Rice dregs are the by-product of rice liquefaction at high temperature to produce starch syrup. The content of rice dreg protein (RDP) in rice dregs is as high as 60%, and its nutritional value is equivalent to rice protein (Amagliani, O'Regan, Kelly, & O'Mahony, 2017; Hou et al., 2017). Rice protein contains approximately 80% gluten, which makes it insoluble under neutral conditions. During the process of high-temperature amylase liquefaction, the rice protein is cross-linked through disulfide bonds to form larger protein aggregates. As a result, the rice protein is seriously denatured in the rice dregs, which is more poorly soluble and more difficult to modify, thereby limiting its application in foods (Zhao et al., 2012).

Many attempts have been made to improve the functional properties of rice protein such as ultrasound (Yang, Wang, Zhang, & Ma, 2020), electron beam irradiation (Li, Wang, Chen, Sun, & Li, 2019), high-pressure homogenization (Wu et al., 2022), glycosylation (Cheng, Wei, Liu, Xu, & Chen, 2021), enzymatic hydrolysis (Xie et al., 2021), and joint modification (He et al., 2021; He, Hu, Woo, Xiong, & Zhao, 2021; Zhang et al., 2021; Zhang et al., 2021). Enzymatic hydrolysis can significantly increase the solubility of rice protein, but the production of bitterness was a concern that must be taken seriously (Murray & Baker, 1952). Physical modification can improve the functional properties of rice protein without bitterness, but the solubilization effect is limited. Currently, there is a growing interest in enhancing protein functional properties using the pH shifting method. This involves simply altering the pH values of protein solutions to severe basic or acidic environments to unfold the protein structure, and neutralizing to refold the proteins. This unfolding-refolding action significantly improved the functional properties of plant proteins, such as rice protein (Dai et al., 2023), soy protein (Jiang, Xiong, & Chen, 2010), alfalfa protein (Nissen et al., 2021), pea protein (Zhu, Huang, Guo, & Chen, 2021) and carob protein isolate (Bengoechea, Ortiz, Guerrero, & Puppo, 2017), and is considered a relatively easy and effective method to enhance protein functionalities.

Some studies in recent years have reported better modification effects of proteins by combining pH shifting and physical processing methods (Jiang et al., 2017; Tan et al., 2021). High-pressure homogenization (HPH) is a simple technology for the modification of proteins with turbulence, cavitation and shear effects, which changes the particle size and conformation of proteins, thus improving the functional properties (Wu et al., 2022). In addition, HPH treatment during pH shifting reduced the quinoa protein aggregate sizes and unfolded the structure (Yildiz & Yıldız, 2023). It may be speculated that pH shifting combined HPH treatment efficiently modify the functional properties of RDP by promoting the depolymerization and refolding effect, which has not been reported to date.

Therefore, the objective of the present study was to investigate the potential of pH shifting combined HPH treatment to improve the functional properties of RDP. The structural differences of RDP treated with pH shifting/HPH treatment, pH shifting combined HPH treatment, and HPH combined pH shifting treatment were compared. Emphasis was placed on determining the mechanism of pH shifting at alkaline conditions followed by HPH prior to neutralization to improve the functional properties of RDP. Moreover, the taste of RDP after various treatments was investigated. The results of this study may provide a simple method to improve the functional properties of rice protein and develop the rice dregs processing industry.