Effects of Alkaline Extraction pH on Amino Acid Compositions, Protein Secondary Structures, Thermal Stability, and Functionalities of Brewer’s Spent Grain Proteins

A high alkaline pH was previously demonstrated to enhance the extraction yield of brewer’s spent grains (BSG) proteins. The effects of extraction pH beyond the extraction yield, however, has not been investigated before. The present work examined the effects of extraction pH (pH 8–12) on BSG proteins’ (1) amino acid compositions, (2) secondary structures, (3) thermal stability, and (4) functionalities (i.e., water/oil holding capacity, emulsifying, and foaming properties). The ideal extraction temperature (60 °C) and BSG-to-solvent ratio (1:20 w/v) for maximizing the extraction yield were first determined to set the conditions for the pH effect study. The results showed that a higher extraction pH led to more balanced compositions between hydrophilic and hydrophobic amino acids and higher proportions of random coils structures indicating increased protein unfolding. This led to superior emulsifying properties of the extracted proteins with more than twofold improvement between pH 8 and a pH larger than 10. The extraction pH, nevertheless, had minimal impact on the water/oil holding capacity, foaming properties, and thermal denaturation propensity of the proteins. The present work demonstrated that a high alkaline pH at pH 11–12 was indeed ideal for both maximizing the extraction yield (37–46 wt.%) and proteins’ functionalities.