ± ± ± ± ± ± ± ± ± ± ± ±

0.24c 0.06b 0.06a 0.07h 0.04f,g 0.05i 0.06j 0.06i 0.12i,j 0.07g 0.09d 0.07e 0.09f

1.57 1.17 1.01 0.19 0.27 0.15 0.13 0.13 0.19 0.19 0.24 0.22 0.15

b* ± ± ± ± ± ± ± ± ± ± ± ± ±

0.06a 0.07b 0.08c 0.07d,e,f 0.06d 0.04e,f 0.05f 0.06f 0.05d,e,f 0.04d,e,f 0.05d 0.06d,e 0.04e,f

4.19 3.55 3.18 0.65 0.6 0.58 0.53 0.45 0.42 0.82 0.77 0.65 0.67

± ± ± ± ± ± ± ± ± ± ± ± ±

0.09a 0.08b 0.07c 0.04e,f 0.03e,f,g 0.06f,g 0.03g 0.04h 0.06h 0.07d 0.06d 0.04e,f 0.07e,f

Values are reported as mean ± standard deviation of three replications. Means followed by the same superscript small letters within a column are not significantly different (p > 0.05).

remarkable improvement in the lightness and E2C1 showed the highest lightness followed by E2C2 having ‘L*’ value 98.84 and 98.07 respectively. A decrease in a* and b* values of the modified starches was also observed. The increase in lightness of the modified starches could be possibly due to reduction of impurities like fibre, lipid, etc. from the native starch which were attached to the starch molecules and were removed due to incorporation of functional groups and cross-linking within the starch molecules during modification. Another possible reason for improved powder colour might be attributed to the use of NaOH during modification process which has a bleaching effect particularly in cross-linked starches (Vanier et al., 2012). 3.9. Colour of taro starch paste The lightness of the hydroxypropylated starch pastes increased compared to the native and other modified starch pastes (Table 6). These results could not be related to the results obtained for lightness of the modified starch powder, but were found to be in accordance with the results for swelling power in the present investigation. E2 showed the highest lightness which may be attributed due to the increased swelling of the starch granules. Cross-linked and dual modified starches showed comparatively lower values of lightness than those of etherified and native starches. Significantly lower values of a* and b* i.e., redness and yellowness were witnessed for all the modified starches when compared to native starch paste, which might be attributed to the reduction of impurities in modified starches as also observed for starch powder. 4. Conclusions Marked differences in the properties of native, single modified and dual-modified starches were observed in the present investigation. Chemical modification of starch using a single method might improve certain functional properties, but at the same time some useful properties are also lost. In the present work it was observed that due to hydroxypropylation the swelling and solubility of native starch was improved whereas the viscosity was found to decrease. Similarly for the cross-linked starches swelling and solubility was found to be less but the pasting properties improved. In the dual-modified starches all the properties were found to be better than that of the native starch. The sequence in which the dual modifications were carried out also affected the properties of starch. Therefore, from the present investigation it can be concluded that specific properties of chemically modified starches to suit specific applications, such as food products requiring both high viscosity

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