A sequential fractionation approach to understanding the physicochemical and functional properties of aqueous and enzyme-assisted aqueous extracted black bean proteins

Black bean proteins were sequentially extracted and characterized to determine the contribution of protein classes to the extractability, thermal behavior, and functionality of protein extracts produced by aqueous (AEP) and enzyme-assisted aqueous extraction processes (EAEP). Osborne fractionation showed that 56% of black bean proteins were water-soluble (albumin-rich), 22% salt-soluble (globulin-rich), 16% alkaline-soluble (glutelin-rich) and 0.65% alcohol-soluble (prolamin-rich). Protein extraction yields of 75 and 81% were achieved by AEP (pH 9.0, 50 °C, 1:10 solids-to-liquid ratio, 60 min) and EAEP (same conditions as AEP with 0.5% (w/w) alkaline protease), respectively. Gel electrophoresis demonstrated differences in the protein molecular weight profile in each fraction, as well as extensive proteolysis of major storage proteins in the EAEP. Proteomics revealed that similar proteins were present among the albumin-, globulin-, and glutelin-rich fractions, but in different proportions to yield distinct protein profiles. AEP and EAEP proteins had similar denaturation temperatures (T_d = 92 °C) but were lower than the albumin- and globulin-rich fractions. There was not a clear association between solubility, emulsifying, and foaming properties for the protein classes, which highlighted the complex nature of the protein extracts, as well as potential impacts of the honogemization process on the surface properties of the proteins. Proteolysis significantly increased solubility at pH 4.5 (50 vs. 93%) and decreased foaming capacity at pH 4.5 and 7.0, while emulsifying capacity was not affected. Overall, the results demonstrate that enzyme-assisted extraction is a feasible method to increase black bean protein extractability while retaining thermal stability and modifying some functional properties.