Effects of pH and Ionic Strength on the Emulsifying Properties of Brewer's Spent Grain Protein Isolate

Brewer's spent grains, a major by-product of the brewing industry, are rich in protein and hold significant potential as plant-based food ingredients. The objective of this study was to explore the effect of pH (6.0, 7.0, 8.0) and ionic strength (0, 0.1, and 0.6 mol/L NaCl) commonly used in food systems on the physicochemical and emulsifying properties of brewer's spent grain protein isolate (BSGP). Results showed that BSGP was primarily composed of two subunits with molecular weights of approximately 35.0 kDa and 20.5 kDa, linked by disulfide bonds. Increasing the pH led to a significant decrease in hydrophobicity (P<0.05), a reduction in free sulfydryl content, and smaller particle size. Protein solubility and emulsifying properties improved with higher pH (P<0.05). From pH6.0 to pH8.0, solubility increased 1.71-fold, emulsifying activity 1.45-fold, and emulsifying stability 1.37-fold. Within the pH range of 4.0 to 7.0, NaCl improved protein solubility. However, at higher pH (6.0 to 8.0), NaCl induced protein unfolding, disrupted protein-water interactions, increased sulfhydryl content, and reduced protein hydrophobicity and fluorescence intensity. This also led to larger particle size, lower Zeta potential (P<0.05), and weakened emulsifying properties. Overall, adjusting pH and NaCl concentrations effectively improved BSGP solubility and modulated emulsification behavior. This study provides a theoretical basis for the utilization of this protein resource in food applications.