Allergenicity and functionality of soy protein subjected to limited enzymatic hydrolysis and Maillard-induced glycation

Abstract

Increasing consumer demand for dietary protein coupled with the need for functional proteins is driving the growth of the soy protein market. However, the expanded use of soy protein is limited by its distinction as one of the “Big 8” food allergens. Thus, there exists a need for a functional, hypoallergenic soy protein ingredient. Previous protein modification approaches have demonstrated varying levels of success in producing a functional, hypoallergenic protein ingredient, with limitations including feasibility, consumer acceptability, loss of functionality, and residual immunoreactivity. A potential solution is to combine two controlled modification approaches, limited enzymatic hydrolysis with partial Maillard-induced glycation. We hypothesize that optimization of limited enzymatic hydrolysis and Maillard-induced glycation conditions will reduce the allergenicity of soy protein, while maintaining its functionality. Thus, the objectives of this study were twofold: (1) to optimize limited enzymatic hydrolysis and partial Maillard-induced glycation conditions to produce a hypoallergenic soy protein, and (2) to determine the effect of limited and targeted hydrolysis and further glycation on protein functionality. Limited (< 8%), targeted hydrolysis was achieved using alcalase, pepsin, and an alcalase/pepsin combination to produce, respectively, ASPH, PSPH, and AP_SPH samples with degrees of hydrolysis (% DH) ranging from 3.8 – 6.8%. Two levels of glycation were achieved, with 68 – 75% remaining free amines, for the mild condition and 55 – 68% for the advanced condition. Changes in immunoreactivity was monitored following qualitative, western blot, and quantitative, ELISA immunoassays, utilizing sera from seven individual with high levels (>3.5 kU/L) of soy protein specific IgE. A significantly higher overall L*a*b* color difference and visible browning confirmed that the advanced condition, compared to the mild condition, had propagated to the final stages of the Maillard reaction. To avoid browning, loss in nutritional quality, and formation of advanced glycation endproducts (AGE) associated with the final stages of the Maillard reaction, the mild glycation conditions were selected for functionality testing. The solubility, emulsification capacity (EC), emulsification activity index (EAI), emulsion stability (ES), gel strength, and water holding capacity (WHC) were tested for soy protein isolate (SPI) and hydrolysate (SPH) samples, as well as the glycated samples and glycation controls. Pepsin targeted the hydrolysis of glycinin, and alcalase targeted the hydrolysis of β-conglycinin. Limited enzymatic hydrolysis significantly reduced immunoreactivity, with reductions in immunoreactivity ranging from 20 – 75%, depending on the enzyme used and the variability among the sera tested. Maillard-induced glycation predominately modified the subunits of β-conglycinin, while having a variable effect on immunoreactivity. In some cases, the negative effect of glycation was due to partial denaturation (unfolding), which exposed epitopes originally hidden in interior of the protein. The solubility, at neutral pH and a 2.5% protein concentration, of SPH samples (66 – 68% protein solubility) decreased in comparison, to that of SPI (96% protein solubility). The effect of limited enzymatic hydrolysis and partial Maillard-induced glycation on emulsification and gelling properties was highly variable with factors such as enzyme choice, % DH, peptide profile, and extent and location of glycation influencing the observed results. For example, while hydrolysis reduced the ES and increased EAI, glycation improved the ES and had an opposite effect on EAI. Similarly, both modifications reduced gel strength of soy protein, while the WHC of PSPH and AP_SPH was improved upon glycation. Overall, this study demonstrated that limited enzymatic hydrolysis and partial Maillard-induced glycation, under specific conditions require further optimization to successfully reduce immunoreactivity (> 90% reduction to claim a hypoallergenic product) and improve functionality. Results highlighted the importance to consider and target individual variability in the immunoreactivity with the various modified proteins, when optimizing conditions to produce a hypoallergenic product. Additional research to optimize modification conditions and to understand the structural impact of these modifications on the protein structure is needed to achieve the goal of developing a hypoallergenic, functional soy protein ingredient.