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Effects of native starch and modified starches on the textural, rheological and microstructural characteristics of soybean protein gel
Journal Pre-proofs Effects of native starch and modified starches on the textural, rheological and microstructural characteristics of soybean protein gel Bin Yu, Fei Ren, Haibo Zhao, Bo Cui, Pengfei Liu PII: DOI: Reference:
S0141-8130(19)33437-3 https://doi.org/10.1016/j.ijbiomac.2019.09.095 BIOMAC 13340
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Please cite this article as: B. Yu, F. Ren, H. Zhao, B. Cui, P. Liu, Effects of native starch and modified starches on the textural, rheological and microstructural characteristics of soybean protein gel, (2019), doi: https://doi.org/ 10.1016/j.ijbiomac.2019.09.095
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Effects of native starch and modified starches on the textural, rheological and microstructural characteristics of soybean protein gel Bin Yua,b,*, Fei Renb, Haibo Zhaoa,b, Bo Cuia,b,*, Pengfei Liua,b a State
Key Laboratory of Biobased Material and Green Papermaking, Qilu
University of Technology (Shandong Academy of Sciences), Jinan, Shandong 250353, China b College
of Food Science and Engineering, Qilu University of Technology (Shandong
Academy of Sciences), Jinan, Shandong 250353, China *Corresponding author at: Qilu University of Technology (Shandong Academy of Sciences), Daxue Road, Changqing District, Ji'nan City, Shandong Province 250353, China. Tel.: +86 0531 89631195; fax: +86 0531 89631195. E-mail address: [email protected] (B. Yu), [email protected] (B. Cui)
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Abstract: The effects of native starch (NS), acetylated starch (AS), and acetylated distarch phosphate (ADSP) on the gel properties of soybean protein thermal gel were investigated using texture analysis, low-field nuclear magnetic resonance (LF-NMR) spectroscopy, dynamic rheometry and scanning electron microscopy. The results of the textural profile analysis showed that 10% ADSP increased the hardness and chewiness of the mixed gel, while NS and AS led to decreases in the textural properties. The results of the LF-NMR analysis indicated that the AS improved the water-holding capacity of the mixed gel due to the transformation of weakly bound water to strongly bound water. During heating and cooling, the rheological profiles of the elastic (G') and viscous modulus (G'') of all the samples exhibited a two-stage pattern of decrease and then increase, and the final values of G' and G'' reached maxima when the ADSP content was 10%. The scanning electron microscopy images showed that the ADSP granules dispersed in the gel network. The integrity of the starch granules was crucial for regulating the properties of the soybean protein gel. These results provided information about the further design and preparation of soybean protein foods containing modified starch.
Keywords: Soybean protein isolate; Modified starch; Gel characteristics.
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1. Introduction Soybean protein is extensively used as a food ingredient because of its high nutritional value, outstanding functional properties, and low cost [1]. The gel properties of soybean protein play a vital role in the regulation of the texture of foods [2-4]. Soybean protein gelation involves the aggregation and gelation of unfolded protein molecular under certain concentration [5]. Physico-chemical parameters applied during soybean protein gelation process, such as heat treatment, pH, and protein concentration, influence the functionalities of soybean protein [6, 7]. Other ingredients in food also affect the gel properties and sensory characteristics of soybean protein [8, 9]. Therefore, blending the soybean protein with other ingredients is a simple and safe method for improving the functional properties of soybean protein [10]. Due to the increasing application of soy proteins in food processing and new product development, considerable studies have been performed on the influence of polysaccharides on gelling properties of soybean protein. In this context, most studies use food gums as model polysaccharides, such as carrageenan, xanthan, and alginate [10, 11]. The interactions between soybean protein and gums are determined by gums concentration, molecular weight, conformation and charge density [12, 13]. The soybean protein gel with different mechanical, rheological properties and microstructure could be prepared by alternating gum type and concentration. However, the main obstacles to the commercial application of gums in soybean products are the poor quality and availability of gums. Starch, a main ingredient in many foods, contributes significantly to the textural properties of the food. Moreover, starch is used as a thickener, stabilizer, gelling agent, 3
and water retaining agent for many industrial applications [14]. However, the low shear resistance, thermal resistance and cooking loss and syneresis of native starch (NS) preclude its use in certain applications in the food industry [15]. To overcome one or more of the shortcomings of NS and meet the demanding technological needs, physical, chemical or enzymatic methods are used to alter the functional and physicochemical properties of the NS [16, 17]. Modified starch (MS) with suitable functional properties provides an economic alternative to other hydrocolloid polysaccharide, which are unstable in quality and availability [18]. MS has been shown by many studies to have no adverse effects and to be safe to use an additive in the processing foods, such as meat, milk, fish products [19-22]. The possible mechanism proposed was that MS as a filler reinforcement agent, filled into the protein network structure, resulting in a homogeneous microstructure [23]. Food containing MS showed a better performance in the final product characteristics than did NS, which demonstrated the possibility of the application of MS to other foods. Although MS is widely used in gel-like foods, there is limited available information on the effect of MS on the gelling properties of heat induced soybean protein gels. Combining soy protein and MS is an alternative approach to developing new products and to lowering cost and ameliorating texture. We speculate that the addition of MS can affect gel properties of soybean protein. Relevant information about the microstructure, textural, and rheological properties of mixed gel of soybean protein and MS can provide a sound scientific basis of formulations and manufacturing method of soybean protein foods. To facilitate the development of soy protein foods containing MS, this study compared the effects of NS, and two common MS (acetylated starch, AS; acetylated distarch phosphate, ADSP) on the textural, rheological and microstructural 4
characteristics of soy protein isolate (SPI), a major commercial soybean protein product, to understand how MS affects the gel formation of soybean protein. This work will provide useful information about the gel properties of soy protein food containing MS. 2. Materials and methods 2.1 Materials Soybean protein isolate (SPI, 91.2% protein, 2.1% ash, 0.5% fat, and 6.2% moisture on a w/w basis) was purchased from Yuwang Group (Dezhou, Shandong, China). Food-grade native corn starch (NS, 88.9% starch, 0.3% protein, 0.1% fat, 0.1% ash, and 11.6% moisture on a w/w basis), acetylated corn starch (AS, 87.1% starch, 0.2% protein, 0.1% fat, 0.2% ash, and 12.4% moisture on a w/w basis) and acetylated distarch phosphate corn starch (ADSP, 86.3% starch, 0.2% protein, 0.1% fat, 0.2% ash, and 13.2% moisture on a w/w basis) were provided by Zhucheng Xingmao Corn Developing Co. Ltd. (Weifang, Shandong, China). The degree of substitution and acetyl content of the AS were 0.07 and 1.45, respectively. The molar substitution and degree of chemical substitution of the ADSP were 0.179 and 0.0052, respectively. All the chemicals were of analytical grade. 2.2 Sample preparation NS, AS or ADSP was added to SPI powder at ratio of 5%, 10%, and 15% (w/w) of the total sample mass, according to preliminary experiments and the reported methods for meat analogues [24]. The uniform powders were dispersed into distilled water to obtain 90% water content. The pH values of the suspensions were adjusted to 7.0 by adding hydrochloric acid or sodium hydroxide (0.1 mol/L). Manual agitation was used to ensure the starch was uniformly dispersed in the viscous mixture 5
suspensions. According the preparation method of SPI gel reported [25], all the samples were moved into metal tubes (inner diameter 30 mm, height 50 mm) and heated in a water bath at 90 °C for 30 min. The samples were cooled with tap water to ambient temperature (20 °C) and stored overnight at 4 °C until further testing. 2.3 Textural profile analysis (TPA) TPA was carried out to analyse the textural characteristics of the mixed gels by a TA-XT Plus texture analyser (Stable Micro Systems, Godalming, UK). The gel was cut into a cylinder (diameter 30 mm, height 30 mm), which was constricted two times to 50% of its initial height at a steady speed of 1 mm/s by a cylinder probe (36 mm diameter). The analysed textural properties of the gel included the hardness, springiness, chewiness, and resilience. Five replicates were conducted for each sample. 2.4 Low-field 1H NMR (LF-NMR) analysis For determination of the water mobility, LF-NMR was carried out on an NMR spectrometer (Niumag Co., Ltd., Shanghai, China) with a 0.5 T permanent magnet. An approximately 3 g sample prepared as described in Section 2.2 was placed into a 15-mm test tube and inserted into the NMR probe. The spin-spin relaxation time (T2) was determined using the Carr-Purcell-Meiboom-Gill (CPMG) sequence with a τ value of 300 μs at 23.2 MHz and 32 °C. T2 and its corresponding area percentage (M2) were calculated according to the LF-NMR relaxation curves by Multi Exp. Inv. analysis software (Niumag Co., Ltd., Shanghai, China). All the measurements were conducted with three replicates for each sample. 2.5 Rheological measurements The rheological properties of SPI/starch mixtures at 90% water content were measured with a Bohlin C-VOR 150 rheometer (Bohlin Instruments Inc., Cranbury, 6
USA) with a 40 mm diameter parallel plate and a 1 mm gap. The linear viscoelastic region was determined by strain sweeps (0.01–100%) at 1 Hz. Temperature ramp tests were carried out at 1 Hz frequency with 1.0% strain in the linear viscoelastic conditions. The SPI/starch mixtures were heated from 25 to 90 °C at 1 °C/min, held for 15 min at 90 °C, cooled down to 25 °C at 1 °C /min and kept at 25 °C for 15 min. Each sample was covered with a thin paraffin oil layer to prevent evaporation losses. All the rheological results were expressed as the storage modulus (G') and loss modulus (G''). 2.6 Scanning electron microscopy Some portions of the gels prepared for the TPA were used for a microstructure analysis. The gels were cut into cubes of approximately 5×5×3 mm and fixed in a 2.5% glutaraldehyde solution for at least 1.5 h. The fixed gels were dehydrated with ascending concentrations of ethanol (50%, 70%, 90%, and 100% twice, 30 min each). The dehydrated gels were transferred onto double-sided tape and then covered by a thin layer of gold. The samples were observed using a Quanta 200 scanning electron microscope (FEI Company, Hillsboro, USA). 2.7 Statistical analysis All measurements were carried out in triplicate except for TPA in quintuplicate. The data was subjected to one-way analysis of variance for statistical significance using SPSS Statistics (version 19.0, SPSS Inc., Chicago, USA). The differences among the means were compared by the Tukey’s multiple comparisons at a significance level of 0.05. 3. Results and Discussion 3.1 Textural properties of the gels 7
