International Journal of Biological Macromolecules 79 (2015) 309–315
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Effect of ␥-irradiation on structure and physico-chemical properties of Amorphophallus paeoniifolius starch Chagam Koteswara Reddy, M. Suriya, P.V. Vidya, K. Vijina, Sundaramoorthy Haripriya ∗ Department of Food Science and Technology, Pondicherry Central University, Puducherry 605014, India
a r t i c l e
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Article history: Received 9 March 2015 Received in revised form 24 April 2015 Accepted 5 May 2015 Available online 12 May 2015 Keywords: Amorphophallus paeoniifolius Starch ␥-Irradiation Viscosity Crystallinity Thermal properties
a b s t r a c t Gamma irradiation is one of the effective techniques able to alter structure and its properties of starch. In this research, the effect of modification in terms of molecular structure and physico-chemical properties of Amorphophallus paeoniifolius starch by ␥-irradiation using 60 Co as ␥-source at doses of 5, 10, 15, 20 and 25 kGy with dose rate 2 kGy/h was studied. Morphology of native and irradiated starches under scanning electron microscope revealed that granules are round, elliptical and polygonal in shape with smooth surfaces; no cracking or roughness was noted on irradiated starches. Amylose content, pH, swelling power of the starches and syneresis of the gelatinized starch were significantly decreased by irradiation; while carboxyl content, solubility, light transmittance and water absorption capacity of the starch granules were raised with increased dose of irradiation. Reduced pasting parameters and changes in FTIR spectrum significantly differed from each other. XRD pattern of irradiated starches showed C-type pattern with intermediate peaks of 2 at 16.92◦ and 18.12◦ , strong peaks of 2 at 23.05◦ and weak peaks of 2 at 14.7◦ , displaying slight decreased in the intensity of peaks in irradiated starches. Irradiation of starches increased gelatinization temperatures and enthalpy value was measured using DSC. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In food processing industries, starch contributes for a variety of vital characteristics that include thickening, gelling, consistency and shelf stability in varied applications [1]. Starch isolated from potato, rice, corn and wheat has been exclusively used to confer structure, texture, consistency and appeal to many food and non-food products [2]. Native starch from different sources have limitations such as low thermal resistance, low shear resistance, low solubility, high viscosity and tendency towards retrogradation leading to staling and syneresis, which may adversely affect the quality of food products. In these conditions, the importance of starch modification demands attention and the approaches could be physical, chemical, or enzymatic methods. The chemical modification methods such as acetylation, oxidation [3] and cross linking [4] are widely applied when paralleled with starch modification by physical methods [5] in food industries. Among physical treatments, irradiation is well accepted since there is no significant raise in temperature, requiring nominal sample preparation, rapidity in process and non-reliance on any type of catalysts [6]. The
∗ Corresponding author. Tel.: +91 9443701906; fax: +91 413 2654621. E-mail address: [email protected] (S. Haripriya). http://dx.doi.org/10.1016/j.ijbiomac.2015.05.003 0141-8130/© 2015 Elsevier B.V. All rights reserved.
modification of starch by irradiation technique is an impending development and the commonly used radiation involves ␥-rays. Irradiation involves the use of ␥-rays, an ionic, non-heat physical treatment, in which, food products are directly exposed to electromagnetic rays. These methods can improve the durability of foods with respect to extension of shelf-life [7]. The food products prepared by using ␥-irradiation have been established to be both nutritionally adequate and safe for human consumption [7,8]. Apart from food industries, starches modified by ␥-irradiation at higher doses are applicable in industries like textile and paper [9]. The free radicals formed by the process of irradiation are involved in the formation of smaller fragments from large granules of starch by breakdown of glycosidic bonds [10,11]. By ␥-irradiation, the resulting smaller fragments from starch molecules showed low molecular weight and pasting viscosity; increase in solubility and acidity [10,12]. Amorphophallus paeoniifolius, popularly known as elephant foot yam in the Indian traditional system and it is herbaceous, perennial, C3 crop, classified under the family Araceae [13]. It is a tropical tuber that primarily originated in the South-East Asian region and is extensively used in Indian cuisines. Elephant foot yam tubers are good sources of carbohydrates, especially starch [14], and present functional properties as food thickeners, though yet to be exploited commercially. Only a few studies have focused on the irradiation of elephant foot yam starch in order to find new food
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applications; hence the present study was undertaken to evaluate the effect of ␥-irradiation on molecular structure and physico-chemical properties of starch from A. paeoniifolius. The modifications in starch properties were identified that would be essential for some applications in the food and non-food such as paper and textile industries. 2. Materials and methods 2.1. Materials The tuber of elephant foot yam (A. paeoniifolius) was purchased from a local market in the Union Territory of Pondicherry, India. The preparation and the method for isolation of starch from elephant foot yam are as described in Reddy et al. [14]. 2.2. Gamma-irradiation of starch The starch samples (150 g/bag) at a moisture content of 10.93% were packed in a polyethylene bag and subjected to irradiation treatment using 60 Co as ␥-source at room temperature (25 ± 1.0 ◦ C). The doses of radiation were controlled at 5, 10, 15, 20 and 25 kGy with a dose rate of 2 kGy/h. To confirm that elephant foot yam starch samples was received the exact dose was measured using ceric-cerous dosimeter and it is placed in the gamma chamber along with the samples. The sample without irradiation treatment were considered as control (native). The irradiation process was done at Central Instrumentation Facility, Pondicherry University, Puducherry, India. 2.3. Physico-chemical properties 2.3.1. Moisture, colour and amylose content The amount of moisture in irradiated starches was calculated in terms of weight loss after heating at 130 ± 2 ◦ C for 2 h using a sample of 2 g. Colour of the native and irradiated starches were analysed using colour scale CIF lab (Hunter Lab Associates Inc.) including the parameters like L* which represents the lightness from 0 (black) to 100 (white); a* and b* represents redness (+a) to greenness (−a) and yellowness (+b) to blueness (−b), respectively; and amylose content of native and irradiated starches was determined using the method of Williams et al. [15]. 2.3.2. Carboxyl content and pH The carboxyl content in irradiated starches was analysed using the procedure described by Chattopadhyay et al. [16] and the pH value of the irradiated starches (40 g/100 mL distilled water) was determined using a digital pH meter calibrated at 25 ◦ C. 2.3.3. Swelling power, water absorption capacity and solubility index The water absorption capacity (WAC), water solubility index (WSI) and swelling power (SP) of native and irradiated starches was analysed respectively, according to the procedure described by Reddy et al. [17]. The WSI and WAC were determined as: WSI = (weight of dry solids in supernatant/weight of dry sample) × 100; WAC = weight of wet sediment/(weight of the dry sample-weight of the dry solids. SP = weight of sediment/weight of dry starch. 2.3.4. Light transmittance Light transmittance of native and irradiated starches was determined using the method described by Wani et al. [18]. Starch suspension (1% w/w dwb) was prepared by heating at 90 ◦ C in a shaking water bath for 30 min and the starch suspension was brought to room temperature. The samples were stored for 120 h
