Effect of temperature, ionic strength and 11S ratio on the rheological properties of heat-induced soy protein gels in relation to network proteins content and aggregates size
Graphical abstract
Introduction
Soy protein is frequently used in the food industry because of its ability to form a gel with desirable sensory and physicochemical characteristics. The physicochemical properties of heat-induced soy protein gels depend on the interactions of the protein molecules taking part in the gel network, and the thermal treatment conditions, such as heating temperature (Nagano et al., 1994b, Sorgentini et al., 1995), pH (M. C. Puppo & Anon, 1998; M. C. Puppo, Lupano, & Anon, 1995; J. M. Renkema, Lakemond, de Jongh, Gruppen, & van Vliet, 2000), ionic strength (Lakemond, de Jongh, Hessing, Gruppen, & Voragen, 2000; M. C. Puppo & Añón, 1999; J. M. Renkema, Gruppen, & Van Vliet, 2002a) and protein composition (J. M. Renkema et al., 2001, Utsumi and Kinsella, 1985b). The ability to gel is the basis for traditional Asian soy products, such as tofu; in addition, soy protein is often applied to improve the texture of sausage. Therefore, a better understanding of the impacts of protein composition and thermal treatment conditions on the physicochemical properties of gels would provide useful insights on the proper application of processing conditions for the development of soy textures foods.
In literatures, it has been extensively reported that the rheological properties of soy protein gels varied with heating treatment conditions. Nagano et al., 1992, Nagano et al., 1994a studied the effect of temperature on the gelation process of 7S and 11S protein by dynamic rheological measurements, and found that with increasing heating temperature gelation time decreased while gelation rate increased. Renkema et al. (2002b) investigated the relationship between storage modulus of soy protein gels and degree of protein denaturation at different heating temperatures ranging from 76 to 94 °C, and observed that higher G′ values were obtained as more protein became denatured. Nagano et al. (1994b) reported that the gelation time became longer and gelation rate became slower for 7S globulin protein with increasing NaCl concentration by measuring the complex shear moduli, which might be due to the fact that denaturation temperature of 7S protein shifted to higher values when increasing ionic strength. The rheological properties of heat-induced soy protein gel can also be affected by the 7S/11S ratio in protein, and higher storage modulus can be obtained in glycinin-rich protein gels (Kohyama and Nishinari, 1993, Nagano et al., 1994a; Renkema et al., 2001) compared to β-conglycinin-rich gels. However, little is known about the relationships between rheological properties of heat-induced soy protein gels and the proportion of network proteins in gel or protein aggregates sizes in solution.
Heat denaturation of soy protein is thought to be a prerequisite for gel formation, and the contribution of subunits to the gel network differs (M. C. Puppo & Anon, 1998; Renkema et al., 2002a, Utsumi and Kinsella, 1985a, Utsumi and Kinsella, 1985b). The protein not involved in network after gelation can be isolated by extraction (Mori et al., 1986, Utsumi et al., 1984), centrifugation (Sorgentini et al., 1995, Utsumi et al., 1984) and diffusion processes (Wu, Hua, Chen, Kong, & Zhang, 2015). In this work, the proteins diffusing out of heat-induced soy protein gels to the buffer were named non-network proteins, the composition of which was analyzed by SDS-PAGE. The left proteins in the gel were named network proteins, and the aim of the present study was to relate rheological properties of gels to network proteins content or composition. Moreover, the effects of ionic strength and 11S ratio on the molecular weight distributions and hydrodynamic radius of heat-induced protein aggregates were investigated by SEC-HPLC and DLS, and the relationship between storage modulus of soy gels and the size of protein aggregates was also discussed.
Section snippets
Materials
Defatted soybean flakes were purchased from Shangdong Gushen Industrial & Commercial Co., Ltd. Soy protein isolates (SPI) were obtained as reported previously (Wu et al., 2015). β-conglycinin-rich and glycinin-rich fractions were isolated from defatted soy flakes as described by Nagano et al. (1992) with some modifications. Defatted soybean flakes was mixed with 10-fold (w/w) distilled water and adjusted to pH 7.0 by 2 M NaOH. After it was stirred for 1 h at 25 °C, the suspension was
Effects of heating temperature
Fig. 1 illustrated the G′ values of 18% (w/v) soy protein isolate gels as a function of heating temperature (85, 90, 95 and 100 °C). The G′ values of gels formed at different temperature were all frequency dependent. Lower G′ values were obtained at lower frequencies compared to at higher frequencies, because at larger experimental time scales (1/ω, ω is the frequency) more protein-protein bonds have the opportunity to become stress-free during the periodic deformation (Renkema et al., 2002a).
Conclusion
The storage moduli of heat-induced soy protein gel were strongly and directly in relation to the amount of network protein and the size of aggregates. For gels formed at the same protein concentration, higher network proteins ratio was found at higher heating temperature, resulting in higher G′ value which might be due to the increase in the amount of strands or thickening of the strands in gel network. However, no correlation between G′ value and Rnet was observed in gels formed at different
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