Elsevier

LWT

Volume 152, December 2021, 112382
LWT

l-Histidine improves solubility and emulsifying properties of soy proteins under various ionic strengths

https://doi.org/10.1016/j.lwt.2021.112382Get rights and content

Highlights

  • Histidine improved soy protein solubility regardless of salt concentrations.

  • Histidine treatment reduced reactive sulfhydryl groups of soy proteins.

  • Histidine decreased surface hydrophobicity of soy proteins.

  • Histidine has different effects on protein structure at different ionic strengths.

  • Histidine improved the emulsifying properties of soy proteins.

Abstract

Soy protein is a commonly used functional ingredient. Despite its versatility, modifications of soy proteins could further enhance its functionalities. The objective of this study is to investigate the effect of l-histidine on physicochemical and functional properties of soy protein isolate (SPI) in the presence of 0, 0.1, and 0.6 M NaCl. The incorporation of l-histidine improved SPI solubility and decreased turbidity regardless of the salt concentrations. Notably, at 0.1 M NaCl, the incorporation of 0.3% l-histidine more than doubled the SPI solubility. Reactive sulfhydryl groups and surface hydrophobicity of the SPI decreased up to 24.2% and 38.6%, respectively, by the l-histidine treatment. The SPI-histidine interactions facilitated exposure of tryptophan residues in the absence of salt while mitigated salt-induced protein unfolding. With no salt, 0.3% l-histidine improved the emulsifying activity and stability of the SPI by 11.5% and 9.2%, respectively. At 0.1 M NaCl, 0.3% l-histidine did not improve the emulsifying activity but increased the emulsion stability by 30.1%. At 0.6 M NaCl, 0.3% l-histidine induced a 25.4% increase in emulsifying activity while no change in emulsion stability. In conclusion, l-histidine can be used as a functional ingredient to enhance the performance of SPI in various food formulations.

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).

References (31)

  • A. Sarkar et al.

    Emulsion stabilization by tomato seed protein isolate: Influence of pH, ionic strength and thermal treatment

    Food Hydrocolloids

    (2016)
  • Y. Wang et al.

    Modification of heat-induced whey protein gels by basic amino acids

    Food Hydrocolloids

    (2020)
  • Y. Zhang et al.

    Heat-induced gel properties of porcine myosin in a sodium chloride solution containing L-lysine and L-histidine

    Lebensmittel-Wissenschaft und -Technologie- Food Science and Technology

    (2017)
  • X. Zhu et al.

    L-Arginine/L-lysine improves emulsion stability of chicken sausage by increasing electrostatic repulsion of emulsion droplet and decreasing the interfacial tension of soybean oil-water

    Food Hydrocolloids

    (2019)
  • N. Alizadeh-Pasdar et al.

    Comparison of protein surface hydrophobicity measured at various pH values using three different fluorescent probes

    Journal of Agricultural and Food Chemistry

    (2000)
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