Texture is an important sensory attribute of food when assessing product acceptability. TPA simulates the chewing of food in the mouth by compressing the product twice [26]. The TPA parameters of the SPI and SPI/starch gels are shown in Table 1. There were significant differences in texture between the SPI/starch mixed gels and SPI gels. The addition of NS and AS led to a decrease in the gel textural parameters of mixed gels for the full range of additive amounts. In the process of gelatinisation, several successive changes occurred [27], and these changes influenced the gelation process of the SPI. AS has a higher swelling and solubility, and a lower gelatinisation temperature than does NS [28]. The NS and AS absorbed more water and swelled, while the SPI absorbed less water, which may have suppressed the aggregation of the SPI during the thermal treatment and formation of hydrogen bond during cooling process. The NS and AS exuded some amylose into the mixed matrix during gelatinisation, which disrupted the SPI gel network formation. As shown in Table 1, the 10% ADSP caused the hardness and chewiness increase to the maximum values of 141.26 g and 114.65 g, respectively. A further increase in the concentration of the ADSP to 15% provoked a significant reduction of the hardness and chewiness (p < 0.05). Cross-linking strengthens the structure of starch granules and limits water absorption [29]. The ADSP, with its lower water absorption capacity and swelling power, did not interfere with the gelation progress of the SPI but improved the hardness and chewiness of the gels as fillers. However, the 15% ADSP induced the formation of a discontinuous gel and decreased the TPA parameters of the mixed gel. The TPA results suggested that the type and content of MS were important factors affecting the properties of soybean protein gel, which was consistent with the report by Sun and Huang [21]. The changes in the textural characteristics of the mixed gel were mainly due to the spacing effect and competitive adsorption of water and starch. 8
3.2 Relaxation properties of the gels The gelation process of hydrocolloid blends and their interactions with water were identified by LF-NMR without destruction of the sample [30]. These results may provide more information about water compartments than do the conventional water holding capacity methods. The water redistribution and mobility of the SPI/starch mixed gels were investigated according to the T2 distributions of the mixed gels obtained from the CPMG decay curves. The T2 distributions, including the peak values and the area percentage associated with the peaks, are shown in Table 2. For each sample, two water components with different T2 ranges were observed and represented as T21 and T22, which indicated the mixed gels contained two water components with different relaxation rates [31]. T21 was the major fraction (accounting for approximately 90% of the total signal) with a relaxation time of 20-27 ms, which corresponded to a less mobile water fraction. T22 was the fraction with a relaxation time of 134-174 ms, corresponding to a more mobile water fraction [32]. The microstructure of a gel determines the distribution of the water fraction [1]. Small intervals in the gel network limited the movement of water, while the water in the large apertures of the mixed gel shown more mobility. As shown in Table 2, the T21 values of the mixed gels were significantly reduced (p < 0.05) after the addition of NS or MS. T21 of the SPI/AS gel decreased from 27.08 to 20.18 ms when the additive content of AS was up to 15%, which was attributed to excellent the swelling capacity induced by the introduction of hydrophilic substituent groups [33]. The SPI/NS and SPI/ADSP gels showed similar results at the same content. Although the differences were not all significant for the additive amount of 15%, the addition of AS to the SPI gel resulted in lower a T21, suggesting that the AS had a higher water holding capacity than did the NS and ADSP. The value of T22 decreased gradually as the amount of the 9
NS and MS, which suggested that the starch content had a significant effect on the value of T22 and more mobile water was also restricted in mixed gels. These phenomena are the results of the formation of hydrogen bonds between the starch and water [34]. The area percentage (M21, M22) of the T21 and T22 are also shown in Table 2. According to previous reports, the relative peak areas of the water factions in the relaxation spectrum were in proportion to their content [35]. Starch addition led to a remarkable increase in M21 and a decrease in M22, which suggested that the starch addition caused the water to become less mobile. Botlan et al. suggested that the water distribution was divided into weakly bound water and strongly bound water [36]. Our findings showed that the NS and MS transformed some weakly bound water to strongly bound water in the mixed gels, mainly because of the excellent water-holding capacity of the starch. The acetylation caused the starch to absorb more moisture, while the cross-linking decreased the hydroxyl groups of the starch without reducing its water retention in the mixed gel. These results indicated that the incorporated AS, NS and ADSP restricted the mobility of water during the gelation of the SPI/starch and influenced the textural properties. 3.3 Rheological properties The strain sweep spectrum of the SPI/starch and SPI dispersions is shown in Fig. 1. The linear viscoelastic region of the samples extended up to more than 3%. There was no obvious change in the linear viscoelastic region as the starch concentration increased. Therefore, a temperature sweep was conducted at a strain of 1.0%. The variations in the heat-induced dynamic viscoelastic properties for the SPI/starch and SPI dispersions during heating and cooling cycle are presented in Fig. 2. G' (Fig. 2 a, c and e) and G'' (Fig. 2 b, d and f) were used to compare the difference in the gelation process of between the SPI/starch and SPI. There was no crossover between G' and 10
G'' throughout the heating and cooling process, and the G' value was higher than the G'' value within the temperature sweep range for all the samples. Thus, the SPI and SPI/starch dispersions had more elastic than viscous properties and might be classified as gels that show the typical rheological behaviour [37]. This observation is consistent with previous findings [38]. The development of G' and G'' of the SPI as a function of time showed a two-stage pattern. G' of the SPI was higher (~2000 Pa) at the beginning of the temperature sweep, and decreased slowly until the temperature reached 74 °C during the cooling stage. The second phase showed a rapid increase in G' or G'', which appeared to reach a limiting value upon further cooling, but no further increase was observed at 25 °C. The decrease in G' caused by heating was consistent with the previous result [39], and it indicating the SPI did not gel during heating [40]. The observed stronger increase in G' and G'' during the cooling stage without a further increase at 25 °C was consistent with previous studies [41]. All the SPI/starch mixtures showed rheological characteristics similar to those of the SPI, indicating that the SPI played a dominant role in the gel network of the SPI/starch mixtures. G' and G'' of the SPI/starch decreased as the starch content increased at the beginning of the heating stage. There were obvious peaks in the rheological curves during the heating stage for the mixtures containing 15% starch. G' and G'' reached maximum values at 63.5, 57.1, 76.0 °C of the heating stage for the NS, AS and ADSP due to the breakdown of the starch granules. During the cooling stage, G' and G'' of all the samples increased sharply and reached a plateau at 25 °C, suggesting that the gel properties were enhanced by cooling. During cooling, the SPI and starch molecules associated and formed gels through noncovalent interactions, such as hydrogen bonds and nonspecific associations [42]. The final value of G' decreased with the additive amounts of NS and AS. SPI/ADSP (10%) had the highest G' and G'', 11
which showed that appropriate ADSP content was an important factor for the improvement of the gel properties. For G'' of the SPI/starch mixtures, only 5% ADSP increased the G'', suggesting the cross-linking reaction suppressed the intermolecular forces between the starch and SPI. The final values of G' and G'' were consistent with the results of the TPA. 3.4 Microstructure of the gels The scanning electron micrographs of the SPI and SPI/starch mixed gels at different concentrations are presented in Fig. 3. The SPI gel showed an inhomogeneous microstructure, which was due to the denaturation and aggregation of the SPI before the heating treatment. The SPI/starch gels showed microstructures similar to that of the SPI gel. All the samples exhibited the continuous phase structure of soybean protein and gelatinised starch granules embedded in the SPI gel matrix. As the starch content increased, more starch granules fragments were distributed in the mixed gel. However, the microstructures of the mixed gels with different starches and contents were obviously different. When the NS content was 15%, the starch granules fragments aggregated and formed a lamellar structure due to the retrogradation of the NS during gel storage at 4 °C. The AS and ADSP showed dispersive granules fragments because the acetylation or cross-linking inhibited the aggregation of gelatinised starch [43]. The NS and AS showed a complete lack of granular integrity, while the ADSP exhibited a round-shaped swollen granule, because the additional covalent bonds of the ADSP enabled the granules to withstand the gelatinisation temperature and maintain the granular integrity [44, 45]. Therefore, the ADSP granules dispersed evenly as a filler in the SPI network after heating and cooling. This finding revealed that the granular integrity is crucial for the characteristics of soybean protein gel. In the process of gel formation, the ADSP, acting as a rigid filler, 12
dispersed in the continuous SPI phase, forming a more compact gel network, which improved the texture and rheological properties. 4. Conclusions This work employed textural profile analysis, dynamic rheological tests, and scanning electron microscopy to provide a better understanding of how NS and MS affect the heat-induced gelatinisation of SPI. The hardness and chewiness of SPI/ADSP gels were significantly improved when the ADSP content was 10%. The excellent effect of AS on the water-holding capacity was attributed to the swelling capacity induced by the introduction of hydrophilic substituent groups. The SPI/ADSP (10%) sample showed higher final values of G' and G'' than did the other samples. The reinforcement texture and rheological properties of the samples with ADSP could be explained by the fact that the ADSP granules absorbed water and swelled during heating with a complete granular structure in the SPI matrix. Amylose and granule fragments released from the gelatinised NS and AS interpenetrated with the SPI network. These results for the SPI/starch gel provide new information that can help create soybean protein food with MS. 5. Acknowledgements This research was supported by Special Funds for Taishan Scholars Project. 6. Conflict of interests We confirm that there is no known conflict of interests associated with this publication.
