Enhancement of Pea Protein Solubility and Thermal Stability for Beverage Applications via Endogenous Maillard-Induced Glycation and Chromatography Purification

Abstract

Growing demands for non-soy, plant protein sources have guided the rapid expansion of the pea protein ingredient market in recent years. Pea protein has emerged as the most prominent alternative to soy protein, as pea is not a major allergen, non-GM, sustainable, and widely available. Accordingly, a variety of pea protein products are now commercially available, suited for a number of different applications. Pea protein generally exhibits inferior functionality compared to soy protein, however, as a result of its intrinsic protein profile and structure, especially following commercial processing. Namely, pea protein exhibits inferior solubility and thermal stability compared to whey and soy proteins, limiting its application in high-protein, RTD beverages. Enhancement of pea protein solubility under acidic conditions and following thermal treatments is, therefore, of interest.Controlled, Maillard-induced glycation is a protein modification technique that has the potential to improve pea protein solubility and thermal stability. While glycation of pea protein has been reported, this process has not been developed for an industrial scale, unreacted carbohydrates are rarely removed, and all previous studies have utilized exogenous saccharides (e.g., pectin, gum Arabic, and corn maltodextrin), presenting concerns regarding application in “clean label” products. The starch-rich by-product of pea protein extraction may be further processed to produce an endogenous reducing saccharide, such as maltodextrin, which may react with protein under controlled glycation conditions. Glycation coupled with purification of partially-glycated pea protein has the potential to produce a highly soluble and thermally stable protein ingredient with added-value, having potential for RTD beverage applications. Therefore, the main objective of this work was to enhance pea protein solubility and thermal stability by producing an endogenously and partially-glycated pea protein (PG-PP) ingredient by completing several goals: (1) develop a method to produce maltodextrin from pea starch with a specific reducing power, (2) initiate and control the early stage of the Maillard reaction to partially-glycate pea protein isolate (PPI) with pea maltodextrin, (3) remove unreacted maltodextrin from the PG-PP via hydrophobic interaction chromatography (HIC) to produce a purified, PG-PP concentrate or isolate, and (4) characterize the effect of glycation coupled with purification on protein structure and the consequent impact on solubility and thermal stability. A method to produce maltodextrin from the pea starch-rich by-product obtained during the production of native pea protein isolate (nPPI) was developed by monitoring maltodextrin dextrose equivalent (DE) in response to hydrolysis time, small saccharide removal, and centrifugation to remove large molecular weight residual starch and fiber. The resulting chain-length distribution was evaluated. Maillard-induced glycation was then confirmed and monitored by assessing changes in color, free amino groups, and protein/glycoprotein profiles upon incubation of the produced maltodextrin with nPPI. Next, removal of unreacted maltodextrin by HIC was confirmed by monitoring maltodextrin elution. The purified PG-PP, along with purified PPI controls and reference samples (nPPI and commercial PPI), were then characterized by assessing their composition and protein/glycoprotein profile. Protein thermal denaturation properties, surface properties (surface hydrophobicity and zeta potential), and secondary structures were evaluated, as well. Lastly, protein solubility and thermal stability and protein digestibility were assessed. The starch-rich by-product was partially hydrolyzed, with hydrolysis conditions optimized to produce maltodextrin with targeted characteristics (DE 15.7, average degree of polymerization 8.3). PPI and maltodextrin were incubated under mild conditions, which initiated and controlled the Maillard reaction to the early stage. PG-PP was formed with minimal browning and protein polymerization, along with moderate free amino group loss. Additionally, PG-PP was purified by removing the majority of unreacted carbohydrates and polymerized proteins via HIC, resulting in a purified water fraction (PW-PG-PP) with a protein content of nearly 60%, reduced surface hydrophilicity, and increased solubility (up to ~91%) and thermal stability at conditions relevant to RTD beverages. Protein digestibility of PW-PG-PP was high and similar to the references. Purification of nPPI control also produced a highly soluble and thermally stable sample with good protein digestibility. HIC removed hydrophobic and polymerized proteins from nPPI, allowing for the fractionation and concentration of hydrophilic proteins in nPPI. This study proved the concept of “clean-label”, endogenous glycation of pea protein, utilizing endogenously produced maltodextrin and controlled Maillard reaction conditions. Additionally, both endogenous glycation coupled with HIC purification, as well and HIC purification alone, greatly improved the solubility and thermal stability of nPPI under acidic conditions and at a high-protein claim concentration (5% protein), with purified samples having nearly the solubility of whey protein, the gold standard for beverages. These processes also largely maintained the digestibility of PPI. Therefore, glycation and HIC purification created pea protein with potential value for application in high-protein, RTD beverages. Moreover, this work uncovered a PPI fractionation process that has the potential to increase specialty pea protein ingredient value, with the water soluble fraction suitable for beverage applications and hydrophobic fraction suitable for meat analogues. This work also provided foundational information, paving the way for future investigation and process optimization for scaled-up glycation and purification of pea protein.