l-Histidine improves solubility and emulsifying properties of soy proteins under various ionic strengths
Introduction
As a versatile functional ingredient, soy proteins are largely used in food products such as bakery products, beverages, dairy-like products, meat products, meat analogs, and protein bars (Singh, Kumar, Sabapathy, & Bawa, 2008). A well sought-after function of soy proteins is their ability to serve as an emulsifying agent. Soy proteins have excellent emulsifying properties among plant proteins. Although inferior to milk proteins as an emulsifier, soy proteins are competitive due to their inexpensive price and applicability in vegetarian and vegan products (Chen, Chen, Yu, & Wu, 2016). Moreover, their emulsifying properties can be further improved by physical, chemical, and biological modifications (Nishinari, Fang, Guo, & Phillips, 2014; Sui, Zhang, & Jiang, 2021).
Among various methods that improve protein functionalities, protein modifications by basic amino acids have attracted considerable attentions lately. Guo, Gao, Zhang, Peng, and Jamali (2021) reported that l-histidine and l-lysine enhanced the emulsifying activity index and emulsion stability index of porcine myofibrillar proteins at both low and high ionic strengths. l-lysine and l-arginine have also been shown to stabilize emulsion in chicken sausage (Zhu et al., 2018; Zhu, Li, Li, Ning, & Zhou, 2019). In addition, l-lysine and l-arginine have demonstrated ability to alleviate oxidation-induced structural changes of myosin and maintain their emulsifying properties (Zhang et al., 2021). Other peripheral evidences supporting the hypothesis that basic amino acids may improve the emulsifying properties of proteins include 1) improved protein solubility (Chen, Chen, et al., 2016; Gao, Wang, Mu, Shi, & Yuan, 2018; Hayakawa, Ito, Wakamatsu, Nishimura, & Hattori, 2009; Li, Zheng, Xu, Zhu, & Zhou, 2018); 2) enhanced electrostatic repulsions (Chen, Chen, et al., 2016; Wang et al., 2020); and 3) increased exposure of hydrophobic amino acid residues (Chen, Chen, et al., 2016; Gao et al., 2018) by basic amino acids.
The previous studies investigated the effects of basic amino acids on muscle and dairy proteins. To our knowledge, no study has examined the effects of l-histidine on the functionalities of plant proteins. The objective of this study is to investigate the effects of l-histidine on the emulsifying properties of soy protein. We tested l-histidine at four different concentrations (0%, 0.1%, 0.2%, 0.3%, w/v). Since l-histidine is expected to alter the pH of the protein solutions, we included the following samples to elucidate whether the outcomes were a result of the pH change or some other effects: 1) a control with 0% l-histidine and with pH adjusted to that of samples containing 0.3% l-histidine and 2) a sample containing 0.3% l-histidine and with pH adjusted to that of the control. Moreover, soy protein-stabilized emulsions are sensitive to changes in ionic strength due to protein precipitation at a low salt concentration and dissolution at a high salt concentration (Xu, Liu, & Zhang, 2015). Therefore, we performed experiments at three salt concentrations (0, 0.1, and 0.6 M NaCl) to investigate the effects of l-histidine treatment under various ionic strengths.
Section snippets
Materials
Soybeans and soybean oil were purchased from a local foods market. l-histidine (99%, biological reagent grade) was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). 5,5′-Dithiobis (2-niteobenoic acid) (DTNB) was purchased from Sangon Biotech Co., Ltd. (Shanghai, China). Magnesium 8-anilino-1-naphthalenesulfonate (ANS-Mg) was purchased from Tokyo Chemical Industry Co., Ltd. (Shanghai, China). All other reagents were of analytical grade.
Extraction of SPI
SPI was prepared by isoelectric
pH changes
Despite being a basic amino acid, l-histidine has an amphoteric imidazole ring as the side chain group that can both donate and accept protons. The pKa of the imidazole ring is 6.04, which gives l-histidine an isoelectric point of 7.59 (Haynes, 2014). Under the test conditions, l-histidine consumes protons and remain protonated, which explains the rise in pH with an increased l-histidine concentration (Table 1). As expected, the pH-elevating effect of l-histidine was less prominent in
Conclusion
In conclusion, the l-histidine treatment enhanced SPI solubility and decreased turbidity of the dispersion, particularly at 0.1 M NaCl. A reduced surface sulfhydryl groups by l-histidine was also observed at 0.1 M NaCl. In addition, l-histidine decreased surface hydrophobicity of the SPI regardless of the salt concentrations. It destabilized the tertiary structure of the SPI in the absence of salt while prevented unfolding of the SPI at 0.1 M and 0.6 M NaCl. These physicochemical changes
CRediT authorship contribution statement
Yaosong Wang: Project administration, Writing – review & editing, Funding acquisition. Tianyi Ma: Conceptualization, Methodology, Data curation, Writing – original draft. Changqi Liu: Writing – review & editing. Fengxian Guo: Conceptualization, Methodology, Resources. Jing Zhao: Supervision, Project administration, Formal analysis, Data curation, Visualization.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (Grant No. 32001746 and No. 31401530) and Natural Science Foundation of Fujian Province (No. 2018J05064).
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