References [1] R. Li, N. Hettiarachchy, S. Rayaprolu, M. Davis, S. Eswaranandam, A. Jha, P. 13
Chen, Improved functional properties of glycosylated soy protein isolate using D-glucose and xanthan gum, J. Food Sci. Tech. 52 (2015) 6067-6072. https://doi.org/10.1007/s13197-014-1681-3. [2] M.B. Barac, M.B. Pesic, S.P. Stanojevic, A.Z. Kostic, V. Bivolarevic, Comparative study of the functional properties of three legume seed isolates: adzuki, pea and soy bean, J. Food Sci. Tech. 52 (2015) 2779-2787. https://doi.org/10.1007/s13197-014-1298-6. [3] O.P. Malav, S. Talukder, P. Gokulakrishnan, S. Chand, Meat analog: a review, Crit. Rev. Food Sci. 55 (2015) 1241-1245. https://doi.org/10.1080/10408398.2012.689381. [4] X. Peng, S. Guo, Texture characteristics of soymilk gels formed by lactic fermentation: A comparison of soymilk prepared by blanching soybeans under different temperatures, Food Hydrocoll. 43 (2015) 58-65. https://doi.org/10.1016/j.foodhyd.2014.04.034. [5] A. Maltais, G.E.Remondetto, M. Subirade, Mechanisms involved in the formation and structure of soya protein cold-set gels: A molecular and supramolecular investigation, Food Hydrocoll. 22 (2008) 550-559. https://doi.org/10.1016/j.foodhyd.2007.01.026. [6] J. Jiang, Y.L. Xiong, J. Chen, pH shifting alters solubility characteristics and thermal stability of soy protein isolate and its globulin fractions in different pH, salt concentration, and temperature conditions, J. Agr. Food Chem. 58 (2010) 8035-8042. https://doi.org/10.1021/jf101045b. 14
[7] A. Maltais, G.E. Remondetto, R. Gonzalez, M. Subirade, Formation of soy protein isolate cold-set gels: protein and salt effects, J. Food Sci. 70 (2005) C67-C73. https://doi.org/10.1111/j.1365-2621.2005.tb09023.x. [8] B. Manion, M. Corredig, Interactions between whey protein isolate and soy protein fractions at oil–water interfaces: effects of heat and concentration of protein in the aqueous phase, J. Food Sci. 71 (2006) E343-E349. https://doi.org/10.1111/j.1750-3841.2006.00160.x. [9] A.M. Siegwein, Y. Vodovotz, E.L. Fisher, Concentration of soy protein isolate affects starch-based confections’ texture, sensory, and storage properties, J. Food Sci. 76 (2011) E422-E428. https://doi.org/10.1111/j.1750-3841.2011.02241.x. [10] Y. Hua, S.W. Cui, Q. Wang, Gelling property of soy protein–gum mixtures, Food Hydrocoll. 17 (2003) 889-894. https://doi.org/10.1016/S0268-005X(03)00110-3. [11] J.A.P. Vilela, Â.L.F. Cavallieri, R.L. Da Cunha, The influence of gelation rate on the physical properties/structure of salt-induced gels of soy protein isolate–gellan gum, Food Hydrocoll. 25 (2011) 1710-1718. https://doi.org/10.1016/j.foodhyd.2011.03.012. [12] S.R. Monteiro, J.A. Lopes-da-Silva, Effect of the molecular weight of a neutral polysaccharide on soy protein gelation, Food Res. Int. 102 (2017) 14-24. https://doi.org/10.1016/j.foodres.2017.09.066 [13] M.S.M. Wee, R. Yusoff, L. Lin, Y.Y. Xu, Effect of polysaccharide concentration and charge density on acid-induced soy protein isolate-polysaccharide gels using HCl, 15
Food Struct. 13 (2017) 45-55. https://doi.org/10.1016/j.foostr.2016.08.001. [14] L.R. Gerits, B. Pareyt, J.A. Delcour, Wheat starch swelling, gelatinization and pasting: Effects of enzymatic modification of wheat endogenous lipids, LWT-Food Sci. Technol. 63 (2015) 361-366. https://doi.org/10.1016/j.lwt.2015.02.035. [15] S. Wattanachant, K.M.A.T. Muhammad, D.M. Hashim, R.A. Rahman, Effect of crosslinking reagents and hydroxypropylation levels on dual-modified sago starch properties, Food Chem. 80 (2003) 463-471. https://doi.org/10.1016/S0308-8146(02)00314-X. [16] B. Kaur, F. Ariffin, R. Bhat, A. A. Karim, Progress in starch modification in the last decade, Food Hydrocoll. 26 (2012) 398-404. https://doi.org/10.1016/j.foodhyd.2011.02.016. [17] N. Masina, Y. E. Choonara, P. Kumar, L.C. du Toit, M. Govender, S. Indermun, V. Pillay, A review of the chemical modification techniques of starch, Carbohyd. Polym. 157 (2017) 1226-1236. https://doi.org/10.1016/j.carbpol.2016.09.094. [18] R.N. Tharanathan, Starch—value addition by modification, Crit. Rev. Food Sci. Nutr. 45 (2005) 371-384. https://doi.org/10.1080/10408390590967702. [19] A.P. de Groot, H.P. Til, V.J. Feron, H.C. Dreef-van Der Meulen, M.I. Willems, Two-year feeding and multigeneration studies in rats on five chemically modified starches, Food Cosmet. Toxicol. 12 (1974) 651-663. https://doi.org/10.1016/0015-6264(74)90236-3. [20] W. Kong, T. Zhang, D. Feng, Y. Xue, Y. Wang, Z. Li, W. Yang, C. Xue, Effects of modified starches on the gel properties of Alaska Pollock surimi subjected to 16
different temperature treatments, Food Hydrocoll. 56 (2016) 20-28. https://doi.org/10.1016/j.foodhyd.2015.11.023. [21] D. Verbeken, K. Bael, O. Thas, K. Dewettinck, Interactions between κ-carrageenan, milk proteins and modified starch in sterilized dairy desserts, Int. Dairy J. 16 (2006) 482-488. https://doi.org/10.1016/j.idairyj.2005.06.006. [22] H. Genccelep, F.T. Saricaoglu, M. Anil, B. Agar, S. Turhan, The effect of starch modification and concentration on steady-state and dynamic rheology of meat emulsions, Food Hydrocoll. 48 (2015) 135-148. https://doi.org/10.1016/j.foodhyd.2015.02.002. [23] F. Sun, Q. Huang, T. Hu, S. Xiong, S. Zhao, Effects and mechanism of modified starches on the gel properties of myofibrillar protein from grass carp, Int. J. Biol. Macromol. 64 (2014) 17-24. https://doi.org/10.1016/j.ijbiomac.2013.11.019. [24] L. Dar-jen, Low-fat meat analogues and methods for making same, United States Patent No. 5676987. [25] S. Isaschar-Ovdat, M. Davidovich-Pinhas, A. Fishman. Modulating the gel properties of soy glycinin by crosslinking with tyrosinase. Food Res. Int. 87 (2016) 42-49. https://doi.org/10.1016/j.foodres.2016.06.018. [26] J. Chen, Food oral processing—A review, Food Hydrocoll. 23 (2009) 1-25. https://doi.org/10.1016/j.foodhyd.2007.11.013. [27] T. Palav, K. Seetharaman, Impact of microwave heating on the physicochemical properties of a starch–water model system, Carbohyd. Polym. 67 (2007) 596-604. https://doi.org/10.1016/j.carbpol.2006.07.006. 17
[28] D.K. Yadav, P.E. Patki, Effect of acetyl esterification on physicochemical properties of chick pea (Cicer arietinum L.) starch, J. Food Sci. Tech. 52 (2015) 1-10. https://doi.org/10.1007/s13197-014-1388-5. [29] G.H. Zheng, H.L. Han, R.S. Bhatty, Functional properties of cross-linked and hydroxypropylated waxy hull-less barley starches, Cereal Chem. 76 (1999) 182-188. https://doi.org/10.1094/CCHEM.1999.76.2.182. [30] A.K. Thybo, I.E. Bechmann, M. Martens, S.B. Engelsen, Prediction of sensory texture of cooked potatoes using uniaxial compression, near infrared spectroscopy and low field 1H NMR spectroscopy, LWT-Food Sci. Technol. 33 (2000) 103-111. https://doi.org/10.1006/fstl.1999.0623. [31] T. Li, X. Rui, W. Li, X. Chen, M. Jiang, M. Dong, Water distribution in tofu and application of T2 relaxation measurements in determination of tofu’s water-holding capacity, J. Agr. Food Chem. 62 (2014) 8594-8601. https://doi.org/10.1021/jf503427m. [32] J.H. Shao, Y.M. Deng, L. Song, A. Batur, N. Jia, D.Y. Liu, Investigation the effects of protein hydration states on the mobility water and fat in meat batters by LF-NMR technique, LWT-Food Sci. Technol. 66 (2016) 1-6. https://doi.org/10.1016/j.lwt.2015.10.008. [33] I.A. Wani, D.S. Sogi, B.S. Gill, Physico-chemical properties of acetylated starches from Indian black gram (Phaseolus mungo L.) cultivars, J. Food Sci. Tech. 47 (2012) 1993-1999. https://doi.org/10.1111/j.1365-2621.2012.03062.x. [34] Z. Fu, J. Chen, S.J. Luo, C.M. Liu, W. Liu, Effect of food additives on starch 18
