Thermal aggregation and gelation of soy globulin at neutral pH

doi:10.1016/j.foodhyd.2016.06.028

Food Hydrocolloids

Volume 61, December 2016, Pages 740-746

https://doi.org/10.1016/j.foodhyd.2016.06.028 Get rights and content

Highlights

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Thermal aggregation of SPI leads to the self-similar aggregates.
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The aggregate structure is independent of temperature and concentration.
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Aggregation and gelation is characterized by an activation energy of 150 kJ/mol.
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The aggregation rate decreases sharply with decreasing concentration.

Abstract

Aggregation of soy globulin was studied in salt free aqueous solution at neutral pH over a wide range of concentrations (0.3–90 g/L) and temperatures (50–95 °C). The structure of the aggregates that were formed during heating was characterized with light scattering. In all cases aggregates with the same self-similar structure were observed that were characterized by a fractal dimension d_f = 2.0. Dynamic light scattering showed that the aggregates were flexible. The aggregate size increased with heating time and the rate of growth was characterized by an Arrhenius temperature dependence up to 85 °C with E_a = 180 kJ/mol independent of the concentration. For a given temperature the aggregation rate increased very strongly with increasing concentration. Gels were formed at concentrations down to 50 g/L and at temperatures down to 50 °C.

Graphical abstract

Introduction

Soybean is currently the most important plant source for proteins in food. The major protein component of soy beans is soy globulin that consists of β-conglycinin and glycinin with molecular weights of ≈2 × 10⁵ g/mol and ≈3.6 × 10⁵ g/mol, respectively (Brooks & Morr, 1985). When soy globulin is heated, the proteins denature, which leads to irreversible aggregation of the proteins via formation of hydrogen and disulfide bonds and hydrophobic interactions (Nishinari, Fang, Guo, & Phillips, 2014). At neutral pH, the denaturation temperature was determined by DSC as about 65 °C and 80 °C for β-conglycinin and glycinin, respectively, but it decreases with increasing pH and increases with increasing ionic strength (Guo et al., 2012, Renkema et al., 2002a).

Thermal aggregation of soy protein isolate (SPI) in aqueous solution has been studied for a long time and may lead to a solution of stable aggregates, a gel or a precipitate, depending on the concentration, heating time, temperature and pH (Guo et al., 2015, Petruccelli and Anon, 1995, Renkema et al., 2000, Renkema and van Vliet, 2002b, Sorgentini et al., 1995). During heating, β-conglycinin and glycinin co-aggregate and form mixed aggregates or gels (Guo et al., 2012, Utsumi et al., 1984). It was found that larger aggregates are formed when SPI is heated longer, at higher temperatures or at higher concentrations. However, the effect of these parameters on the aggregation and gelation rate and on the aggregate structure has not yet been studied quantitatively. For a full understanding of the thermal aggregation process of soy globulin in aqueous solution it is necessary to investigate the structure of the aggregates during their growth as a function of the concentration and the temperature.

The objective of the work presented here was to investigate the aggregation and gelation process of soy globulins in aqueous solution at neutral pH. The structure of the aggregates was characterized with static and dynamic light scattering, and the rate of aggregation and gelation was determined by light scattering and rheology. We have studied the structure of the aggregates as a function of heating time for up to one week over a wide range of concentrations up to 90 g/L and temperatures up to 95 °C. Studying the systems during extremely long heating times allowed us to show that soy globulins aggregate at much lower concentrations and temperatures than was realized in the literature. Furthermore, it allowed us to cover a wide range of temperatures and concentrations and to accurately determine the effects of these parameters on the rate of aggregation and gelation and on the structure of the aggregates.

Section snippets

Materials

Soy protein isolate was obtained from defatted soybean flakes (Yuwang Group, China) by precipitation at pH 4.5 using the procedure detailed elsewhere (Chen, Lin, Sun, & Zhao, 2014) The SPI powder contained 94% protein (Kjeldahl, N × 6.25). Analysis by SDS-page showed that the sample contained about 60% glycinin and 40% β-conglycinin. A stock solution was prepared in salt-free Milli-Q water with 3 mM NaN₃ added in order to avoid bacterial growth. A small fraction of large aggregates was removed

Interaction of native soy globulin

The interaction between the native proteins as a function of the concentration was studied using light scattering measurements. Fig. 1 shows the concentration dependence of I_r/KC at q → 0 as a function the concentration. From the values at low concentration we find that M_w = 3 × 10⁶ g/mol, which is larger than expected for a mixture of glycinin and β-conglycinin. Recently, we demonstrated that this is caused by spontaneous association of native soy globulin leading to larger aggregates at

Conclusion

Heating soy protein isolate in aqueous solution at neutral pH leads to the formation of flexible self-similar aggregates characterized by a fractal dimension of 2. The structure of the aggregates is independent of heating time, heating temperature (50–85 °C) and protein concentration (50–90 g/L). For C ≥ 50 g/L, aggregation leads to the formation of a percolating network at a critical gel time. The temperature dependence of the aggregation and gelation rate up to 85 °C is determined by the

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Thermal aggregation and gelation of soy globulin at neutral pH

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Interaction of native soy globulin

Conclusion

Food Hydrocolloids

Food Hydrocolloids

International Journal of Biological Macromolecules

Food Hydrocolloids

Journal of Biotechnology

Food Hydrocolloids

Food Hydrocolloids

Archives of Biochemistry and Biophysics

Dynamic light scattering with applications to chemistry, biology, and physics

Current aspects of soy protein fractionation and nomenclature

Journal of the American Oil Chemists Society

Dynamic light scattering: The method and some applications

Light scattering: Principles and development

Stable and pH-sensitive protein nanogels made by self-assembly of heat denatured soy protein

Journal of Agricultural and Food Chemistry

Reaction kinetics of the denaturation of whey proteins in milk

Journal of Food Science