Role of protein and lipid oxidation in hardening of high-protein bars during storage

Protein bar hardening negatively impacts shelf life, quality, and consumer acceptance. Although oxidation is known to negatively affect the flavor and texture of foods, the specific roles of lipid and protein oxidation in bar hardening have not been thoroughly investigated. Furthermore, most research has concentrated on dairy proteins, with a notable lack of studies addressing the hardening of plant-based protein bars. We investigated the role of protein and lipid oxidation, Maillard reactions, moisture loss, protein aggregation, and microstructural changes in the hardening of pea, whey, and rice protein bars over a storage period of 6 weeks (hardness increased 7.2×, 5.4×, and 4.4×, respectively). Changes in tryptophan fluorescence, free sulfhydryl content (e.g., loss of 57% for pea and 44% for whey), and carbonyl content demonstrated that pea and whey bars underwent protein oxidation. Lipid oxidation also occurred, demonstrated by increased peroxide and thiobarbituric acid-reactive substance values. Rice bars, however, did not undergo oxidation. Mass spectrometry indicated greater Maillard-reaction-related protein glycations formed in pea and whey bars (6.9% and 7.7%, respectively) than in rice bars (2.1%). SDS-PAGE revealed that pea and whey, but not rice, proteins aggregated during storage. Overall, this study found that moisture loss, protein and lipid oxidation, Maillard reactions, and protein aggregation correlated with bar hardening. Chemical changes may cause protein aggregation, resulting in hardening. Likely because of rice proteins' innate insolubility and disulfide linkages, rice protein bars were less susceptible to chemical changes and aggregation and hardened more slowly than whey and pea protein bars. PRACTICAL APPLICATION: This study shows that lipid and protein oxidation are correlated with protein bar hardening in both pea and whey protein bars. Additionally, this work suggests that rice protein bars may harden more slowly than pea and whey bars. These findings suggest that potential strategies to prevent bar hardening and extend shelf life include (1) adding antioxidants to prevent oxidation and (2) using rice proteins to partially or fully substitute other protein isolates.