retrogradation: A review, Starch-Starke 67 (2015) 69-78. https://doi.org/10.1002/star.201300278. [35] C.H. Tang, X.Y. Wang, X.Q. Yang, L. Li, Formation of soluble aggregates from insoluble commercial soy protein isolate by means of ultrasonic treatment and their gelling properties, J. Food Eng. 92 (2009) 432-437. https://doi.org/10.1016/j.jfoodeng.2008.12.017. [36] D. Le Botlan, Y. Rugraff, C. Martin, P. Colonna, Quantitative determination of bound water in wheat starch by time domain NMR spectroscopy, Carbohyd. Res. 308 (1998) 29-36. https://doi.org/10.1016/S0008-6215(98)00068-8. [37] C. Rodriguez-Abreu, D.P. Acharya, K. Aramaki, H. Kunieda, Structure and rheology of direct and reverse liquid-crystal phases in a block copolymer/water/oil system, Colloid. Surface. A 269 (2005) 59-66. https://doi.org/10.1016/j.colsurfa.2005.06.061. [38] J. Ahmed, H. S. Ramaswamy, I. Alli, Thermorheological characteristics of soybean protein isolate, J. Food Sci. 71 (2006) E158-E163. https://doi.org/10.1111/j.1365-2621.2006.tb15629.x. [39] J.Y. Li, A.I. Yeh, K.L. Fan, Gelation characteristics and morphology of corn starch/soy protein concentrate composites during heating, J. Food Eng. 78 (2007) 1240-1247. https://doi.org/10.1016/j.jfoodeng.2005.12.043. [40] T.V. Vliet, A.H. Martin, M.A Bos, Gelation and interfacial behaviour of vegetable proteins, Curr. Opin. Colloid Interface Sci. 7 (2002) 462-468. https://doi.org/10.1016/S1359-0294(02)00078-X. 19
[41] J.H. Zhu, X.Q. Yang, I. Ahmad, Y. Jiang, X.Y. Wang, L.Y. Wu, Effect of guar gum on the rheological, thermal and textural properties of soybean β-conglycinin gel, Int. J. Food Sci. Tech. 44 (2009) 1314-1322. https://doi.org/10.1111/j.1365-2621.2009.01960.x. [42] D. Jia, Q. Huang, S. Xiong, Chemical interactions and gel properties of black carp actomyosin affected by MTGase and their relationships, Food Chem. 196 (2016) 1180-1187. https://doi.org/10.1016/j.foodchem.2015.10.030. [43] O.V. López, N.E. Zaritzky, M.A. García, Physicochemical characterization of chemically modified corn starches related to rheological behavior, retrogradation and film forming capacity, J. Food Eng. 99 (2010) 160-168. https://doi.org/10.1016/j.jfoodeng.2010.03.041. [44] H.L. Lee, B. Yoo, Dynamic rheological and thermal properties of acetylated sweet potato starch, Starch-Starke 61 (2009) 407-413. https://doi.org/10.1002/star.200800109. [45] J. Juansang, C. Puttanlek, V. Rungsardthong, S. Puncha-arnon, D. Uttapap, Effect of gelatinisation on slowly digestible starch and resistant starch of heat-moisture treated and chemically modified canna starches, Food Chem. 131 (2012) 500-507. https://doi.org/10.1016/j.foodchem.2011.09.013.
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Samples
Table 1 Textural properties of SPI and SPI/starch gels Textural properties Starch content Hardness (g) Springiness Chewiness (g) (%, w/w)
Resilience
SPI
0
107.89 ± 1.19c
0.93 ± 0.01a
93.23 ± 0.97b
0.76 ± 0.02a
SPI/NS
5 10 15
101.27 ± 2.37d 99.75 ± 2.48d 89.20 ± 1.36f
0.91 ± 0.02ab 0.90 ± 0.02bc 0.89 ± 0.01cd
90.48 ± 0.95c 83.84 ± 1.17d 74.23 ± 1.20f
0.72 ± 0.01b 0.72 ± 0.01bc 0.70 ± 0.02bcde
SPI/AS
5 10 15
98.38 ± 1.60d 92.22 ± 2.00ef 87.89 ± 1.66f
0.92 ± 0.01ab 0.91 ± 0.01ab 0.87 ± 0.01de
92.24 ± 1.03bc 77.50 ± 1.44e 74.93 ± 1.12f
0.71 ± 0.01bcd 0.70 ± 0.02cdef 0.69 ± 0.01def
SPI/ADSP
5 129.81 ± 2.41b 0.94 ± 0.02a 113.86 ± 1.86a 0.71 ± 0.01bcd 10 141.26 ± 3.22a 0.88 ± 0.01cd 114.65 ± 1.29a 0.69 ± 0.01ef 15 96.76 ± 1.71de 0.85 ± 0.03e 82.37 ± 1.27d 0.68 ± 0.01g Values are means of quintuplicate determination ± standard deviation. Means with different letters in the same column are significantly different (p < 0.05). SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
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Table 2 LF-NMR relaxation time and area percentage of SPI and SPI/starch gels Samples SPI
Starch content (%, w/w) 0
Relaxation time (ms) T21 T22 a 27.08 ± 0.73 173.98 ± 0.67a
Area percentage (%) M21 M22 f 92.82±0.64 7.18 ± 0.64a
SPI/NS
5 10 15
24.96 ± 0.42b 22.38 ± 0.53cde 21.55 ± 0.38def
168.64 ± 0.75b 161.34 ± 0.83d 155.33 ± 1.08f
94.69±0.33de 96.71±0.34bc 98.84±0.30a
5.31±0.64bc 3.29±0.34de 1.16±0.30f
SPI/AS
5 10 15
23.78 ± 0.39bc 21.96 ± 0.33de 20.18 ± 0.44f
165.81 ± 1.07c 154.48 ± 1.11f 134.86 ± 0.85h
94.21±0.33def 96.56±0.38bc 98.02±0.81ab
5.79±0.33abc 3.44±0.38de 1.98±0.81ef
SPI/ADSP
5 10 15
24.89 ± 0.8b 22.82 ± 0.17cd 21.31 ± 0.52ef
167.26 ± 0.92bc 93.48±0.55ef 158.76 ± 0.71e 95.41±0.63cd 151.32 ± 0.66g 97.12±0.34b
6.52±0.55ab 4.60±0.63cd 2.88±0.34e
Values are means of triplicate determination ± standard deviation. Means with different letters in the same column are significantly different (p < 0.05). SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
22
Figure Captions:
Fig.1 Strain sweep for the SPI and SPI/starch mixtures at 25 °C and 1Hz. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate. Fig.2 Storage modulus (Gʹ) (left column) and loss modulus (Gʹʹ) (right column) for the SPI and SPI/starch mixtures as function time during a heating and cooling cycle. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate. Fig.3 SEM photographs of the SPI and SPI/starch gels at 1000-fold magnification:
(a) pure SPI gel; (b-d) SPI/NS gels with NS mass ratio of 5%, 10%, and 15%; (e-g) SPI/AS gels with AS mass ratio of 5%, 10%, and 15%; (h-j) SPI/ADSP gels with ADSP mass ratio of 5%, 10%, and 15%. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
23
Fig.1 Strain sweep for the SPI and SPI/starch mixtures at 25 °C and 1Hz. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
24
Fig.2 Storage modulus (Gʹ) (left column) and loss modulus (Gʹʹ) (right column) for the SPI and SPI/starch mixtures as function time during a heating and cooling cycle. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
25
a
SPI
b
d
c
NS
NS NS
e
f
g
AS
AS AS
i
h
j
ADSP
ADSP
ADSP
Fig.3 SEM photographs of SPI and SPI/starch gels at 1000-fold magnification: (a) pure SPI gel; (b-d) SPI/NS gels with NS mass ratio of 5%, 10%, and 15%; (e-g) SPI/AS gels with AS mass ratio of 5%, 10%, and 15%; (h-j) SPI/ADSP gels with ADSP mass ratio of 5%, 10%, and 15%. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate. 26
27
S0141-8130(19)33437-3 https://doi.org/10.1016/j.ijbiomac.2019.09.095 BIOMAC 13340
To appear in: Received Date: Revised Date: Accepted Date:
28 May 2019 10 September 2019 12 September 2019
Please cite this article as: B. Yu, F. Ren, H. Zhao, B. Cui, P. Liu, Effects of native starch and modified starches on the textural, rheological and microstructural characteristics of soybean protein gel, (2019), doi: https://doi.org/ 10.1016/j.ijbiomac.2019.09.095
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Effects of native starch and modified starches on the textural, rheological and microstructural characteristics of soybean protein gel Bin Yua,b,*, Fei Renb, Haibo Zhaoa,b, Bo Cuia,b,*, Pengfei Liua,b a State
Key Laboratory of Biobased Material and Green Papermaking, Qilu
University of Technology (Shandong Academy of Sciences), Jinan, Shandong 250353, China b College
of Food Science and Engineering, Qilu University of Technology (Shandong
Academy of Sciences), Jinan, Shandong 250353, China *Corresponding author at: Qilu University of Technology (Shandong Academy of Sciences), Daxue Road, Changqing District, Ji'nan City, Shandong Province 250353, China. Tel.: +86 0531 89631195; fax: +86 0531 89631195. E-mail address: [email protected] (B. Yu), [email protected] (B. Cui)
1
Abstract: The effects of native starch (NS), acetylated starch (AS), and acetylated distarch phosphate (ADSP) on the gel properties of soybean protein thermal gel were investigated using texture analysis, low-field nuclear magnetic resonance (LF-NMR) spectroscopy, dynamic rheometry and scanning electron microscopy. The results of the textural profile analysis showed that 10% ADSP increased the hardness and chewiness of the mixed gel, while NS and AS led to decreases in the textural properties. The results of the LF-NMR analysis indicated that the AS improved the water-holding capacity of the mixed gel due to the transformation of weakly bound water to strongly bound water. During heating and cooling, the rheological profiles of the elastic (G') and viscous modulus (G'') of all the samples exhibited a two-stage pattern of decrease and then increase, and the final values of G' and G'' reached maxima when the ADSP content was 10%. The scanning electron microscopy images showed that the ADSP granules dispersed in the gel network. The integrity of the starch granules was crucial for regulating the properties of the soybean protein gel. These results provided information about the further design and preparation of soybean protein foods containing modified starch.