at 4 ◦ C in a refrigerator and the transmittance was analyzed every 24 h by measuring absorbance at 640 nm against a water blank with UV–Visible Spectrophotometer (UV-1800, Shimadzu, North America). 2.3.5. Syneresis Syneresis of native and irradiated starches was determined using the method described by Wani et al. [18]. Starch suspensions (2% w/w dwb) were heated at 90 ◦ C for 30 min in a shaking water bath and the starch samples were stored for 120 h at 4 ◦ C. Syneresis was calculated as percentage of water released after centrifugation at 3000 × g for 10 min. Syneresis (g/100 g) = (Weight of water released/Weight of gel) × 100. 2.4. Pasting properties Pasting properties of native and irradiated starches was determined using the method described by Reddy et al. [19]. Pasting properties of starches were determined using a Rapid ViscoAnalyzer (RVA starch master 2, Newport Scientific, Warriewood, NSW, Australia) using 3.5 g of sample dispersed in 25 mL of water. Then it was heated at 50 ◦ C with continuous stirring for 10 s and held for 1 min. The temperature was elevated to 95 ◦ C (heating rate 6 ◦ C/min) for 7.3 min and held for 5 min and finally cooled to 50 ◦ C (Cooling rate 6 ◦ C/min). The following parameters including pasting temperature, peak viscosity; breakdown viscosity, final viscosity and setback viscosity were examined from viscoamylographs. 2.5. Morphological characteristics The morphological characteristics native and irradiated starches were evaluated using scanning electron microscope (HITACHI Model S-3000H). The powdered samples were sprinkled on a double-sided sticky tape placed on aluminium stubs and covered with carbon coating layer. 2.6. X-ray diffraction The X-ray patterns of native and irradiated starches was analysed with an X-ray diffractometer (Shimadzu XRD 7000) with Cu K␣ value of 1.54060 radiation at a speed of 2◦ /min, diffraction angle of 2 at 4◦ and 50◦ at 40 kV and 30 mA. The total area under the curve and the area under each prominent peak was measured using OriginPro software and the percentage crystallinity was assessed by using the following formula: Crystallinity (%) = (Area under peaks/Total area) × 100. 2.7. Fourier transform infrared (FTIR) spectroscopy The FTIR spectra of native and irradiated starches was recorded on an FTIR Spectrophotometer (Thermo Nicolet Model: 6700, UK) at room temperature. The starch samples was mixed with KBr and made into pellets before measurement. Standardization of instrument was carried out using KBr pellet as blank and the spectra were recorded within the range of 500–4000 cm−1 . 2.8. Thermal analysis Thermal properties of native and irradiated starches was determined using the method described by Reddy et al. [19]. The thermal properties of starches were analyzed using Differential Scanning Colourimetry (TA-Q20 DSC). Water (14 L) was added with a micro syringe to starch samples (6 mg dwb) in the DSC pans, which were then sealed, reweighed and kept at room temperature for 24 h to ensure equilibration of the samples and water. The samples were scanned from 35 ◦ C to 150 ◦ C, at heating rate of 10 ◦ C/min and an
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empty pan was used a reference. The values of onset (To ), peak (Tp ) and final (Tf ) gelatinization temperatures and enthalpy (H) were obtained from the thermographs of the samples using Universal Analysis 2000 3.9A software.
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3.1.2. Carboxyl content and pH of starches The levels of carboxyl content and pH values for native and irradiated starches is shown in Table 1. The amount of carboxyl groups ranged from 0.27 to 2.56 (5 kGy to 25 kGy) which showed significant (p ≤ 0.05) difference among the irradiation doses. The starch was degraded by gamma irradiation causes the generation and transformation of free radicals and follows the low-molecular products with the number of carboxylic acids and aldehydes [25]. This variation in the carboxyl content in irradiated starches could be due to the formation of acids like formic, acetic, pyruvic and glucuronic acids by the action of free radicals [26] and these results were comparable with other studies [8,24]. The pH value significantly differed in irradiated starches than native (0 kGy) with increased dose of gamma irradiation. Lowest value of pH (3.39) was observed in 25 kGy irradiated starch and highest for native starch (6.14) and the pH values ranged from 3.39 to 6.14; and pH was decreased by increased dose of gamma irradiation. The decrease in pH could be due to breakdown in glycosidic linkages in the starch by the action of free radical generated by irradiation.
2.9. Statistical analysis All analyses were carried out in triplicates. The data was subjected to one way ANOVA to analyze the significant difference in all data and Duncan’s Multiple Range Test (DMRT) (p ≤ 0.05) to analyze the significant difference between mean values of samples using SPSS 18 software (SPSS Institute Inc., Cary, NC, USA).
3. Results and discussion 3.1. Physico-chemical properties 3.1.1. Effect of -irradiation on colour and moisture content of starches Colour parameters including L* (lightness to darkness), a* (redness to greenness) and b* (yellowness to blueness) values of native (0 kGy) and irradiated elephant foot yam starches are represented in Table 1. The lightness (L*) value ranged from 93.5 to 96.4, which showed significant difference among native and irradiated starches. a* value of irradiated starches differed from native starch with highest redness (a*) value, (0.31) being observed in 5 kGy starch and then decreased; these results are comparable with previous reports for irradiation starches from sago [20] and horse chestnut [21]. The yellowness (b*) value significantly differed from native to irradiated starches with increased dose of radiation and the values ranged from 1.07 (0 kGy) to 4.81 (25 kGy). The increase in b* values with increase in irradiation doses has also been reported for sago [20], corn [22] and arrowhead [23] starches. The increase in yellowness value may be due to the caramelization that would have occurred in monosaccharides; these monosaccharides are derived from starch polysaccharides by gamma irradiation. Moisture content of native and irradiated starches is represented in Table 1. The amount of moisture was significantly decreased from 10.93% (0 kGy) to 7.59% (25 kGy). From the results, a declining trend was observed in moisture content of irradiated starches with increased dosage of ␥-irradiation. The decrease in the moisture content of irradiated starch might be due to the penetration of radiation energy by the gamma rays which caused the loss of moisture from the irradiated starch [24]. The decrease in the moisture content may improve the shelf life of starch by preventing the invasion of microorganisms.
3.1.3. Amylose content of starches Decrease in amylose content of irradiated elephant foot yam starches were observed with increase doses of ␥-irradiation and the results are represented in Table 1. These values ranged from 28.3% at native (0 kGy) to 19.3% at 25 kGy and the lowest amylose content was noted in irradiated (25 kGy) starch. This could be due to depolymerisation of starch granules by ␥-irradiation and decreased binding capacity with iodine molecules; these results are comparable with gamma irradiation of starches from arrowhead, potato and beans [7,23]. Yu and Wang [10] proposed that the decrease of amylose content initiated from the breakage or cleavage of partly branched long chains in amylopectin during gamma irradiation.
3.1.4. Water absorption capacity and water solubility index of starches The values of WAC and WSI of native (0 kGy) and irradiated starches from elephant foot yam is represented in Table 1. The WAC of native and irradiated starches ranged from 2.31% to 3.51%; and observed lowest WAC in native (2.31%) and highest (3.51%) in 25 kGy of irradiated starch. This increased WAC of ␥irradiated starches could be either due to the interruption of starch molecules by the formation of smaller molecules like dextrin and monosaccharides, which have more affinity with water molecules when compared to starch polymers [12]. Previous studies also report increase in WAC of starch of arrowhead [23] by gamma irradiation.
