Ultrasound heat treatment effects on structure and acid-induced cold set gel properties of soybean protein isolate
Graphical abstract
Introduction
As an important byproduct of the soybean oil industry, soybean protein isolate (SPI) is used in the food industry due to its solubility, emulsification and gelation properties. It is of significance for the effective utilization of protein resources and the improvement of food quality (Chen, Wang, et al., 2019). Tofu is a soybean product prepared using the gelation ability of soybean protein. The preparation technology of traditional soybean protein gels usually involves two heating steps: the first heating of soybean protein at neutral pH and the second heating of soybean protein with the addition of coagulants such as calcium sulfate (CaSO4) and/or glucono-δ-lactone (GDL) (Lin et al., 2016). The protons released by coagulants can neutralize the negative charge of the soybean protein polymer, which then forms the three-dimensional network structure mainly though hydrogen bonds, ion interactions and hydrophobic interactions (Liu & Kuo, 2011). However, the second heating during preparation of soybean protein gels limits its application as a controlled delivery carrier to provide heat-sensitive nutrients such as some vitamins (Yoshimatsu et al., 2014) and probiotics (de Vos et al., 2010). Recently, the soybean protein cold set gels formed at low temperatures after some pretreatment such as ultrasound or heating have been studied. The cold set gels can protect the heat-sensitive compounds added into the gels, and reduce the use of heating equipment and energy consumption (Maltais et al., 2009). However, traditional soybean protein cold set gels have poor gel strength and water holding capacity (WHC). Therefore, it may be beneficial to improve the properties of soybean protein cold set gels to broaden its application in the food field.
Some physical methods such as ultrasound, homogenization, high pressure and pulsed electric field have been used to modify the structure and improve the functional properties of proteins (Xue et al., 2018; Zhang et al., 2017). High intensity ultrasound with low frequency (~20–100 kHz) can produce strong cavitation effects and mechanical forces, which can be used to change the physicochemical and functional properties of soybean proteins (Chen, Ma, et al., 2019). Arzeni et al. (2012) showed that ultrasound could change the emulsification, solubility and thermal gelation properties of SPI. Hu et al. (2015) reported that ultrasound changed the three-dimensional network structure of transglutaminase induced SPI gel, leading to the increase of hydrophobicity and the improvement of gel strength. They also found that the beneficial properties of SPI thermal gels induced by GDL (Hu, Fan, et al., 2013) and CaSO4 (Hu, Li-Chan, et al., 2013) were increased with ultrasound. Ultrasound treatment can also improve the gelation properties of other proteins such as whey protein (Shen, Zhao, & Guo, 2017) and myofibrillar protein (Amiri et al., 2018).
The SPI needs to be fully unfolded and denatured by the heat treatment before the formation of GDL-induced SPI cold set gel (GSCG), and the degree of denaturation and aggregation of SPI will affect its subsequent gelation behavior (Liu & Kuo, 2011). The denaturation and aggregation of SPI can also be affected by ultrasound (Jambrak et al., 2009), which might improve gel properties. Nevertheless, there are few reports about the effects of ultrasound heat treatment on SPI structural changes and the properties of GSCG. In this study, the secondary and tertiary structures of SPI and physicochemical properties of GSCG with ultrasound heat treatment were investigated, and the relationship between SPI structures and GSCG properties was also evaluated, which will provide information about how ultrasound heat treatment affects SPI gel behavior.
Section snippets
Materials
SPI, containing 92.4% crude protein as measured using the Kjeldahl method and a conversion factor of 6.25 (Kamizake et al., 2003), was provided by Harbin High-Tech Ltd. (Harbin, Heilongjiang, China). GDL was purchased through Hui Yang Biological Technology Co. (Jinan, Shandong, China). Potassium bromide (KBr), 1-anilino-8-naphtalene-sulfonate (ANS), sodium dodecyl sulfate (SDS) and 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) were purchased from Sigma Aldrich Co. (St. Louis, MO, USA). All
FTIR spectroscopy
FTIR spectroscopy of SPI with various treatments are shown in Fig. 1. The characteristic absorption peaks for SPI were seen at 1658 cm-1 (amide I, CO stretching vibration), 1542 cm-1 (amide II, N–H bending vibration) and 1405 cm-1 (amide III, C–N stretching vibration), which were consistent with the results of Huang et al. (2012). The absorption peak intensity and shape of heated SPI were similar to that of control SPI. The strength of the absorption peaks at amide I and amide II were increased
Conclusions
Ultrasound heat treatment of SPI resulted in more disordered secondary structure and less compact tertiary conformation, which increased H0 and free –SH. However, the formation of GSCG mainly involved hydrophobic interactions rather than disulfide bonds due to the limitation of sulfhydryl group oxidation during acidification at low temperature. Ultrasound heat treatment changed the particle size of SPI to affect its gelation properties. The 15 min ultrasound heat treated GSCG had the highest
Author statement
Chengbin Zhao: Conceptualization, Methodology, Writing-Original Draft. Zejun Chu: Validation, Data curation. Zhenchi Miao: Software. Jing Liu: Visualization. Jingsheng Liu: Supervision. Xiuying Xu: Formal analysis. Yuzhu Wu: Data curation. Baokun Qi: Writing-Reviewing and Editing, Funding acquisition. Jiannan Yan: Formal analysis, Investigation.
Declaration of competing interest
The authors confirm that they have no conflicts of interest with respect to the work described in this manuscript.
Acknowledgments
The authors gratefully acknowledge the financial support provided by the Science and Technology Development Program of Jilin Province of China (20190103121JH), the Postdoctoral Merit-based Program of Jilin Province of China, the Natural Science Foundation of Heilongjiang Province of China (LH 2019C032) and the National Grain Industry Leading Talent Program of China (LL2018201).
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