Keywords: Soybean protein isolate; Modified starch; Gel characteristics.
2
1. Introduction Soybean protein is extensively used as a food ingredient because of its high nutritional value, outstanding functional properties, and low cost [1]. The gel properties of soybean protein play a vital role in the regulation of the texture of foods [2-4]. Soybean protein gelation involves the aggregation and gelation of unfolded protein molecular under certain concentration [5]. Physico-chemical parameters applied during soybean protein gelation process, such as heat treatment, pH, and protein concentration, influence the functionalities of soybean protein [6, 7]. Other ingredients in food also affect the gel properties and sensory characteristics of soybean protein [8, 9]. Therefore, blending the soybean protein with other ingredients is a simple and safe method for improving the functional properties of soybean protein [10]. Due to the increasing application of soy proteins in food processing and new product development, considerable studies have been performed on the influence of polysaccharides on gelling properties of soybean protein. In this context, most studies use food gums as model polysaccharides, such as carrageenan, xanthan, and alginate [10, 11]. The interactions between soybean protein and gums are determined by gums concentration, molecular weight, conformation and charge density [12, 13]. The soybean protein gel with different mechanical, rheological properties and microstructure could be prepared by alternating gum type and concentration. However, the main obstacles to the commercial application of gums in soybean products are the poor quality and availability of gums. Starch, a main ingredient in many foods, contributes significantly to the textural properties of the food. Moreover, starch is used as a thickener, stabilizer, gelling agent, 3
and water retaining agent for many industrial applications [14]. However, the low shear resistance, thermal resistance and cooking loss and syneresis of native starch (NS) preclude its use in certain applications in the food industry [15]. To overcome one or more of the shortcomings of NS and meet the demanding technological needs, physical, chemical or enzymatic methods are used to alter the functional and physicochemical properties of the NS [16, 17]. Modified starch (MS) with suitable functional properties provides an economic alternative to other hydrocolloid polysaccharide, which are unstable in quality and availability [18]. MS has been shown by many studies to have no adverse effects and to be safe to use an additive in the processing foods, such as meat, milk, fish products [19-22]. The possible mechanism proposed was that MS as a filler reinforcement agent, filled into the protein network structure, resulting in a homogeneous microstructure [23]. Food containing MS showed a better performance in the final product characteristics than did NS, which demonstrated the possibility of the application of MS to other foods. Although MS is widely used in gel-like foods, there is limited available information on the effect of MS on the gelling properties of heat induced soybean protein gels. Combining soy protein and MS is an alternative approach to developing new products and to lowering cost and ameliorating texture. We speculate that the addition of MS can affect gel properties of soybean protein. Relevant information about the microstructure, textural, and rheological properties of mixed gel of soybean protein and MS can provide a sound scientific basis of formulations and manufacturing method of soybean protein foods. To facilitate the development of soy protein foods containing MS, this study compared the effects of NS, and two common MS (acetylated starch, AS; acetylated distarch phosphate, ADSP) on the textural, rheological and microstructural 4
characteristics of soy protein isolate (SPI), a major commercial soybean protein product, to understand how MS affects the gel formation of soybean protein. This work will provide useful information about the gel properties of soy protein food containing MS. 2. Materials and methods 2.1 Materials Soybean protein isolate (SPI, 91.2% protein, 2.1% ash, 0.5% fat, and 6.2% moisture on a w/w basis) was purchased from Yuwang Group (Dezhou, Shandong, China). Food-grade native corn starch (NS, 88.9% starch, 0.3% protein, 0.1% fat, 0.1% ash, and 11.6% moisture on a w/w basis), acetylated corn starch (AS, 87.1% starch, 0.2% protein, 0.1% fat, 0.2% ash, and 12.4% moisture on a w/w basis) and acetylated distarch phosphate corn starch (ADSP, 86.3% starch, 0.2% protein, 0.1% fat, 0.2% ash, and 13.2% moisture on a w/w basis) were provided by Zhucheng Xingmao Corn Developing Co. Ltd. (Weifang, Shandong, China). The degree of substitution and acetyl content of the AS were 0.07 and 1.45, respectively. The molar substitution and degree of chemical substitution of the ADSP were 0.179 and 0.0052, respectively. All the chemicals were of analytical grade. 2.2 Sample preparation NS, AS or ADSP was added to SPI powder at ratio of 5%, 10%, and 15% (w/w) of the total sample mass, according to preliminary experiments and the reported methods for meat analogues [24]. The uniform powders were dispersed into distilled water to obtain 90% water content. The pH values of the suspensions were adjusted to 7.0 by adding hydrochloric acid or sodium hydroxide (0.1 mol/L). Manual agitation was used to ensure the starch was uniformly dispersed in the viscous mixture 5
suspensions. According the preparation method of SPI gel reported [25], all the samples were moved into metal tubes (inner diameter 30 mm, height 50 mm) and heated in a water bath at 90 °C for 30 min. The samples were cooled with tap water to ambient temperature (20 °C) and stored overnight at 4 °C until further testing. 2.3 Textural profile analysis (TPA) TPA was carried out to analyse the textural characteristics of the mixed gels by a TA-XT Plus texture analyser (Stable Micro Systems, Godalming, UK). The gel was cut into a cylinder (diameter 30 mm, height 30 mm), which was constricted two times to 50% of its initial height at a steady speed of 1 mm/s by a cylinder probe (36 mm diameter). The analysed textural properties of the gel included the hardness, springiness, chewiness, and resilience. Five replicates were conducted for each sample. 2.4 Low-field 1H NMR (LF-NMR) analysis For determination of the water mobility, LF-NMR was carried out on an NMR spectrometer (Niumag Co., Ltd., Shanghai, China) with a 0.5 T permanent magnet. An approximately 3 g sample prepared as described in Section 2.2 was placed into a 15-mm test tube and inserted into the NMR probe. The spin-spin relaxation time (T2) was determined using the Carr-Purcell-Meiboom-Gill (CPMG) sequence with a τ value of 300 μs at 23.2 MHz and 32 °C. T2 and its corresponding area percentage (M2) were calculated according to the LF-NMR relaxation curves by Multi Exp. Inv. analysis software (Niumag Co., Ltd., Shanghai, China). All the measurements were conducted with three replicates for each sample. 2.5 Rheological measurements The rheological properties of SPI/starch mixtures at 90% water content were measured with a Bohlin C-VOR 150 rheometer (Bohlin Instruments Inc., Cranbury, 6
USA) with a 40 mm diameter parallel plate and a 1 mm gap. The linear viscoelastic region was determined by strain sweeps (0.01–100%) at 1 Hz. Temperature ramp tests were carried out at 1 Hz frequency with 1.0% strain in the linear viscoelastic conditions. The SPI/starch mixtures were heated from 25 to 90 °C at 1 °C/min, held for 15 min at 90 °C, cooled down to 25 °C at 1 °C /min and kept at 25 °C for 15 min. Each sample was covered with a thin paraffin oil layer to prevent evaporation losses. All the rheological results were expressed as the storage modulus (G') and loss modulus (G''). 2.6 Scanning electron microscopy Some portions of the gels prepared for the TPA were used for a microstructure analysis. The gels were cut into cubes of approximately 5×5×3 mm and fixed in a 2.5% glutaraldehyde solution for at least 1.5 h. The fixed gels were dehydrated with ascending concentrations of ethanol (50%, 70%, 90%, and 100% twice, 30 min each). The dehydrated gels were transferred onto double-sided tape and then covered by a thin layer of gold. The samples were observed using a Quanta 200 scanning electron microscope (FEI Company, Hillsboro, USA). 2.7 Statistical analysis All measurements were carried out in triplicate except for TPA in quintuplicate. The data was subjected to one-way analysis of variance for statistical significance using SPSS Statistics (version 19.0, SPSS Inc., Chicago, USA). The differences among the means were compared by the Tukey’s multiple comparisons at a significance level of 0.05. 3. Results and Discussion 3.1 Textural properties of the gels 7
Texture is an important sensory attribute of food when assessing product acceptability. TPA simulates the chewing of food in the mouth by compressing the product twice [26]. The TPA parameters of the SPI and SPI/starch gels are shown in Table 1. There were significant differences in texture between the SPI/starch mixed gels and SPI gels. The addition of NS and AS led to a decrease in the gel textural parameters of mixed gels for the full range of additive amounts. In the process of gelatinisation, several successive changes occurred [27], and these changes influenced the gelation process of the SPI. AS has a higher swelling and solubility, and a lower gelatinisation temperature than does NS [28]. The NS and AS absorbed more water and swelled, while the SPI absorbed less water, which may have suppressed the aggregation of the SPI during the thermal treatment and formation of hydrogen bond during cooling process. The NS and AS exuded some amylose into the mixed matrix during gelatinisation, which disrupted the SPI gel network formation. As shown in Table 1, the 10% ADSP caused the hardness and chewiness increase to the maximum values of 141.26 g and 114.65 g, respectively. A further increase in the concentration of the ADSP to 15% provoked a significant reduction of the hardness and chewiness (p < 0.05). Cross-linking strengthens the structure of starch granules and limits water absorption [29]. The ADSP, with its lower water absorption capacity and swelling power, did not interfere with the gelation progress of the SPI but improved the hardness and chewiness of the gels as fillers. However, the 15% ADSP induced the formation of a discontinuous gel and decreased the TPA parameters of the mixed gel. The TPA results suggested that the type and content of MS were important factors affecting the properties of soybean protein gel, which was consistent with the report by Sun and Huang [21]. The changes in the textural characteristics of the mixed gel were mainly due to the spacing effect and competitive adsorption of water and starch. 8