Table 1 Moisture, colour parameters, carboxyl content, pH, amylose, water absorption capacity, water solubility index and relative crystallinity of ␥-irradiated elephant foot yam starches. Parameter
Dose of irradiation 0 kGy
Moisture (%) L* a* b* Carboxyl pH Amylose (%) WAC (%) WSI (%) RC (%)
10.93 ± 95.5 ± 0.16 ± 1.07 ± 0.0f 6.14 ± 28.3 ± 2.31 ± 1.98 ± 19.37 ±
5 kGy 0.1a 0.9d 0.03d 0.01e 0.02a 0.3a 0.5d 0.1f 0.1a
10.02 95.7 0.31 2.59 0.27 4.57 26.23 2.63 2.12 19.32
10 kGy ± ± ± ± ± ± ± ± ± ±
0.2b 1.2c 0.04a 0.05d 0.01e 0.02b 0.4b 0.4cd 0.2e 0.2a
9.12 96.4 0.06 3.38 0.68 4.06 24.32 2.87 2.42 19.12
± ± ± ± ± ± ± ± ± ±
15 kGy 0.1c 1.1a 0.01e 0.04c 0.02d 0.02c 0.4c 0.6bc 0.1d 0.1a
8.91 95.2 0.18 4.3 1.29 3.67 21.98 3.23 2.78 18.01
± ± ± ± ± ± ± ± ± ±
20 kGy 0.2d 1.1e 0.02c 0.03b 0.03c 0.02d 0.3d 0.5ab 0.2c 0.2a
8.09 93.5 0.22 4.28 2.1 3.5 20.56 3.34 3.52 18.09
± ± ± ± ± ± ± ± ± ±
25 kGy 0.2e 0.8f 0.01b 0.01b 0.03b 0.02de 0.6e 0.4a 0.2b 0.2a
7.59 95.8 0.16 4.81 2.56 3.39 19.31 3.51 3.85 18.19
± ± ± ± ± ± ± ± ± ±
0.1f 1.4b 0.01d 0.05a 0.03a 0.03e 0.8f 0.3a 0.1a 0.1a
L*, lightness to darkness; a*, redness to greenness; b*, blueness to yellowness; WAC, water absorption capacity; WSI, water solubility index; RC, relative crystallinity. All data were means of triplicates. Values with the same superscripts in a row did not differ significantly (p ≤ 0.05) by DMRT.
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Fig. 1. Swelling power of ␥-irradiated elephant foot yam starches.
The WSI of starch was increased significantly with increased dose of gamma irradiation and these values ranged from 1.98% to 3.51%. Highest WSI was observed in 25 kGy of irradiated starch. The elevation of solubility index may be due to the depolymerisation of starch granules [25] and increased levels of small molecules like monosaccharaides upon ␥-irradiation [26], these resulted molecules have higher affinity for hydration when compared with native starch molecules. The ␥-irradiation process decreases the inter-chain hydrogen bonds in starch molecules and increases the hydrogen bonds with water, which enhanced the solubility of starch treated by ␥-irradiation [27] and these results were similar with other studies [12,26]. 3.1.5. Swelling power of starches The SP of native (0 kGy) and irradiated starches from elephant foot yam is represented in Fig. 1. Swelling power indicates the ability of starch to trap and hold water within its structure [28]. The SP of native and irradiated starches was evaluated between 50 ◦ C to 90 ◦ C with 10 ◦ C intervals. The results suggested that the SP of irradiated starches decreased with increased dose of irradiation when compared with native starch. At initial temperature (50 ◦ C), there was not much difference in the swelling power of native and irradiated starches, and the values ranged from 2.38% to 2.61% (native starch). When the temperature was raised up to 90 ◦ C, the SP of native starch (2.61–8.16%) significantly differed from irradiated starches and highest swelling power was found in native starch (8.16%) at 90 ◦ C; but in irradiated starches even when the temperature was increased, there was a reduction in the swelling power. At 90 ◦ C, lowest swelling power value was noted in 25 kGy irradiated starch (3.24%). Reduced swelling power in starch molecules may have occurred by depolymerisation of starch by ␥-irradiation. Subsequently amylopectin percentage is predominantly responsible for swelling [29] and this decreased swelling power ability may be interrelated to high reduction of amylopectin molecules with ␥-irradiation. Similar results were reported for other starches by gamma irradiation [20,30]. 3.1.6. Syneresis of starches The syneresis properties of native and irradiated starches are represented in Fig. 2. Stable decrease in syneresis was found with increased dose of ␥-irradiation in elephant foot yam starch, and the storage for 120 h at 4 ◦ C showed an increase in syneresis. Native starch exhibited maximum syneresis after 24 h of storage (86.6–89.3%) when compared with all other irradiated starches. Lowest syneresis value was found in 25 kGy irradiated starch from 24 h to 120 h; the values ranged from 71.5% to 78.8% at 120 h storage period. This could be due to the decreased amylose content by
Fig. 2. Syneresis of ␥-irradiated elephant foot yam starches.
irradiation and affinity of water molecules with amylopectin molecules [7]. 3.1.7. Light transmittance of starches The light transmittance properties of native and irradiated starches are represented in Fig. 3. Transmittance is the fraction of incident light at a specified wavelength when passed through a sample. A decreased light transmittance at 640 nm was observed in native starch and irradiated starches when kept for 72 h storage at 4 ◦ C, and which stayed almost constant up to 120 h. The results exhibited reduced levels of branched molecules, disintegration of starch molecules and amylose/amylopectin chain lengths after irradiation. The reduction in transmittance of starch paste was a consequence of retrogradation of starch molecules which rises with storage time. The reorganization of amylose molecule form aggregates that decreases light transmittance of starch pastes [31]. 3.2. Pasting properties of starches The pasting properties of native and irradiated starches are shown in Table 2. The pasting properties like peak, holding, final, set back and break down viscosities considerably decreased with increased dose of irradiation. The peak viscosity was highest for native starch (1985 cP) and there was a steady decrease from 792 cP to 99 cP with increasing dose of radiation from 5 kGy to 25 kGy. Peak viscosity mainly explains the swelling of starch granules [32]; it correlates with the reduction of swelling power due to the disintegration of starch granules, lessening in integrity and rigidity of starch granules due to breakdown of glycosidic bonds by ␥irradiation [7]. The steep fall in the hold viscosity was noted from 2374 cP (native starch) to 16 cP (25 kGy) with increasing irradiation. The setback and final viscosities are primarily due to a re-ordering
Fig. 3. Light transmittance of ␥-irradiated elephant foot yam starches.
