Effects of Different Industrial Heating Processes of Milk on Site-Specific Protein Modifications and Their Relationship to in Vitro and in Vivo Digestibility

Heating processes are applied to milk and dairy products to ensure their microbiological safety and shelf lives. However, how differences in "industrial" thermal treatments affect protein digestibility is still equivocal. In this study, raw milk was subjected to pasteurization, three kinds of ultra-high-temperature (UHT) treatment, and in-can sterilization and was investigated by in vitro and in vivo digestion and proteomic methods. In-can sterilized milk, followed by UHT milk samples, showed a rapid decrease in protein bands during the course of digestion. However, protein digestibility determined by a Kjeldahl procedure showed insignificant differences. Proteomic analysis revealed that lactulosyllysine, which reflects a decrease in protein digestibility, in α-lactalbumin, β-lactoglobulin, and caseins was higher in in-can sterilized milk, followed by UHT milk samples. Thus, industrial heating may improve the digestibility of milk proteins by denaturation, but the improvement is likely to be offset by heat-derived modifications involved in decreased protein digestibility.