Thermal, Pasting and Gel Textural Properties of Commercial Starches from Different Botanical Sources

Volume 4 • Issue 4 • 1000161 J Bioproces Biotechniq ISSN:2155-9821 JBPBT, an open access journal *Corresponding author: Bo Zhang, Institute of Agro-Products Processing Science & Technology, And Chinese Academy of Agricultural Sciences, Key Laboratory of Agricultural Product Processing, Ministry of Agriculture, P.O. Box 5109, Beijing 100193, People ’s Republic of China, Tel: 86 01062815846; Fax: 86 01062895141; E-mail: zjzb1978@126.com


Introduction
Starch is the most abundant reserve carbohydrate in plants and a valuable ingredient in the food industry because it is widely used as a thickener, a gelling agent, a bulking agent and a water retention agent [1]. The applications of starch in food systems are primarily governed by its gelatinization, pasting, solubility, swelling and digestibility properties.
The physicochemical properties of starches depend on the botanical source from which they are isolated. The major botanical and commercial sources of starches are cereals, tubers, roots, and legumes. Many comparisons of the physicochemical and functional properties of starches from different crops have been published. Starches from cultivars of wheat [2], corn [3], potato [4], sweet potato [5], cassava [6], and mung bean [7] have been shown to vary in terms of starch composition and properties. Cereal starches contain a significant quantity of phospholipids, whereas corn and rice starches generally show higher transition temperatures than wheat and potato starches, and the highest enthalpy change values are found in potato and wheat starches. The free fatty acids in rice and corn starches contribute to their higher transition temperatures [8]. The starches from cassava and potato share similarities: these produce relatively bland pastes with higher viscosity, better clarity, and lower retrogradation rates than the starches derived from cereals. Normal cassava and potato starches have a high swelling power and a dispersed volume fraction compared with starches from other tropical root and tuber crops [9]. Tuber starches have been shown to be more susceptible than legume or cereal starches to heat-moisture treatment [10,11]. Potato starch exhibits higher swelling power, solubility, paste clarity, and viscosity than wheat, rice, and corn starches. Potato starch shows a higher tendency toward retrogradation compared with cereal starches [8]. In general, legume starches are characterized by a high amylose content (24-65%) and are reportedly more viscous than cereal starches [12,13].
Starch gelatinization refers to the disruption of the molecular order within the starch granules when heated in the presence of water. Evidence of the loss of an organized structure includes irreversible granule swelling, loss of birefringence, and crystallinity. Gelatinization is an energy-absorbing process that can be monitored through Differential Scanning calorimetry (DSC) and has been used for the study of starches [14]. In the DSC thermogram, thermal stability of samples can be monitored by the onset temperature (T o ) or peak transition temperature (T p ), while the transformed proportion is reflected by the area under the endothermic peak, representing the enthalpy change (∆H). The sharpness of the transition peak measured as width at half peak height (∆T 1/2 ), is an index of the co-operativity of the transition from native to denatured state [15]. A study of the thermal properties of starch can guide the processing and the utilization of starch and provide information about its structure.
form a viscous paste. The setback viscosity indicates the synaeresis of starch upon the cooling of the cooked starch pastes [17].
The gelling ability of starch is important in food manufacturing. The textural properties of starch gels are crucial criteria that are used to evaluate the performance of starch in a food system.
Although lots of studies have been conducted on starch systems, few studies have compared commercial starches originating from different starch-rich crops using the same experimental methods. Usually, the cultivars of normal starches for research are certain and with high purity. The commercial starches are usually mixed with several cultivars, and their purities are different. However, starches used for industry are mostly commercial grade. Thus, the present investigation focuses on the comparison of commercial starch samples from different origins, i.e. wheat, maize corn, potato, sweet potato, cassava, mung bean, and pea, to document the differences in their thermal, pasting, and gel textural properties. These comparisons will aid the selection of the appropriate variety for end-use suitability.

Materials
Commercial grade wheat starch, corn starch, potato starch, sweet potato starch, cassava starch, mung bean starch, pea starch, amylose from potato, and amylopectin from waxy corn were acquired in a market. The total starch contents, measured by a third-party detection institution using the Chinese standard method GB/T 5009.9-2003, are specified in Table 1.