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Table 2 Pasting properties of ␥-irradiated elephant foot yam starches. Parameter
Dose of irradiation 0 kGy
Pasting properties Pasting temp (◦ C) Pasting point (cP) Peak viscosity (cP) Hold viscosity (cP) Final viscosity (cP) Break down (cP) Set back (cP)
85.5 17 1985 1374 2148 610 745
5 kGy
± ± ± ± ± ± ±
1.0ab 1b 10a 15a 12a 6b 8a
84.4 17 792 183 268 616 84
± ± ± ± ± ± ±
10 kGy 0.5c 1b 5b 4b 6b 4a 2b
86.3 17 266 35 53 231 18
± ± ± ± ± ± ±
0.4a 1b 4c 3c 5c 3c 2c
15 kGy
20 kGy
85.2 ± 0.6bc 16 ± 1b 192 ± 4d 21 ± 2d 34 ± 3d 171 ± 3d 12 ± 1d
85.8 20 112 17 26 94 8
± ± ± ± ± ± ±
25 kGy 0.4ab 1a 2e 1e 3e 4e 1e
85.2 19 99 16 21 83 6
± ± ± ± ± ± ±
0.2bc 1a 3f 1e 2f 3f 1f
All data were means of triplicates. Values with the same superscripts in a row did not differ significantly (p ≤ 0.05) by DMRT.
or polymerization of amylose and long linear amylopectin. Final viscosity and set back was decreased from 2148 cP and 745 cP to 21 cP and 6 cP respectively with increased dose of irradiation, which could be due to the reordering or polymerization of leaked amylose, and long linear amylopectin undergo depolymerisation [20] leading to a significant decrease in the setback and final viscosities. Lowest break down viscosity (83 cP) was found in 25 kGy irradiated starch and highest in native starch (610 cP), which could be attributed to breaking of starch granules. These results were comparable with other starches treated with gamma irradiation [7,30]. 3.3. Morphological characteristics Morphological characteristics of native and irradiated starches using scanning electron microscopy are represented in Fig. 4. SEM studies have revealed that the elephant foot yam starch (native) granules are round, elliptical and polygonal in shape with smooth surfaces, and no obvious effects or signs of damage on the surface. After ␥-irradiation, the starch granules were not visually differed from the non-irradiated starch granules and it was recommended that ␥-irradiation process retained the original shapes and sizes of starch granules. Moreover, there was no granular cracking or roughness on the surface of starch granules caused by ␥-irradiation.
Similar results were reported for irradiation of cowpea [30] and maize [27] starch granules. From the microscopic observation, that ␥-irradiation damage to elephant foot yam starch granules might exist only in the form of changes to the structure of the starch molecules.
3.4. X-ray diffraction of starches The XRD patterns of native and irradiated starches are represented in Fig. 5. All irradiated starches exhibited C-type pattern (mixture of A and B-type starch pattern) with intermediate peaks of 2 at 16.92◦ and 18.12◦ , strong peaks of 2 at 23.05◦ and weak peaks of 2 at 14.7◦ when compared with native (0 kGy) starch (B-type pattern). The irradiated elephant foot yam starch exhibited change in diffraction pattern compared to native starch. These diffraction patterns of starch was differed from other reports for starches by gamma irradiation [7]. The relative crystallinity (RC) of elephant foot yam starch decreased from 19.37% (non-irradiated starch) to 18.19% (25 kGy) which may be due to decreased amylopectin content following irradiation. The decreased relative crystalline were also observed in irradiation of potato and bean starches [7,33]. RC mainly affected by length of amylopectin chains, orientation of double helices in crystallite [34].
Fig. 4. Scanning electron micrographs (SEM) of ␥-irradiated elephant foot yam starches.
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Table 3 Thermal properties of ␥-irradiated elephant foot yam starches. Parameter
Dose of irradiation 0 kGy
Thermal properties T0 (◦ C) TP (◦ C) TC (◦ C) H gel (J/g) PHI (J/g C) R (◦ C)
61.41 68.75 84.28 15.28 2.06 22.02
5 kGy ± ± ± ± ± ±
0.02a 0.1c 0.06b 0.04c 0.01a 0.12c
60.33 74.74 84.18 15.47 1.07 23.8
10 kGy ± ± ± ± ± ±
0.07bc 0.5a 0.05b 0.02a 0.01c 0.02ab
60.08 72.8 84.46 15.19 1.46 24.23
± ± ± ± ± ±
15 kGy 0.06cd 0.1b 0.06a 0.01d 0.05b 0.06a
59.86 69.06 83.5 15.09 1.55 24.07
± ± ± ± ± ±
20 kGy 0.05d 0.05c 0.08d 0.01e 0.02b 0.01a
60.17 67.49 83.61 14.93 2.04 23.33
± ± ± ± ± ±
25 kGy 0.03bc 0.05d 0.04c 0.04f 0.02a 0.06b
60.49 72.68 83.71 15.4 1.24 23.2
± ± ± ± ± ±
0.04b 0.1b 0.05c 0.01b 0.01bc 0.01b
T0 , onset temperature; TP , peak temperature; TC , conclusion temperature; Hgel , enthalpy of gelatinization; PHI, peak height index (( gel/(TP − T0 )); R, gelatinization range (TC − T0 ). All data were means of triplicates. Values with the same superscripts in a row did not differ significantly (p ≤ 0.05) by DMRT.
of gamma-irradiation might be cause destruction of crystalline regions and amorphous regions of starch granules. 3.6. Thermal properties of starches
Fig. 5. XRD patterns of ␥-irradiated elephant foot yam starches.
Thermal properties of native and irradiated starches are represented in Table 3. The gelatinization temperatures (To , Tp and Tc ) and gelatinization enthalpy of native starch was 61.4 ◦ C, 68.7 ◦ C, 84.2 ◦ C and 15.28 J/g, respectively. The reduction of gelatinization temperatures (To , Tp and Tc ) and enthalpy (H) initiated by ␥irradiation did not significantly differ from native to 25 kGy, which is similar with other irradiation studies [27,35]. The starches with longer amylopectin and greater crystallinity exhibited higher gelatinization temperature and enthalpy [36,37]. In addition, Hoover and Manuel [28] informed that the reduction in enthalpy value was principally due to the interruption of the double helices rather than longer range distraction of crystallinity.
3.5. FTIR spectroscopy
4. Conclusion
The FTIR spectra of native and irradiated starches are represented in Fig. 6. Irradiated elephant foot yam starches were subjected to FTIR analysis to recognize the interruption of glycosidic bonds and reduced short-range crystalline order. After ␥-irradiation, the spectra of ␥-irradiated starches when compared, revealed that no new functional groups were added by ␥-irradiation process. The increased intensities of the band at 1155 cm−1 caused by ␥-irradiation (10 kGy and 15 kGy) exhibited alteration in the ordered structure of irradiated starches. It demonstrated that the low levels of ␥-irradiation do not make changes in the crystalline structure of starch molecules. From the crystalline structure determined by X-ray diffraction and FTIR spectroscopy, these results showed that high doses
The present research reveals that the properties of elephant foot yam starches are altered and significantly affected by ␥-irradiation. Irradiation treatment of elephant foot yam starch resulted in reduced pasting properties and changes in FTIR spectrum with increased dose of ␥-irradiation. Amylose content, pH, swelling power and syneresis values significantly decreased; carboxyl content, solubility, light transmittance and water absorption capacity increased upon increased dosage of irradiation on starches. There was significant difference between the XRD pattern of native and irradiated starches, only showing slight decreased intensity of peaks in irradiated starches. Irradiation process increased gelatinization temperatures and enthalpy of elephant foot yam starch. However modification of elephant foot yam starch by ␥-irradiation is a rapid and non-heat process compared to other modification processes and also these irradiated starch products can be used in food industries due to decreased syneresis and other functional properties instead of native starch. Acknowledgements The authors are grateful to the help rendered by technologists and technicians of gamma chamber and other necessary instruments at the Central Instrumentation Facility, Pondicherry Central University, Puducherry. This project is financially supported by the Department of Food Science and Technology, Pondicherry Central University, India. References
Fig. 6. FTIR spectrum of ␥-irradiated elephant foot yam starches.