3.2 Relaxation properties of the gels The gelation process of hydrocolloid blends and their interactions with water were identified by LF-NMR without destruction of the sample [30]. These results may provide more information about water compartments than do the conventional water holding capacity methods. The water redistribution and mobility of the SPI/starch mixed gels were investigated according to the T2 distributions of the mixed gels obtained from the CPMG decay curves. The T2 distributions, including the peak values and the area percentage associated with the peaks, are shown in Table 2. For each sample, two water components with different T2 ranges were observed and represented as T21 and T22, which indicated the mixed gels contained two water components with different relaxation rates [31]. T21 was the major fraction (accounting for approximately 90% of the total signal) with a relaxation time of 20-27 ms, which corresponded to a less mobile water fraction. T22 was the fraction with a relaxation time of 134-174 ms, corresponding to a more mobile water fraction [32]. The microstructure of a gel determines the distribution of the water fraction [1]. Small intervals in the gel network limited the movement of water, while the water in the large apertures of the mixed gel shown more mobility. As shown in Table 2, the T21 values of the mixed gels were significantly reduced (p < 0.05) after the addition of NS or MS. T21 of the SPI/AS gel decreased from 27.08 to 20.18 ms when the additive content of AS was up to 15%, which was attributed to excellent the swelling capacity induced by the introduction of hydrophilic substituent groups [33]. The SPI/NS and SPI/ADSP gels showed similar results at the same content. Although the differences were not all significant for the additive amount of 15%, the addition of AS to the SPI gel resulted in lower a T21, suggesting that the AS had a higher water holding capacity than did the NS and ADSP. The value of T22 decreased gradually as the amount of the 9
NS and MS, which suggested that the starch content had a significant effect on the value of T22 and more mobile water was also restricted in mixed gels. These phenomena are the results of the formation of hydrogen bonds between the starch and water [34]. The area percentage (M21, M22) of the T21 and T22 are also shown in Table 2. According to previous reports, the relative peak areas of the water factions in the relaxation spectrum were in proportion to their content [35]. Starch addition led to a remarkable increase in M21 and a decrease in M22, which suggested that the starch addition caused the water to become less mobile. Botlan et al. suggested that the water distribution was divided into weakly bound water and strongly bound water [36]. Our findings showed that the NS and MS transformed some weakly bound water to strongly bound water in the mixed gels, mainly because of the excellent water-holding capacity of the starch. The acetylation caused the starch to absorb more moisture, while the cross-linking decreased the hydroxyl groups of the starch without reducing its water retention in the mixed gel. These results indicated that the incorporated AS, NS and ADSP restricted the mobility of water during the gelation of the SPI/starch and influenced the textural properties. 3.3 Rheological properties The strain sweep spectrum of the SPI/starch and SPI dispersions is shown in Fig. 1. The linear viscoelastic region of the samples extended up to more than 3%. There was no obvious change in the linear viscoelastic region as the starch concentration increased. Therefore, a temperature sweep was conducted at a strain of 1.0%. The variations in the heat-induced dynamic viscoelastic properties for the SPI/starch and SPI dispersions during heating and cooling cycle are presented in Fig. 2. G' (Fig. 2 a, c and e) and G'' (Fig. 2 b, d and f) were used to compare the difference in the gelation process of between the SPI/starch and SPI. There was no crossover between G' and 10
G'' throughout the heating and cooling process, and the G' value was higher than the G'' value within the temperature sweep range for all the samples. Thus, the SPI and SPI/starch dispersions had more elastic than viscous properties and might be classified as gels that show the typical rheological behaviour [37]. This observation is consistent with previous findings [38]. The development of G' and G'' of the SPI as a function of time showed a two-stage pattern. G' of the SPI was higher (~2000 Pa) at the beginning of the temperature sweep, and decreased slowly until the temperature reached 74 °C during the cooling stage. The second phase showed a rapid increase in G' or G'', which appeared to reach a limiting value upon further cooling, but no further increase was observed at 25 °C. The decrease in G' caused by heating was consistent with the previous result [39], and it indicating the SPI did not gel during heating [40]. The observed stronger increase in G' and G'' during the cooling stage without a further increase at 25 °C was consistent with previous studies [41]. All the SPI/starch mixtures showed rheological characteristics similar to those of the SPI, indicating that the SPI played a dominant role in the gel network of the SPI/starch mixtures. G' and G'' of the SPI/starch decreased as the starch content increased at the beginning of the heating stage. There were obvious peaks in the rheological curves during the heating stage for the mixtures containing 15% starch. G' and G'' reached maximum values at 63.5, 57.1, 76.0 °C of the heating stage for the NS, AS and ADSP due to the breakdown of the starch granules. During the cooling stage, G' and G'' of all the samples increased sharply and reached a plateau at 25 °C, suggesting that the gel properties were enhanced by cooling. During cooling, the SPI and starch molecules associated and formed gels through noncovalent interactions, such as hydrogen bonds and nonspecific associations [42]. The final value of G' decreased with the additive amounts of NS and AS. SPI/ADSP (10%) had the highest G' and G'', 11
which showed that appropriate ADSP content was an important factor for the improvement of the gel properties. For G'' of the SPI/starch mixtures, only 5% ADSP increased the G'', suggesting the cross-linking reaction suppressed the intermolecular forces between the starch and SPI. The final values of G' and G'' were consistent with the results of the TPA. 3.4 Microstructure of the gels The scanning electron micrographs of the SPI and SPI/starch mixed gels at different concentrations are presented in Fig. 3. The SPI gel showed an inhomogeneous microstructure, which was due to the denaturation and aggregation of the SPI before the heating treatment. The SPI/starch gels showed microstructures similar to that of the SPI gel. All the samples exhibited the continuous phase structure of soybean protein and gelatinised starch granules embedded in the SPI gel matrix. As the starch content increased, more starch granules fragments were distributed in the mixed gel. However, the microstructures of the mixed gels with different starches and contents were obviously different. When the NS content was 15%, the starch granules fragments aggregated and formed a lamellar structure due to the retrogradation of the NS during gel storage at 4 °C. The AS and ADSP showed dispersive granules fragments because the acetylation or cross-linking inhibited the aggregation of gelatinised starch [43]. The NS and AS showed a complete lack of granular integrity, while the ADSP exhibited a round-shaped swollen granule, because the additional covalent bonds of the ADSP enabled the granules to withstand the gelatinisation temperature and maintain the granular integrity [44, 45]. Therefore, the ADSP granules dispersed evenly as a filler in the SPI network after heating and cooling. This finding revealed that the granular integrity is crucial for the characteristics of soybean protein gel. In the process of gel formation, the ADSP, acting as a rigid filler, 12
dispersed in the continuous SPI phase, forming a more compact gel network, which improved the texture and rheological properties. 4. Conclusions This work employed textural profile analysis, dynamic rheological tests, and scanning electron microscopy to provide a better understanding of how NS and MS affect the heat-induced gelatinisation of SPI. The hardness and chewiness of SPI/ADSP gels were significantly improved when the ADSP content was 10%. The excellent effect of AS on the water-holding capacity was attributed to the swelling capacity induced by the introduction of hydrophilic substituent groups. The SPI/ADSP (10%) sample showed higher final values of G' and G'' than did the other samples. The reinforcement texture and rheological properties of the samples with ADSP could be explained by the fact that the ADSP granules absorbed water and swelled during heating with a complete granular structure in the SPI matrix. Amylose and granule fragments released from the gelatinised NS and AS interpenetrated with the SPI network. These results for the SPI/starch gel provide new information that can help create soybean protein food with MS. 5. Acknowledgements This research was supported by Special Funds for Taishan Scholars Project. 6. Conflict of interests We confirm that there is no known conflict of interests associated with this publication.