Thermal properties of starches
The thermal characteristics of the starches were studied using a differential scanning calorimeter (DSC, Q-200, TA Instruments, USA). The starch was mixed with water in a vial to yield a 50% moisture paste. The vial was then capped and stored at 4°C overnight. After allowing the vial to reach room temperature, the moisture-equilibrated starch was conditioned in hermetic aluminum TA pans, weighed (approximately 10 mg) using a precision balance (± 0.01 mg, Analytical Plus, Mettler Toledo) and heated at a rate of 10°C min -1 from 20°C to 120°C in an inert atmosphere (50 mL min -1 of dry N 2 ). The reference was a void aluminum TA pan. The onset temperature (T o ), the peak temperature (T p ), the enthalpy change (∆H) and the width at half-peak height (∆T 1/2 ) were computed from the curves using the Universal Analysis Program (Version 1.9 D; TA Instruments). The enthalpies were calculated on a starch dry-weight basis.

Pasting properties of starches
According to AACC 76-21 STD2 [18], the pasting properties of the starches were evaluated using the Micro Visco-Amylo-Graph (Brabender, Germany). The viscosity profiles of the starches were recorded using starch suspensions (9.21%, w/w; 100 g total weight). The heating and cooling cycle program to which the samples were subjected was the following: heating from 30°C to 95°C at a rate of 6°C min -1 , holding 95°C for 5 min, cooling from 95°C to 50°C at a rate of 6°C min -1 , and holding at 50°C for 2 min. The parameters recorded were the pasting temperature (P Temp ), peak viscosity (PV), final viscosity (FV), breakdown viscosity (BV), and setback viscosity (SV). All of the measurements were replicated twice.

Textural properties of starch gels
A starch emulsion (9.21%, w/w) was heated from 30°C to 95°C at a rate of 6°C min -1 , maintained at 95°C for 20 min in the Micro Visco-Amylo-Graph, poured into small beakers and stored at 4°C to cause gelation. The textural properties of the starch gels were measured as described by Sandhu and Singh [17]. The gels' textural properties were evaluated through a texture profile analysis (TPA) using the TA-XT2i texture analyzer (Stable MicroSystems, Surrey, England). The gel was compressed at a rate of 0.5 mm s -1 to a distance of 10 mm with a cylindrical plunger (diameter=10 mm). The compression was repeated twice to generate a force-time curve from which the hardness (the height of the first peak) and springiness (the ratio between the recovered height after the first compression and the original gel height) were determined. The negative area of the curve during the retraction of the probe was termed the adhesiveness. The cohesiveness was calculated as the ratio of the area under the second peak to the area under the first peak. Five repeated measurements were performed for each sample and their average was used for the analysis.

Statistical analysis
One-way Analysis of Variance (ANOVA), Pearson correlation coefficients (r) for the relationships between all of the properties, and a Principal Component Analysis (PCA) were performed using the PASW Statistics 18.0 software. The comparisons between treatments were evaluated using Duncan's test. The significance level was set to 95% for all of the data analyses.

Thermal properties of starches
The DSC curves of the starches are shown in Figure 1  temperatures (onset, T o ; peak, T p ), the enthalpy change (∆H) and the width at half-peak height (∆T 1/2 ) of the gelatinization process are shown in Table 2. A significant difference was observed among the varieties of starches. Wheat starch had the lowest gelatinization temperatures. Sweet potato starch had the highest gelatinization temperatures, indicating that more energy is required to initiate the gelatinization of this starch. The ∆H for the various starches ranged from 0.49 to 5.29J/g. The ∆T 1/2 of mung bean starch (12.14°C) was considerably higher than that of the other starches. The corn, sweet potato, and cassava starches showed the highest transition temperatures and the lowest ∆H.

Pasting properties of starches
The pasting properties of the various starches are summarized in Figure 2 and Table 3. A significant difference in the pasting properties was observed among the different starches was observed. The corn and amylopectin starches have a substantial higher pasting temperature than the other starches. The peak viscosities (PV) of the various starches

Gel texture properties of starch gels
The textural properties of starch gels, which were determined    using a texture analyzer, are shown in Table 4. As shown, the textural parameters of starches varied significantly. The legume starches showed the highest hardness (701.94 g for pea starch, and 587.31 g for mung bean starch), whereas the cassava starch gel showed the lowest hardness (51.37 g). The cohesiveness of the starches varied from 0.34 to 0.49, and wheat starch, cassava starch and amylose exhibited slightly higher cohesiveness than the other starches. The springiness of the starches varied from 0.73 to 1.00, with mung bean starch having the highest springiness. The adhesiveness of cassava starch and amylose were not detected, whereas that of the other starches varied from -28.72 to -363.91 g•s, with pea starch exhibited the highest value, and tuber and root starches presenting the lowest values.