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Effect of ␥-irradiation on structure and physico-chemical properties of Amorphophallus paeoniifolius starch Chagam Koteswara Reddy, M. Suriya, P.V. Vidya, K. Vijina, Sundaramoorthy Haripriya ∗ Department of Food Science and Technology, Pondicherry Central University, Puducherry 605014, India
a r t i c l e
i n f o
Article history: Received 9 March 2015 Received in revised form 24 April 2015 Accepted 5 May 2015 Available online 12 May 2015 Keywords: Amorphophallus paeoniifolius Starch ␥-Irradiation Viscosity Crystallinity Thermal properties
a b s t r a c t Gamma irradiation is one of the effective techniques able to alter structure and its properties of starch. In this research, the effect of modification in terms of molecular structure and physico-chemical properties of Amorphophallus paeoniifolius starch by ␥-irradiation using 60 Co as ␥-source at doses of 5, 10, 15, 20 and 25 kGy with dose rate 2 kGy/h was studied. Morphology of native and irradiated starches under scanning electron microscope revealed that granules are round, elliptical and polygonal in shape with smooth surfaces; no cracking or roughness was noted on irradiated starches. Amylose content, pH, swelling power of the starches and syneresis of the gelatinized starch were significantly decreased by irradiation; while carboxyl content, solubility, light transmittance and water absorption capacity of the starch granules were raised with increased dose of irradiation. Reduced pasting parameters and changes in FTIR spectrum significantly differed from each other. XRD pattern of irradiated starches showed C-type pattern with intermediate peaks of 2 at 16.92◦ and 18.12◦ , strong peaks of 2 at 23.05◦ and weak peaks of 2 at 14.7◦ , displaying slight decreased in the intensity of peaks in irradiated starches. Irradiation of starches increased gelatinization temperatures and enthalpy value was measured using DSC. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In food processing industries, starch contributes for a variety of vital characteristics that include thickening, gelling, consistency and shelf stability in varied applications [1]. Starch isolated from potato, rice, corn and wheat has been exclusively used to confer structure, texture, consistency and appeal to many food and non-food products [2]. Native starch from different sources have limitations such as low thermal resistance, low shear resistance, low solubility, high viscosity and tendency towards retrogradation leading to staling and syneresis, which may adversely affect the quality of food products. In these conditions, the importance of starch modification demands attention and the approaches could be physical, chemical, or enzymatic methods. The chemical modification methods such as acetylation, oxidation [3] and cross linking [4] are widely applied when paralleled with starch modification by physical methods [5] in food industries. Among physical treatments, irradiation is well accepted since there is no significant raise in temperature, requiring nominal sample preparation, rapidity in process and non-reliance on any type of catalysts [6]. The
∗ Corresponding author. Tel.: +91 9443701906; fax: +91 413 2654621. E-mail address: [email protected] (S. Haripriya). http://dx.doi.org/10.1016/j.ijbiomac.2015.05.003 0141-8130/© 2015 Elsevier B.V. All rights reserved.
modification of starch by irradiation technique is an impending development and the commonly used radiation involves ␥-rays. Irradiation involves the use of ␥-rays, an ionic, non-heat physical treatment, in which, food products are directly exposed to electromagnetic rays. These methods can improve the durability of foods with respect to extension of shelf-life [7]. The food products prepared by using ␥-irradiation have been established to be both nutritionally adequate and safe for human consumption [7,8]. Apart from food industries, starches modified by ␥-irradiation at higher doses are applicable in industries like textile and paper [9]. The free radicals formed by the process of irradiation are involved in the formation of smaller fragments from large granules of starch by breakdown of glycosidic bonds [10,11]. By ␥-irradiation, the resulting smaller fragments from starch molecules showed low molecular weight and pasting viscosity; increase in solubility and acidity [10,12]. Amorphophallus paeoniifolius, popularly known as elephant foot yam in the Indian traditional system and it is herbaceous, perennial, C3 crop, classified under the family Araceae [13]. It is a tropical tuber that primarily originated in the South-East Asian region and is extensively used in Indian cuisines. Elephant foot yam tubers are good sources of carbohydrates, especially starch [14], and present functional properties as food thickeners, though yet to be exploited commercially. Only a few studies have focused on the irradiation of elephant foot yam starch in order to find new food
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applications; hence the present study was undertaken to evaluate the effect of ␥-irradiation on molecular structure and physico-chemical properties of starch from A. paeoniifolius. The modifications in starch properties were identified that would be essential for some applications in the food and non-food such as paper and textile industries. 2. Materials and methods 2.1. Materials The tuber of elephant foot yam (A. paeoniifolius) was purchased from a local market in the Union Territory of Pondicherry, India. The preparation and the method for isolation of starch from elephant foot yam are as described in Reddy et al. [14]. 2.2. Gamma-irradiation of starch The starch samples (150 g/bag) at a moisture content of 10.93% were packed in a polyethylene bag and subjected to irradiation treatment using 60 Co as ␥-source at room temperature (25 ± 1.0 ◦ C). The doses of radiation were controlled at 5, 10, 15, 20 and 25 kGy with a dose rate of 2 kGy/h. To confirm that elephant foot yam starch samples was received the exact dose was measured using ceric-cerous dosimeter and it is placed in the gamma chamber along with the samples. The sample without irradiation treatment were considered as control (native). The irradiation process was done at Central Instrumentation Facility, Pondicherry University, Puducherry, India. 2.3. Physico-chemical properties 2.3.1. Moisture, colour and amylose content The amount of moisture in irradiated starches was calculated in terms of weight loss after heating at 130 ± 2 ◦ C for 2 h using a sample of 2 g. Colour of the native and irradiated starches were analysed using colour scale CIF lab (Hunter Lab Associates Inc.) including the parameters like L* which represents the lightness from 0 (black) to 100 (white); a* and b* represents redness (+a) to greenness (−a) and yellowness (+b) to blueness (−b), respectively; and amylose content of native and irradiated starches was determined using the method of Williams et al. [15]. 2.3.2. Carboxyl content and pH The carboxyl content in irradiated starches was analysed using the procedure described by Chattopadhyay et al. [16] and the pH value of the irradiated starches (40 g/100 mL distilled water) was determined using a digital pH meter calibrated at 25 ◦ C. 2.3.3. Swelling power, water absorption capacity and solubility index The water absorption capacity (WAC), water solubility index (WSI) and swelling power (SP) of native and irradiated starches was analysed respectively, according to the procedure described by Reddy et al. [17]. The WSI and WAC were determined as: WSI = (weight of dry solids in supernatant/weight of dry sample) × 100; WAC = weight of wet sediment/(weight of the dry sample-weight of the dry solids. SP = weight of sediment/weight of dry starch. 2.3.4. Light transmittance Light transmittance of native and irradiated starches was determined using the method described by Wani et al. [18]. Starch suspension (1% w/w dwb) was prepared by heating at 90 ◦ C in a shaking water bath for 30 min and the starch suspension was brought to room temperature. The samples were stored for 120 h
