Protein Modifications in High Protein-Oil and Protein-Oil-Sugar Systems at Low Water Activity

Physicochemical and thermal properties of high protein systems during storage at 20 and 40 A degrees C were investigated for 14 weeks. Component interactions of whey protein isolate (WPI)-olive oil (OO), WPI-sunflower oil (SO) (75:25), WPI-(glucose-fructose; G-F) (45:40), WPI-OO-(G-F), and WPI-SO-(G-F) (45:15:40) systems at low water contents during storage were derived from differential scanning calorimetry (DSC), colorimetric, water activity (a(w)), reducing and nonreducing SDS-PAGE electrophoresis data. The degree of unsaturation of oil affected color (yellowness) and microstructure of the systems as well as variations in water migration and nonenzymatic browning kinetics (NEB) during storage. These effects were evident in the SO systems. All systems at 40 A degrees C showed changes in protein conformation to those favoring hydrophobic interactions with oil. These systems showed decreased a(w), insolubilization, hardening as a result of carbonyl-amine polymerization and covalent cross-linking of proteins in the NEB. The DSC data showed a protein hydration transition for rehumidified-WPI, WPI-oil, WPI-sugar, and WPI-oil-sugar. The rehumidified-WPI and WPI-oil also showed a(w)-dependent denaturation endotherms (irreversible transition) for alpha-lactalbumin and beta-lactoglobulin at higher temperatures (T). The WPI-sugar and WPI-oil-sugar showed an exotherm for the browning reaction (irreversible transition) at T-onset similar to 90 A degrees C. An exothermic protein hydration in the systems containing sugar was storage time-dependent, and indicated changes of protein conformation. The presence of oil in WPI-oil-sugar caused an increase in the glass transition of sugars during storage, especially for SO. The WPI-(G-F) and WPI-oil-(G-F) showed broadened glass transition during a reheating scan in DSC that was a result of polymerization in protein, oil, and sugar components mixture. Stability of high protein systems is dependent on hydration and reactions in both hydrophilic and hydrophobic phases.