Deamidation of soy protein by protein-glutaminase: Process evaluation and effect of deamidation on protein functional properties and flavor-protein interactions

Flavor is a major determinant of the consumer acceptance of a food product. The availability of a flavor compound for sensory perception is greatly influenced by its interaction with non-volatile food constituents including fats, carbohydrates and proteins. The binding of flavor compounds to soy protein can be problematic since it can lead to flavor fade (loss of flavor or lowering flavor intensity) and hence a decline in product quality. These flavor-protein binding interactions can be altered by changing the conformation of the proteins. While chemical deamidation of soy protein isolate was previously found to decrease flavor-protein binding, the use of an enzymatic method for deamidation is generally more desirable since it is substrate specific, can be conducted under mild reaction conditions, and is perceived as natural and safe. Optimization of the enzymatic deamidation of soy protein isolate (SPI) by protein-glutaminase (PG) was successfully carried out using response surface methodology (RSM) to obtain a deamidated SPI with high degree of deamidation (DD) and an acceptably low degree of hydrolysis (DH). The deamidated SPI had enhanced solubility in both acidic and neutral conditions, improved emulsification properties, increased foaming capacity, but decreased foaming stability over the resting time. The effects of PG deamidation on flavor binding properties of SPI under aqueous conditions were evaluated by a modified equilibrium dialysis technique. It was found that partial deamidation (43.7% DD) decreased overall binding affinity for selected carbonyl containing flavor compounds (vanillin and maltol). The thermodynamic parameters of binding indicated that the flavor-protein interactions were spontaneous and that the nature of the interactions shifted from entropy to enthalpy driven after deamidation. Deamidation of soy protein appears to change the mechanism of binding from hydrophobic interactions and/or covalent bonding (Shiff-base formation) to weaker van der Waals forces or hydrogen bonding. The effect of PG deamidation on protein solubility and flavor binding potential of soymilk was studied. The sensory characteristics on aroma of deamidated soymilk (DSM) did not differ from those of the control soymilk (treated without PG; CSM). Protein solubility in the DSM was enhanced at weakly acidic conditions (pH 5.0). DSM had lower flavor binding potential than the CSM as evidenced by the fact that the odor detection thresholds for the flavor compounds vanillin and maltol were approximately 5 and 3 fold lower, respectively, in DSM than in CSM. The sigmoidal relationship of dose-response curves relating concentration of flavor compounds to aroma intensity demonstrated that DSM had lower flavor binding potential than CSM. The n exponents from Stevens’s power law indicated that vanilla and cotton candy intensities increased, as a function of vanillin or maltol concentration, at a higher rate in DSM than in CSM. The findings of this study can lead to the development of technology to produce proteins with improved functional properties and potentially decreased problems associated with flavor-protein interactions, especially with carbonyl containing flavor compounds. The information about binding mechanisms caused by modification of binding sites in protein by PG will allow the food industry to produce protein ingredients, from soybean as well as from other sources, with improved functional properties and potentially decreased flavor fade problem, especially for use in acidic protein-fortified foods and beverages