at 4 ◦ C in a refrigerator and the transmittance was analyzed every 24 h by measuring absorbance at 640 nm against a water blank with UV–Visible Spectrophotometer (UV-1800, Shimadzu, North America). 2.3.5. Syneresis Syneresis of native and irradiated starches was determined using the method described by Wani et al. [18]. Starch suspensions (2% w/w dwb) were heated at 90 ◦ C for 30 min in a shaking water bath and the starch samples were stored for 120 h at 4 ◦ C. Syneresis was calculated as percentage of water released after centrifugation at 3000 × g for 10 min. Syneresis (g/100 g) = (Weight of water released/Weight of gel) × 100. 2.4. Pasting properties Pasting properties of native and irradiated starches was determined using the method described by Reddy et al. [19]. Pasting properties of starches were determined using a Rapid ViscoAnalyzer (RVA starch master 2, Newport Scientific, Warriewood, NSW, Australia) using 3.5 g of sample dispersed in 25 mL of water. Then it was heated at 50 ◦ C with continuous stirring for 10 s and held for 1 min. The temperature was elevated to 95 ◦ C (heating rate 6 ◦ C/min) for 7.3 min and held for 5 min and finally cooled to 50 ◦ C (Cooling rate 6 ◦ C/min). The following parameters including pasting temperature, peak viscosity; breakdown viscosity, final viscosity and setback viscosity were examined from viscoamylographs. 2.5. Morphological characteristics The morphological characteristics native and irradiated starches were evaluated using scanning electron microscope (HITACHI Model S-3000H). The powdered samples were sprinkled on a double-sided sticky tape placed on aluminium stubs and covered with carbon coating layer. 2.6. X-ray diffraction The X-ray patterns of native and irradiated starches was analysed with an X-ray diffractometer (Shimadzu XRD 7000) with Cu K␣ value of 1.54060 radiation at a speed of 2◦ /min, diffraction angle of 2 at 4◦ and 50◦ at 40 kV and 30 mA. The total area under the curve and the area under each prominent peak was measured using OriginPro software and the percentage crystallinity was assessed by using the following formula: Crystallinity (%) = (Area under peaks/Total area) × 100. 2.7. Fourier transform infrared (FTIR) spectroscopy The FTIR spectra of native and irradiated starches was recorded on an FTIR Spectrophotometer (Thermo Nicolet Model: 6700, UK) at room temperature. The starch samples was mixed with KBr and made into pellets before measurement. Standardization of instrument was carried out using KBr pellet as blank and the spectra were recorded within the range of 500–4000 cm−1 . 2.8. Thermal analysis Thermal properties of native and irradiated starches was determined using the method described by Reddy et al. [19]. The thermal properties of starches were analyzed using Differential Scanning Colourimetry (TA-Q20 DSC). Water (14 L) was added with a micro syringe to starch samples (6 mg dwb) in the DSC pans, which were then sealed, reweighed and kept at room temperature for 24 h to ensure equilibration of the samples and water. The samples were scanned from 35 ◦ C to 150 ◦ C, at heating rate of 10 ◦ C/min and an
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empty pan was used a reference. The values of onset (To ), peak (Tp ) and final (Tf ) gelatinization temperatures and enthalpy (H) were obtained from the thermographs of the samples using Universal Analysis 2000 3.9A software.
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3.1.2. Carboxyl content and pH of starches The levels of carboxyl content and pH values for native and irradiated starches is shown in Table 1. The amount of carboxyl groups ranged from 0.27 to 2.56 (5 kGy to 25 kGy) which showed significant (p ≤ 0.05) difference among the irradiation doses. The starch was degraded by gamma irradiation causes the generation and transformation of free radicals and follows the low-molecular products with the number of carboxylic acids and aldehydes [25]. This variation in the carboxyl content in irradiated starches could be due to the formation of acids like formic, acetic, pyruvic and glucuronic acids by the action of free radicals [26] and these results were comparable with other studies [8,24]. The pH value significantly differed in irradiated starches than native (0 kGy) with increased dose of gamma irradiation. Lowest value of pH (3.39) was observed in 25 kGy irradiated starch and highest for native starch (6.14) and the pH values ranged from 3.39 to 6.14; and pH was decreased by increased dose of gamma irradiation. The decrease in pH could be due to breakdown in glycosidic linkages in the starch by the action of free radical generated by irradiation.
2.9. Statistical analysis All analyses were carried out in triplicates. The data was subjected to one way ANOVA to analyze the significant difference in all data and Duncan’s Multiple Range Test (DMRT) (p ≤ 0.05) to analyze the significant difference between mean values of samples using SPSS 18 software (SPSS Institute Inc., Cary, NC, USA).
3. Results and discussion 3.1. Physico-chemical properties 3.1.1. Effect of -irradiation on colour and moisture content of starches Colour parameters including L* (lightness to darkness), a* (redness to greenness) and b* (yellowness to blueness) values of native (0 kGy) and irradiated elephant foot yam starches are represented in Table 1. The lightness (L*) value ranged from 93.5 to 96.4, which showed significant difference among native and irradiated starches. a* value of irradiated starches differed from native starch with highest redness (a*) value, (0.31) being observed in 5 kGy starch and then decreased; these results are comparable with previous reports for irradiation starches from sago [20] and horse chestnut [21]. The yellowness (b*) value significantly differed from native to irradiated starches with increased dose of radiation and the values ranged from 1.07 (0 kGy) to 4.81 (25 kGy). The increase in b* values with increase in irradiation doses has also been reported for sago [20], corn [22] and arrowhead [23] starches. The increase in yellowness value may be due to the caramelization that would have occurred in monosaccharides; these monosaccharides are derived from starch polysaccharides by gamma irradiation. Moisture content of native and irradiated starches is represented in Table 1. The amount of moisture was significantly decreased from 10.93% (0 kGy) to 7.59% (25 kGy). From the results, a declining trend was observed in moisture content of irradiated starches with increased dosage of ␥-irradiation. The decrease in the moisture content of irradiated starch might be due to the penetration of radiation energy by the gamma rays which caused the loss of moisture from the irradiated starch [24]. The decrease in the moisture content may improve the shelf life of starch by preventing the invasion of microorganisms.
3.1.3. Amylose content of starches Decrease in amylose content of irradiated elephant foot yam starches were observed with increase doses of ␥-irradiation and the results are represented in Table 1. These values ranged from 28.3% at native (0 kGy) to 19.3% at 25 kGy and the lowest amylose content was noted in irradiated (25 kGy) starch. This could be due to depolymerisation of starch granules by ␥-irradiation and decreased binding capacity with iodine molecules; these results are comparable with gamma irradiation of starches from arrowhead, potato and beans [7,23]. Yu and Wang [10] proposed that the decrease of amylose content initiated from the breakage or cleavage of partly branched long chains in amylopectin during gamma irradiation.
3.1.4. Water absorption capacity and water solubility index of starches The values of WAC and WSI of native (0 kGy) and irradiated starches from elephant foot yam is represented in Table 1. The WAC of native and irradiated starches ranged from 2.31% to 3.51%; and observed lowest WAC in native (2.31%) and highest (3.51%) in 25 kGy of irradiated starch. This increased WAC of ␥irradiated starches could be either due to the interruption of starch molecules by the formation of smaller molecules like dextrin and monosaccharides, which have more affinity with water molecules when compared to starch polymers [12]. Previous studies also report increase in WAC of starch of arrowhead [23] by gamma irradiation.
