| Being one of the advance technologies in food processing, ultra high pressure processing has received intensive interest in the field of food research. China has listed ultra high pressure processing as one of the top priority technology for research and development for the food industry. Whey proteins is highly digestible and rich in calcium, hence an ideal source in formulating food for the elderly. Whey protein, a major by-product from cheese processing, is often discarded as wastewater effluent and thus cause serious problem to environmental pollution. In the present study, high pressure processing was used to treat whey protein isolate for forming gels; the in vitro protein digestibility and calcium bioavailability of the high pressure-induced gels were studied.The digestibility of high pressure-induced whey protein (14%,w/v) gel was (78.28%, 400 MPa, 30 min) significant higher than those of native whey protein (64.25%, 14%,w/v) and heat-induced gel (58.50%, 80℃,30min) (p﹤0.05). Results from electrophoresis (SDS-PAGE and Tricine-SDS-PAGE) showed that, bovine serum albumin(BSA) andα-lactalbumin were easily digested by pepsin but notβ-lactoglobulin, however, was digested in later stage by trypsin; Pepsin digestion resulted in peptides with molecular weight between 1000-6000 Da; digestion with trypsin further reduced the molecular weight of the peptides. Size exclusion chromatography analysis also showed that, compared to native whey protein and the heat-induced gel, the hydrolysates derived from high pressure-induced gel contained lower amount of peptides with molecular weight between 800-1300Da. The results from in vivo method were in agreement with those from in vitro method.The level of high pressure (200-600MPa), pressuring time (10-30min) and pH value of the whey protein solution (pH5.0-9.0) exerted significant effect on the in vitro protein digestibility (p﹤0.05). Increment in pressure, pressuring time and pH value, enhanced the in vitro protein digestibility of the high pressure-induced gels. Results from scanning electron microscopy showed that there were significant differences in the microstructure of the high pressure-induced gels. Hence, the differences in microstructure among the high pressure-induced gels might result in the variation of protein digestibility as observed.The calcium bioavailability of high pressure-induced whey protein (14%,w/v) gel (400 MPa, 30 min) was 15.18%, which was significantly higher than those of native whey protein (12.08%,14%,w/v) and heat-induced whey protein gel (11.76%,80℃, 30min). Size exclusion chromatography on the dialysates generated three peaks, where the second peak (corresponding to a molecular weight of 5 000-6 000 Da) demonstrating the highest calcium level. This dialysate fraction was found to contain the highest level of carboxyl and imidazole groups.No significant differences were found in the calcium bioavailability of gels formed under different pressures (200-600MPa) (p﹤0.05). After supplementing the high pressure induced-gels (200-600MPa) with 0.015 M calcium ion, the calcium-binding capacity of the respective dialysates from high pressure-induced gels was significant enhanced (p﹤0.05). The higher calcium binding capacity of the dialysates, however, did not lead to better calcium bioavailability, as values of calcium bioavailability was significant reduced(p﹤0.05).WPI/chocolate mixed gels were prepared by adding whey protein (12-20%) to chocolate powers (12%) followed by pressurizing at 400MPa for 30min. Significant differences were observed in the textural and sensory properties among the high pressure-induced WPI/chocolate mixed gels. The high pressure-induced WPI/chocolate mixed gels with 18% of whey protein was the most acceptable. The microbiological quality (total numbers of colony, coliform, mold, yeast numbers of pathogenic bacteria) conformed to the national standard(GB 9678.1-94).The present study showed that, the high pressure-induced whey protein gels demonstrated good protein digestibility and calcium bioavailability, and is an ideal source for development foods for the elderly. so it can be developed as one kind of food.
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