References [1] R. Li, N. Hettiarachchy, S. Rayaprolu, M. Davis, S. Eswaranandam, A. Jha, P. 13
Chen, Improved functional properties of glycosylated soy protein isolate using D-glucose and xanthan gum, J. Food Sci. Tech. 52 (2015) 6067-6072. https://doi.org/10.1007/s13197-014-1681-3. [2] M.B. Barac, M.B. Pesic, S.P. Stanojevic, A.Z. Kostic, V. Bivolarevic, Comparative study of the functional properties of three legume seed isolates: adzuki, pea and soy bean, J. Food Sci. Tech. 52 (2015) 2779-2787. https://doi.org/10.1007/s13197-014-1298-6. [3] O.P. Malav, S. Talukder, P. Gokulakrishnan, S. Chand, Meat analog: a review, Crit. Rev. Food Sci. 55 (2015) 1241-1245. https://doi.org/10.1080/10408398.2012.689381. [4] X. Peng, S. Guo, Texture characteristics of soymilk gels formed by lactic fermentation: A comparison of soymilk prepared by blanching soybeans under different temperatures, Food Hydrocoll. 43 (2015) 58-65. https://doi.org/10.1016/j.foodhyd.2014.04.034. [5] A. Maltais, G.E.Remondetto, M. Subirade, Mechanisms involved in the formation and structure of soya protein cold-set gels: A molecular and supramolecular investigation, Food Hydrocoll. 22 (2008) 550-559. https://doi.org/10.1016/j.foodhyd.2007.01.026. [6] J. Jiang, Y.L. Xiong, J. Chen, pH shifting alters solubility characteristics and thermal stability of soy protein isolate and its globulin fractions in different pH, salt concentration, and temperature conditions, J. Agr. Food Chem. 58 (2010) 8035-8042. https://doi.org/10.1021/jf101045b. 14
[7] A. Maltais, G.E. Remondetto, R. Gonzalez, M. Subirade, Formation of soy protein isolate cold-set gels: protein and salt effects, J. Food Sci. 70 (2005) C67-C73. https://doi.org/10.1111/j.1365-2621.2005.tb09023.x. [8] B. Manion, M. Corredig, Interactions between whey protein isolate and soy protein fractions at oil–water interfaces: effects of heat and concentration of protein in the aqueous phase, J. Food Sci. 71 (2006) E343-E349. https://doi.org/10.1111/j.1750-3841.2006.00160.x. [9] A.M. Siegwein, Y. Vodovotz, E.L. Fisher, Concentration of soy protein isolate affects starch-based confections’ texture, sensory, and storage properties, J. Food Sci. 76 (2011) E422-E428. https://doi.org/10.1111/j.1750-3841.2011.02241.x. [10] Y. Hua, S.W. Cui, Q. Wang, Gelling property of soy protein–gum mixtures, Food Hydrocoll. 17 (2003) 889-894. https://doi.org/10.1016/S0268-005X(03)00110-3. [11] J.A.P. Vilela, Â.L.F. Cavallieri, R.L. Da Cunha, The influence of gelation rate on the physical properties/structure of salt-induced gels of soy protein isolate–gellan gum, Food Hydrocoll. 25 (2011) 1710-1718. https://doi.org/10.1016/j.foodhyd.2011.03.012. [12] S.R. Monteiro, J.A. Lopes-da-Silva, Effect of the molecular weight of a neutral polysaccharide on soy protein gelation, Food Res. Int. 102 (2017) 14-24. https://doi.org/10.1016/j.foodres.2017.09.066 [13] M.S.M. Wee, R. Yusoff, L. Lin, Y.Y. Xu, Effect of polysaccharide concentration and charge density on acid-induced soy protein isolate-polysaccharide gels using HCl, 15
Food Struct. 13 (2017) 45-55. https://doi.org/10.1016/j.foostr.2016.08.001. [14] L.R. Gerits, B. Pareyt, J.A. Delcour, Wheat starch swelling, gelatinization and pasting: Effects of enzymatic modification of wheat endogenous lipids, LWT-Food Sci. Technol. 63 (2015) 361-366. https://doi.org/10.1016/j.lwt.2015.02.035. [15] S. Wattanachant, K.M.A.T. Muhammad, D.M. Hashim, R.A. Rahman, Effect of crosslinking reagents and hydroxypropylation levels on dual-modified sago starch properties, Food Chem. 80 (2003) 463-471. https://doi.org/10.1016/S0308-8146(02)00314-X. [16] B. Kaur, F. Ariffin, R. Bhat, A. A. Karim, Progress in starch modification in the last decade, Food Hydrocoll. 26 (2012) 398-404. https://doi.org/10.1016/j.foodhyd.2011.02.016. [17] N. Masina, Y. E. Choonara, P. Kumar, L.C. du Toit, M. Govender, S. Indermun, V. Pillay, A review of the chemical modification techniques of starch, Carbohyd. Polym. 157 (2017) 1226-1236. https://doi.org/10.1016/j.carbpol.2016.09.094. [18] R.N. Tharanathan, Starch—value addition by modification, Crit. Rev. Food Sci. Nutr. 45 (2005) 371-384. https://doi.org/10.1080/10408390590967702. [19] A.P. de Groot, H.P. Til, V.J. Feron, H.C. Dreef-van Der Meulen, M.I. Willems, Two-year feeding and multigeneration studies in rats on five chemically modified starches, Food Cosmet. Toxicol. 12 (1974) 651-663. https://doi.org/10.1016/0015-6264(74)90236-3. [20] W. Kong, T. Zhang, D. Feng, Y. Xue, Y. Wang, Z. Li, W. Yang, C. Xue, Effects of modified starches on the gel properties of Alaska Pollock surimi subjected to 16
different temperature treatments, Food Hydrocoll. 56 (2016) 20-28. https://doi.org/10.1016/j.foodhyd.2015.11.023. [21] D. Verbeken, K. Bael, O. Thas, K. Dewettinck, Interactions between κ-carrageenan, milk proteins and modified starch in sterilized dairy desserts, Int. Dairy J. 16 (2006) 482-488. https://doi.org/10.1016/j.idairyj.2005.06.006. [22] H. Genccelep, F.T. Saricaoglu, M. Anil, B. Agar, S. Turhan, The effect of starch modification and concentration on steady-state and dynamic rheology of meat emulsions, Food Hydrocoll. 48 (2015) 135-148. https://doi.org/10.1016/j.foodhyd.2015.02.002. [23] F. Sun, Q. Huang, T. Hu, S. Xiong, S. Zhao, Effects and mechanism of modified starches on the gel properties of myofibrillar protein from grass carp, Int. J. Biol. Macromol. 64 (2014) 17-24. https://doi.org/10.1016/j.ijbiomac.2013.11.019. [24] L. Dar-jen, Low-fat meat analogues and methods for making same, United States Patent No. 5676987. [25] S. Isaschar-Ovdat, M. Davidovich-Pinhas, A. Fishman. Modulating the gel properties of soy glycinin by crosslinking with tyrosinase. Food Res. Int. 87 (2016) 42-49. https://doi.org/10.1016/j.foodres.2016.06.018. [26] J. Chen, Food oral processing—A review, Food Hydrocoll. 23 (2009) 1-25. https://doi.org/10.1016/j.foodhyd.2007.11.013. [27] T. Palav, K. Seetharaman, Impact of microwave heating on the physicochemical properties of a starch–water model system, Carbohyd. Polym. 67 (2007) 596-604. https://doi.org/10.1016/j.carbpol.2006.07.006. 17
[28] D.K. Yadav, P.E. Patki, Effect of acetyl esterification on physicochemical properties of chick pea (Cicer arietinum L.) starch, J. Food Sci. Tech. 52 (2015) 1-10. https://doi.org/10.1007/s13197-014-1388-5. [29] G.H. Zheng, H.L. Han, R.S. Bhatty, Functional properties of cross-linked and hydroxypropylated waxy hull-less barley starches, Cereal Chem. 76 (1999) 182-188. https://doi.org/10.1094/CCHEM.1999.76.2.182. [30] A.K. Thybo, I.E. Bechmann, M. Martens, S.B. Engelsen, Prediction of sensory texture of cooked potatoes using uniaxial compression, near infrared spectroscopy and low field 1H NMR spectroscopy, LWT-Food Sci. Technol. 33 (2000) 103-111. https://doi.org/10.1006/fstl.1999.0623. [31] T. Li, X. Rui, W. Li, X. Chen, M. Jiang, M. Dong, Water distribution in tofu and application of T2 relaxation measurements in determination of tofu’s water-holding capacity, J. Agr. Food Chem. 62 (2014) 8594-8601. https://doi.org/10.1021/jf503427m. [32] J.H. Shao, Y.M. Deng, L. Song, A. Batur, N. Jia, D.Y. Liu, Investigation the effects of protein hydration states on the mobility water and fat in meat batters by LF-NMR technique, LWT-Food Sci. Technol. 66 (2016) 1-6. https://doi.org/10.1016/j.lwt.2015.10.008. [33] I.A. Wani, D.S. Sogi, B.S. Gill, Physico-chemical properties of acetylated starches from Indian black gram (Phaseolus mungo L.) cultivars, J. Food Sci. Tech. 47 (2012) 1993-1999. https://doi.org/10.1111/j.1365-2621.2012.03062.x. [34] Z. Fu, J. Chen, S.J. Luo, C.M. Liu, W. Liu, Effect of food additives on starch 18
