Study On The Preparation And In Vitro Digestion Of Whey Protein Isolate Emulsions/Cold-set Gels

Food protein has a number of important features such as emulsifying, ligand binding and gelation properties. Their assemblies are widely used as delivery systems for bioactive compounds in aqueous food products. There is a growing interest among food scientists on oil- in- water(O/W) emulsions and their assemblies. Clarify the changes and mechanisms of protein-stabilized emulsions in physical properties during digestion will be conducive to design and develop emulsion-based delivery system. In addition, the development of protein cold-set gels overcomes the limitations of heat- induced gels and can be applied as carriers for heat-sensitive molecules and bioactive compounds. However, there are few reports on nano-/micro- gels while protein cold-set gels have been widely studied. Design and develop novel protein assembly system by combining its emulsifying and gelation properties will be beneficial to expand the development and utilization of protein resources. The objective of this study was to investigate the preparation and in vitro digestion of emulsions/cold-set gels prepared by using whey protein isolate(WPI).Firstly, stable O/W emulsion was prepared by using WPI as an emulsifier. The changes in the physicochemical properties of emulsions occurring during digestion in simulated gastric fluid(SGF) or simulated intestinal fluid(SIF) under various conditions were analysed. The presence of PBS led the emulsion become more stable. Adding bile salts made the ζ-potential become more negative and the content of interface protein decreased, which indicated that replacement happend on the interface layer. In acidic simulated gastric conditions, WPI-stabilized emulsions showed resisitance to pepsin over time during 6.5 h of gastric digestion. In simulated intestinal conditions, deeper hydrolysis took place in WPI-stabilized emulsions, leading to a change in SDS-PAGE. After sequential digestion in SGF and SIF, protein-stabilized O/W emulsions were completely hydrolysis. Microdroplets formed as oil droplets aggregated. The encapsulation efficiency of vitamin E or resveratrol was 76.66% and 39.00%,respectively. After sequential digestion in SGF and SIF, vitamin E was significantly reduced while resveratrol was much more stable in intestinal condition. Compared the decreasing rate in digestive solution and subnatant, it can be found that emulsion protected bioactive compounds during the gastrointestinal digestion.To further enhance the encapsulation efficiency of bioactive compounds, we choose protein gels as carrier systems. The effect of preheating te mperature and concentration of protein or Ca2+ on WPI aggregation was discussed to find the optimum conditions of preparing WPI microgels as delivery systems. The results showed that, WPI microgel can be formed as pre- heating temperature above 65℃. Microgels obtained at a concentration of 5 mmol/L CaCl2 has uniform particle size distributions after pre-heating at 75℃ and 85℃.Finally, WPI self- supporting emulsion gels were prepared by using a cold gelation method. The impact of Ca2+ concentration and oil content was investigated. Study found that improve the oil content can significantly reduce the gelation time, syneresis, water holding capacity(WHC) and gel loss rate of WPI emulsion gels and enhance gel hardness and adhesiveness. The structural properties of WPI emulsion gels did not affect by Ca2+ concentration when it higher than 20 mmol/L. Encapsulation can be efficiently achieved by WPI cold-set gels and emulsion gels. The encapsulation efficiency of resveratrol in cold-set gel was more than 98%. After digestion in SGF, gels were substantially digested which may led resveratrol to be released entirely.