Table 1 Moisture, colour parameters, carboxyl content, pH, amylose, water absorption capacity, water solubility index and relative crystallinity of ␥-irradiated elephant foot yam starches. Parameter
Dose of irradiation 0 kGy
Moisture (%) L* a* b* Carboxyl pH Amylose (%) WAC (%) WSI (%) RC (%)
10.93 ± 95.5 ± 0.16 ± 1.07 ± 0.0f 6.14 ± 28.3 ± 2.31 ± 1.98 ± 19.37 ±
5 kGy 0.1a 0.9d 0.03d 0.01e 0.02a 0.3a 0.5d 0.1f 0.1a
10.02 95.7 0.31 2.59 0.27 4.57 26.23 2.63 2.12 19.32
10 kGy ± ± ± ± ± ± ± ± ± ±
0.2b 1.2c 0.04a 0.05d 0.01e 0.02b 0.4b 0.4cd 0.2e 0.2a
9.12 96.4 0.06 3.38 0.68 4.06 24.32 2.87 2.42 19.12
± ± ± ± ± ± ± ± ± ±
15 kGy 0.1c 1.1a 0.01e 0.04c 0.02d 0.02c 0.4c 0.6bc 0.1d 0.1a
8.91 95.2 0.18 4.3 1.29 3.67 21.98 3.23 2.78 18.01
± ± ± ± ± ± ± ± ± ±
20 kGy 0.2d 1.1e 0.02c 0.03b 0.03c 0.02d 0.3d 0.5ab 0.2c 0.2a
8.09 93.5 0.22 4.28 2.1 3.5 20.56 3.34 3.52 18.09
± ± ± ± ± ± ± ± ± ±
25 kGy 0.2e 0.8f 0.01b 0.01b 0.03b 0.02de 0.6e 0.4a 0.2b 0.2a
7.59 95.8 0.16 4.81 2.56 3.39 19.31 3.51 3.85 18.19
± ± ± ± ± ± ± ± ± ±
0.1f 1.4b 0.01d 0.05a 0.03a 0.03e 0.8f 0.3a 0.1a 0.1a
L*, lightness to darkness; a*, redness to greenness; b*, blueness to yellowness; WAC, water absorption capacity; WSI, water solubility index; RC, relative crystallinity. All data were means of triplicates. Values with the same superscripts in a row did not differ significantly (p ≤ 0.05) by DMRT.
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Fig. 1. Swelling power of ␥-irradiated elephant foot yam starches.
The WSI of starch was increased significantly with increased dose of gamma irradiation and these values ranged from 1.98% to 3.51%. Highest WSI was observed in 25 kGy of irradiated starch. The elevation of solubility index may be due to the depolymerisation of starch granules [25] and increased levels of small molecules like monosaccharaides upon ␥-irradiation [26], these resulted molecules have higher affinity for hydration when compared with native starch molecules. The ␥-irradiation process decreases the inter-chain hydrogen bonds in starch molecules and increases the hydrogen bonds with water, which enhanced the solubility of starch treated by ␥-irradiation [27] and these results were similar with other studies [12,26]. 3.1.5. Swelling power of starches The SP of native (0 kGy) and irradiated starches from elephant foot yam is represented in Fig. 1. Swelling power indicates the ability of starch to trap and hold water within its structure [28]. The SP of native and irradiated starches was evaluated between 50 ◦ C to 90 ◦ C with 10 ◦ C intervals. The results suggested that the SP of irradiated starches decreased with increased dose of irradiation when compared with native starch. At initial temperature (50 ◦ C), there was not much difference in the swelling power of native and irradiated starches, and the values ranged from 2.38% to 2.61% (native starch). When the temperature was raised up to 90 ◦ C, the SP of native starch (2.61–8.16%) significantly differed from irradiated starches and highest swelling power was found in native starch (8.16%) at 90 ◦ C; but in irradiated starches even when the temperature was increased, there was a reduction in the swelling power. At 90 ◦ C, lowest swelling power value was noted in 25 kGy irradiated starch (3.24%). Reduced swelling power in starch molecules may have occurred by depolymerisation of starch by ␥-irradiation. Subsequently amylopectin percentage is predominantly responsible for swelling [29] and this decreased swelling power ability may be interrelated to high reduction of amylopectin molecules with ␥-irradiation. Similar results were reported for other starches by gamma irradiation [20,30]. 3.1.6. Syneresis of starches The syneresis properties of native and irradiated starches are represented in Fig. 2. Stable decrease in syneresis was found with increased dose of ␥-irradiation in elephant foot yam starch, and the storage for 120 h at 4 ◦ C showed an increase in syneresis. Native starch exhibited maximum syneresis after 24 h of storage (86.6–89.3%) when compared with all other irradiated starches. Lowest syneresis value was found in 25 kGy irradiated starch from 24 h to 120 h; the values ranged from 71.5% to 78.8% at 120 h storage period. This could be due to the decreased amylose content by
Fig. 2. Syneresis of ␥-irradiated elephant foot yam starches.
irradiation and affinity of water molecules with amylopectin molecules [7]. 3.1.7. Light transmittance of starches The light transmittance properties of native and irradiated starches are represented in Fig. 3. Transmittance is the fraction of incident light at a specified wavelength when passed through a sample. A decreased light transmittance at 640 nm was observed in native starch and irradiated starches when kept for 72 h storage at 4 ◦ C, and which stayed almost constant up to 120 h. The results exhibited reduced levels of branched molecules, disintegration of starch molecules and amylose/amylopectin chain lengths after irradiation. The reduction in transmittance of starch paste was a consequence of retrogradation of starch molecules which rises with storage time. The reorganization of amylose molecule form aggregates that decreases light transmittance of starch pastes [31]. 3.2. Pasting properties of starches The pasting properties of native and irradiated starches are shown in Table 2. The pasting properties like peak, holding, final, set back and break down viscosities considerably decreased with increased dose of irradiation. The peak viscosity was highest for native starch (1985 cP) and there was a steady decrease from 792 cP to 99 cP with increasing dose of radiation from 5 kGy to 25 kGy. Peak viscosity mainly explains the swelling of starch granules [32]; it correlates with the reduction of swelling power due to the disintegration of starch granules, lessening in integrity and rigidity of starch granules due to breakdown of glycosidic bonds by ␥irradiation [7]. The steep fall in the hold viscosity was noted from 2374 cP (native starch) to 16 cP (25 kGy) with increasing irradiation. The setback and final viscosities are primarily due to a re-ordering
Fig. 3. Light transmittance of ␥-irradiated elephant foot yam starches.