retrogradation: A review, Starch-Starke 67 (2015) 69-78. https://doi.org/10.1002/star.201300278. [35] C.H. Tang, X.Y. Wang, X.Q. Yang, L. Li, Formation of soluble aggregates from insoluble commercial soy protein isolate by means of ultrasonic treatment and their gelling properties, J. Food Eng. 92 (2009) 432-437. https://doi.org/10.1016/j.jfoodeng.2008.12.017. [36] D. Le Botlan, Y. Rugraff, C. Martin, P. Colonna, Quantitative determination of bound water in wheat starch by time domain NMR spectroscopy, Carbohyd. Res. 308 (1998) 29-36. https://doi.org/10.1016/S0008-6215(98)00068-8. [37] C. Rodriguez-Abreu, D.P. Acharya, K. Aramaki, H. Kunieda, Structure and rheology of direct and reverse liquid-crystal phases in a block copolymer/water/oil system, Colloid. Surface. A 269 (2005) 59-66. https://doi.org/10.1016/j.colsurfa.2005.06.061. [38] J. Ahmed, H. S. Ramaswamy, I. Alli, Thermorheological characteristics of soybean protein isolate, J. Food Sci. 71 (2006) E158-E163. https://doi.org/10.1111/j.1365-2621.2006.tb15629.x. [39] J.Y. Li, A.I. Yeh, K.L. Fan, Gelation characteristics and morphology of corn starch/soy protein concentrate composites during heating, J. Food Eng. 78 (2007) 1240-1247. https://doi.org/10.1016/j.jfoodeng.2005.12.043. [40] T.V. Vliet, A.H. Martin, M.A Bos, Gelation and interfacial behaviour of vegetable proteins, Curr. Opin. Colloid Interface Sci. 7 (2002) 462-468. https://doi.org/10.1016/S1359-0294(02)00078-X. 19
[41] J.H. Zhu, X.Q. Yang, I. Ahmad, Y. Jiang, X.Y. Wang, L.Y. Wu, Effect of guar gum on the rheological, thermal and textural properties of soybean β-conglycinin gel, Int. J. Food Sci. Tech. 44 (2009) 1314-1322. https://doi.org/10.1111/j.1365-2621.2009.01960.x. [42] D. Jia, Q. Huang, S. Xiong, Chemical interactions and gel properties of black carp actomyosin affected by MTGase and their relationships, Food Chem. 196 (2016) 1180-1187. https://doi.org/10.1016/j.foodchem.2015.10.030. [43] O.V. López, N.E. Zaritzky, M.A. García, Physicochemical characterization of chemically modified corn starches related to rheological behavior, retrogradation and film forming capacity, J. Food Eng. 99 (2010) 160-168. https://doi.org/10.1016/j.jfoodeng.2010.03.041. [44] H.L. Lee, B. Yoo, Dynamic rheological and thermal properties of acetylated sweet potato starch, Starch-Starke 61 (2009) 407-413. https://doi.org/10.1002/star.200800109. [45] J. Juansang, C. Puttanlek, V. Rungsardthong, S. Puncha-arnon, D. Uttapap, Effect of gelatinisation on slowly digestible starch and resistant starch of heat-moisture treated and chemically modified canna starches, Food Chem. 131 (2012) 500-507. https://doi.org/10.1016/j.foodchem.2011.09.013.
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Samples
Table 1 Textural properties of SPI and SPI/starch gels Textural properties Starch content Hardness (g) Springiness Chewiness (g) (%, w/w)
Resilience
SPI
0
107.89 ± 1.19c
0.93 ± 0.01a
93.23 ± 0.97b
0.76 ± 0.02a
SPI/NS
5 10 15
101.27 ± 2.37d 99.75 ± 2.48d 89.20 ± 1.36f
0.91 ± 0.02ab 0.90 ± 0.02bc 0.89 ± 0.01cd
90.48 ± 0.95c 83.84 ± 1.17d 74.23 ± 1.20f
0.72 ± 0.01b 0.72 ± 0.01bc 0.70 ± 0.02bcde
SPI/AS
5 10 15
98.38 ± 1.60d 92.22 ± 2.00ef 87.89 ± 1.66f
0.92 ± 0.01ab 0.91 ± 0.01ab 0.87 ± 0.01de
92.24 ± 1.03bc 77.50 ± 1.44e 74.93 ± 1.12f
0.71 ± 0.01bcd 0.70 ± 0.02cdef 0.69 ± 0.01def
SPI/ADSP
5 129.81 ± 2.41b 0.94 ± 0.02a 113.86 ± 1.86a 0.71 ± 0.01bcd 10 141.26 ± 3.22a 0.88 ± 0.01cd 114.65 ± 1.29a 0.69 ± 0.01ef 15 96.76 ± 1.71de 0.85 ± 0.03e 82.37 ± 1.27d 0.68 ± 0.01g Values are means of quintuplicate determination ± standard deviation. Means with different letters in the same column are significantly different (p < 0.05). SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
21
Table 2 LF-NMR relaxation time and area percentage of SPI and SPI/starch gels Samples SPI
Starch content (%, w/w) 0
Relaxation time (ms) T21 T22 a 27.08 ± 0.73 173.98 ± 0.67a
Area percentage (%) M21 M22 f 92.82±0.64 7.18 ± 0.64a
SPI/NS
5 10 15
24.96 ± 0.42b 22.38 ± 0.53cde 21.55 ± 0.38def
168.64 ± 0.75b 161.34 ± 0.83d 155.33 ± 1.08f
94.69±0.33de 96.71±0.34bc 98.84±0.30a
5.31±0.64bc 3.29±0.34de 1.16±0.30f
SPI/AS
5 10 15
23.78 ± 0.39bc 21.96 ± 0.33de 20.18 ± 0.44f
165.81 ± 1.07c 154.48 ± 1.11f 134.86 ± 0.85h
94.21±0.33def 96.56±0.38bc 98.02±0.81ab
5.79±0.33abc 3.44±0.38de 1.98±0.81ef
SPI/ADSP
5 10 15
24.89 ± 0.8b 22.82 ± 0.17cd 21.31 ± 0.52ef
167.26 ± 0.92bc 93.48±0.55ef 158.76 ± 0.71e 95.41±0.63cd 151.32 ± 0.66g 97.12±0.34b
6.52±0.55ab 4.60±0.63cd 2.88±0.34e
Values are means of triplicate determination ± standard deviation. Means with different letters in the same column are significantly different (p < 0.05). SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
22
Figure Captions:
Fig.1 Strain sweep for the SPI and SPI/starch mixtures at 25 °C and 1Hz. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate. Fig.2 Storage modulus (Gʹ) (left column) and loss modulus (Gʹʹ) (right column) for the SPI and SPI/starch mixtures as function time during a heating and cooling cycle. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate. Fig.3 SEM photographs of the SPI and SPI/starch gels at 1000-fold magnification:
(a) pure SPI gel; (b-d) SPI/NS gels with NS mass ratio of 5%, 10%, and 15%; (e-g) SPI/AS gels with AS mass ratio of 5%, 10%, and 15%; (h-j) SPI/ADSP gels with ADSP mass ratio of 5%, 10%, and 15%. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
23
Fig.1 Strain sweep for the SPI and SPI/starch mixtures at 25 °C and 1Hz. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
24
Fig.2 Storage modulus (Gʹ) (left column) and loss modulus (Gʹʹ) (right column) for the SPI and SPI/starch mixtures as function time during a heating and cooling cycle. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate.
25
a
SPI
b
d
c
NS
NS NS
e
f
g
AS
AS AS
i
h
j
ADSP
ADSP
ADSP
Fig.3 SEM photographs of SPI and SPI/starch gels at 1000-fold magnification: (a) pure SPI gel; (b-d) SPI/NS gels with NS mass ratio of 5%, 10%, and 15%; (e-g) SPI/AS gels with AS mass ratio of 5%, 10%, and 15%; (h-j) SPI/ADSP gels with ADSP mass ratio of 5%, 10%, and 15%. SPI, soybean protein isolate; NS, native starch; AS, acetylated starch; ADSP, acetylated distarch phosphate. 26
27