In this study we investigated changes in the major proteins of pasteurized milk (85 °C/ 30 s) during storage (at 4 °C for 7 days) and the effects on the milk protein digestibility during in vitro gastric digestion. The results revealed that there were two major noticeable changes of protein interactions. The first was the heat-induced interaction of whey proteins with casein micelles leading to an increase in the micelle size by 17-34 nm. The second appears to be caused by progress protein aggregation that was most evident between 5 and 7 days of storage. The protein aggregation visible by TEM, led to another measurable increase of the micelle size by 18-42 nm, and reduction in the milk flow behavior index. The amounts of κ-CN and β-lg increased gradually in the micelle phase during the storage suggesting that re-association of β-lg/κ-CN complexes with the micelles, resulting in changes in the surface hydrophobicity of proteins in pasteurised milk during storage. An in vitro gastric model was used to investigate the resulting effects of these different store times on protein digestibility. The digesta collected at various time points throughout the gastric digestion were studied using SDS-PAGE, ELISA and LC-MS/MS. Caseins showed a rapid hydrolysis compared to β-lg. There were minor changes in milk proteins after 3 days storage. However, a comprehensive hydrolysis of proteins was observed in pasteurised milk upon 7 days storage. The peptide profiles showed significant differences between the samples. Nevertheless, some common traits after digestion were identified.
Food, Nutrition, and Health
Ph.D. candidate, School of Chemical Engineering and Technology, Harbin Institute of Technology, China
Xing Li is a Ph.D. student in the School of Chemical Engineering and Technology, Harbin Institute of Technology. He received his BS in Food science at Jilin University in 2014. His research is focusd on the dairy chemistry, dairy processing and dairy nutrition.