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Table 2 Pasting properties of ␥-irradiated elephant foot yam starches. Parameter
Dose of irradiation 0 kGy
Pasting properties Pasting temp (◦ C) Pasting point (cP) Peak viscosity (cP) Hold viscosity (cP) Final viscosity (cP) Break down (cP) Set back (cP)
85.5 17 1985 1374 2148 610 745
5 kGy
± ± ± ± ± ± ±
1.0ab 1b 10a 15a 12a 6b 8a
84.4 17 792 183 268 616 84
± ± ± ± ± ± ±
10 kGy 0.5c 1b 5b 4b 6b 4a 2b
86.3 17 266 35 53 231 18
± ± ± ± ± ± ±
0.4a 1b 4c 3c 5c 3c 2c
15 kGy
20 kGy
85.2 ± 0.6bc 16 ± 1b 192 ± 4d 21 ± 2d 34 ± 3d 171 ± 3d 12 ± 1d
85.8 20 112 17 26 94 8
± ± ± ± ± ± ±
25 kGy 0.4ab 1a 2e 1e 3e 4e 1e
85.2 19 99 16 21 83 6
± ± ± ± ± ± ±
0.2bc 1a 3f 1e 2f 3f 1f
All data were means of triplicates. Values with the same superscripts in a row did not differ significantly (p ≤ 0.05) by DMRT.
or polymerization of amylose and long linear amylopectin. Final viscosity and set back was decreased from 2148 cP and 745 cP to 21 cP and 6 cP respectively with increased dose of irradiation, which could be due to the reordering or polymerization of leaked amylose, and long linear amylopectin undergo depolymerisation [20] leading to a significant decrease in the setback and final viscosities. Lowest break down viscosity (83 cP) was found in 25 kGy irradiated starch and highest in native starch (610 cP), which could be attributed to breaking of starch granules. These results were comparable with other starches treated with gamma irradiation [7,30]. 3.3. Morphological characteristics Morphological characteristics of native and irradiated starches using scanning electron microscopy are represented in Fig. 4. SEM studies have revealed that the elephant foot yam starch (native) granules are round, elliptical and polygonal in shape with smooth surfaces, and no obvious effects or signs of damage on the surface. After ␥-irradiation, the starch granules were not visually differed from the non-irradiated starch granules and it was recommended that ␥-irradiation process retained the original shapes and sizes of starch granules. Moreover, there was no granular cracking or roughness on the surface of starch granules caused by ␥-irradiation.
Similar results were reported for irradiation of cowpea [30] and maize [27] starch granules. From the microscopic observation, that ␥-irradiation damage to elephant foot yam starch granules might exist only in the form of changes to the structure of the starch molecules.
3.4. X-ray diffraction of starches The XRD patterns of native and irradiated starches are represented in Fig. 5. All irradiated starches exhibited C-type pattern (mixture of A and B-type starch pattern) with intermediate peaks of 2 at 16.92◦ and 18.12◦ , strong peaks of 2 at 23.05◦ and weak peaks of 2 at 14.7◦ when compared with native (0 kGy) starch (B-type pattern). The irradiated elephant foot yam starch exhibited change in diffraction pattern compared to native starch. These diffraction patterns of starch was differed from other reports for starches by gamma irradiation [7]. The relative crystallinity (RC) of elephant foot yam starch decreased from 19.37% (non-irradiated starch) to 18.19% (25 kGy) which may be due to decreased amylopectin content following irradiation. The decreased relative crystalline were also observed in irradiation of potato and bean starches [7,33]. RC mainly affected by length of amylopectin chains, orientation of double helices in crystallite [34].
Fig. 4. Scanning electron micrographs (SEM) of ␥-irradiated elephant foot yam starches.
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Table 3 Thermal properties of ␥-irradiated elephant foot yam starches. Parameter
Dose of irradiation 0 kGy
Thermal properties T0 (◦ C) TP (◦ C) TC (◦ C) H gel (J/g) PHI (J/g C) R (◦ C)
61.41 68.75 84.28 15.28 2.06 22.02
5 kGy ± ± ± ± ± ±
0.02a 0.1c 0.06b 0.04c 0.01a 0.12c
60.33 74.74 84.18 15.47 1.07 23.8
10 kGy ± ± ± ± ± ±
0.07bc 0.5a 0.05b 0.02a 0.01c 0.02ab
60.08 72.8 84.46 15.19 1.46 24.23
± ± ± ± ± ±
15 kGy 0.06cd 0.1b 0.06a 0.01d 0.05b 0.06a
59.86 69.06 83.5 15.09 1.55 24.07
± ± ± ± ± ±
20 kGy 0.05d 0.05c 0.08d 0.01e 0.02b 0.01a
60.17 67.49 83.61 14.93 2.04 23.33
± ± ± ± ± ±
25 kGy 0.03bc 0.05d 0.04c 0.04f 0.02a 0.06b
60.49 72.68 83.71 15.4 1.24 23.2
± ± ± ± ± ±
0.04b 0.1b 0.05c 0.01b 0.01bc 0.01b
T0 , onset temperature; TP , peak temperature; TC , conclusion temperature; Hgel , enthalpy of gelatinization; PHI, peak height index (( gel/(TP − T0 )); R, gelatinization range (TC − T0 ). All data were means of triplicates. Values with the same superscripts in a row did not differ significantly (p ≤ 0.05) by DMRT.
of gamma-irradiation might be cause destruction of crystalline regions and amorphous regions of starch granules. 3.6. Thermal properties of starches
Fig. 5. XRD patterns of ␥-irradiated elephant foot yam starches.
Thermal properties of native and irradiated starches are represented in Table 3. The gelatinization temperatures (To , Tp and Tc ) and gelatinization enthalpy of native starch was 61.4 ◦ C, 68.7 ◦ C, 84.2 ◦ C and 15.28 J/g, respectively. The reduction of gelatinization temperatures (To , Tp and Tc ) and enthalpy (H) initiated by ␥irradiation did not significantly differ from native to 25 kGy, which is similar with other irradiation studies [27,35]. The starches with longer amylopectin and greater crystallinity exhibited higher gelatinization temperature and enthalpy [36,37]. In addition, Hoover and Manuel [28] informed that the reduction in enthalpy value was principally due to the interruption of the double helices rather than longer range distraction of crystallinity.
3.5. FTIR spectroscopy
4. Conclusion
The FTIR spectra of native and irradiated starches are represented in Fig. 6. Irradiated elephant foot yam starches were subjected to FTIR analysis to recognize the interruption of glycosidic bonds and reduced short-range crystalline order. After ␥-irradiation, the spectra of ␥-irradiated starches when compared, revealed that no new functional groups were added by ␥-irradiation process. The increased intensities of the band at 1155 cm−1 caused by ␥-irradiation (10 kGy and 15 kGy) exhibited alteration in the ordered structure of irradiated starches. It demonstrated that the low levels of ␥-irradiation do not make changes in the crystalline structure of starch molecules. From the crystalline structure determined by X-ray diffraction and FTIR spectroscopy, these results showed that high doses
The present research reveals that the properties of elephant foot yam starches are altered and significantly affected by ␥-irradiation. Irradiation treatment of elephant foot yam starch resulted in reduced pasting properties and changes in FTIR spectrum with increased dose of ␥-irradiation. Amylose content, pH, swelling power and syneresis values significantly decreased; carboxyl content, solubility, light transmittance and water absorption capacity increased upon increased dosage of irradiation on starches. There was significant difference between the XRD pattern of native and irradiated starches, only showing slight decreased intensity of peaks in irradiated starches. Irradiation process increased gelatinization temperatures and enthalpy of elephant foot yam starch. However modification of elephant foot yam starch by ␥-irradiation is a rapid and non-heat process compared to other modification processes and also these irradiated starch products can be used in food industries due to decreased syneresis and other functional properties instead of native starch. Acknowledgements The authors are grateful to the help rendered by technologists and technicians of gamma chamber and other necessary instruments at the Central Instrumentation Facility, Pondicherry Central University, Puducherry. This project is financially supported by the Department of Food Science and Technology, Pondicherry Central University, India. References
Fig. 6. FTIR spectrum of ␥-irradiated elephant foot yam starches.
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