The impact of moisture-induced aggregation of soy protein isolate and hydrolysate during storage on product and nutritional quality

Abstract

With the increased interest in protein-based foods in the United States, soy protein isolate (SPI) and hydrolysate (SPH) have become widely used in the food industry. However, during processing and distribution proteins/peptides tend to aggregate when introduced to increased temperatures and relative humidity. When aggregation occurs in a dry protein powder, there is a decrease in processability, product quality, and consumer acceptability. It is therefore necessary to characterize the covalent and non-covalent interactions involved in moisture-induced aggregation as well as their reaction kinetics to aid in the development of technologies to limit aggregation.The objectives of this study were twofold: (1) to characterize the physicochemical changes of moisture-induced protein/peptide aggregation in SPI and SPH powders during storage as a function of water activity (a_w) and temperature and (2) to determine the nutritional and physiological changes of SPI and SPH during storage when subjected to various aw and temperatures.The effects of a_w and temperature during the storage of SPI, SPH, and a 50:50 mixture on several physicochemical characteristics were investigated. Three temperatures (25, 35, and 45°C) and 8 a_w (ranging from 0.05 to 0.79) were employed for the storage study. Prior to storage, intrinsic properties as well as moisture sorption isotherms were determined for each protein powder. Over time, the color change was assessed in a non-destructive manner; also, samples were collected at predetermined time points to monitor the aggregation and resulting changes. Change in color, % solubility, protein/peptide profile, loss of free amino groups, formation of fluorescent Maillard compounds, and denaturation were monitored using well-established methods of analysis. Reaction kinetics were used when possible to better understand the effects of storage parameters and sample types, while also allowing for better predictions at other storage parameters. Lastly, changes in in vitro digestibility and angiotensin-converting enzyme (ACE) inhibitory activity were assessed to determine protein aggregation's effects on nutritional and physiological changes.Storage at a_w above the monolayer moisture value of the soy protein ingredients resulted in the formation of insoluble aggregates. Hydrophobic interactions, disulfide bonds, and covalent linkages induced by the Maillard reaction lead to the observed aggregation; these interactions were most pronounced at 45°C and a_w>0.59. Even though there was a minimal amount of reducing sugars in the protein powders, the development of fluorescent compounds, change in color, and loss of free amino groups indicated that the Maillard reaction occurred. Soy protein hydrolysate was the most susceptible to the Maillard reaction, but SPI and 50/50 had larger decreases in % solubility over time at a_w>0.33 and 45°C. The aggregates formed during storage, however, did not have a significant effect on the % digestibility and ACE inhibitory activity of soy proteins/peptides. These findings can be used to help predict changes in food matrices with similar a_w during storage for optimal utilization of soy protein ingredients.