Pearson correlations among the various properties of the starches
Several significant correlations were observed between the gelatinization, and the pasting and the gel texture properties of the starches (Table 5). Positive interrelationships between the gelatinization parameters were also observed. The T o was correlated with the T p and ∆H (r=0.994 and -0.845, respectively, p<0.01), and the T p was negatively correlated with the ∆H (r=-0.832, p<0.01). The adhesiveness was negatively correlated with the PV, BV, and SV (P<0.05), and the hardness was positively correlated with the springiness (r=0.830, p<0.01).

Principal component analysis of the various properties of the starches
A principal component analysis (PCA) was conducted on four of the tested attributes. The results of the PCA (Figure 3) indicated that 35.64% of the variation in the data can be explained by PC1, 23.54% can be explained by PC2, 18.85% can be explained by PC3, and 9.96% can be explained by PC4. The variables that are found close to each other in pairs or groups indicate a positive correlation. The variables describing the pasting properties, specifically PV, FV, SV, and BV, were correlated with each other ( Figure 3A). The loading plot shows that the pasting properties are correlated with the total starch content. The T o is correlated with the T p . The hardness is correlated with the springiness ( Figure 3B).
The potato starch and cassava starch presented high positive scores in PC1 and were located near the PC1 axis (Figure 3a). The sweet potato starch and mung bean starches had positive scores in the PC2 and were located near the PC2 axis. The potato, pea, and mung bean starches had positive scores in PC3 (Figure 3b).
The comparison of the score and loading plots ( Figure 3A

Discussion
The value of T o shows the internal structure of the granule during its disintegrationes, which results in the release of polysaccharide into the surrounding medium [19]. The heat capacity plays a major role in this process because it directly reflects the molecular structure [20].    Higher enthalpies and temperatures indicate a higher degree of granule crystallinity [21]. According to our results, T o and T p are negatively correlated with the ∆H (n=9, r=-0.845 and -0.832, p<0.01).

Variety P Temp (°C) PV(mPa•s) FV(mPa•s) BV(mPa•s) SV(mPa•s)
Singh et al. [13] studied the thermal and pasting properties of 13 different black gram cultivars. The T o varied from 66.1 to 71.3 °C (the moisture content was 70%); the PV varied from 422 to 514 RVU; and the BV varied from 134 to 212 RVU. The PV, and BV were negatively correlated with the T o (n=13, r=-0.794 and -0.835, p<0.01). The P Temp was positively correlated with the T o (n=13, r=0.961, p<0.01). During the pasting process, the starch granules first swell; and this swelling is followed by the melting of crystals and then the paste becoming viscous.
The variations in ∆H may represent differences in the bonding forces between the double helices that form the amylopectin crystallites, which alter the alignment of hydrogen bonds within the starch molecules [17]. According to previous results, the PV, FV, BV, and SV of starch emulsions are slightly negatively correlated with the ∆H (n=9; r=-0.743, -0.623, -0.733, and -0.611, respectively; P<0.05), and the P Temp and the hardness of the starch gel are positively correlated with ∆H (r=0.620, p<0.05; r=0.44, p<0.01) [22]. Bao et al. [23] observed a significant positive correlation between ∆H and cohesiveness (n=127, r=0.234, P<0.05) in rice starch.

Conclusion
In the present study, variations in the thermal, pasting and gel texture properties were observed between different varieties of starches. Wheat starch showed the lowest transition temperatures (the onset temperature, T o and the peak temperature, T p ). The root starches (sweet potato starch, cassava starch) showed the highest transition temperatures and the lowest enthalpy change values. Mung bean starch showed the slowest transition speed. The tuber and root starches showed higher viscosities (peak viscosity, final viscosity, breakdown viscosity and setback viscosity) and lower adhesiveness. The legume starches showed the highest hardness values. The properties of amylose and amylopectin largely depended on their source.
The peak viscosity, final viscosity, breakdown viscosity and setback viscosity were positively correlated with each other and were positively correlated with the starch content. The onset temperature was negatively correlated with the enthalpy change. The pasting temperature was positively correlated with onset temperature and the final viscosity, setback viscosity, and pasting temperature were negatively correlated with the enthalpy change.