Abstract: The effect of heat treatment on the nutritional value of fresh raw milk and reconstituted milk were studied by in vitro simulated digestion method. The treated samples were detected by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), size exclusion high performance liquid chromatography, scanning electron microscopy and mass spectrometry. The results showed that some milk proteins were broken down during simulated gastric digestion, and casein showed the highest degree of breakdown. The level of digestion of casein in fresh milk by the gastric juice was higher than that of other heat-treated samples. After digestion with the intestinal juice, milk proteins were completely broken down into free amino acids and small peptides, and the digestibility of fresh milk was higher than that of other heat-treated samples. Milk proteins were mainly digested into peptides with molecular weight of less than 1 500 Da, which can be easily absorbed by the body. It was observed by scanning electron microscopy (SEM) that heat treatment induced denaturation and aggregation of milk proteins and that the particle size of reconstituted milk was larger than that of other milk samples with increasing heating temperature. Analysis by mass spectrometry (MS) and high performance liquid chromatography (HPLC) revealed that greater degree of heat treatment resulted in higher reaction degree in Maillard reaction.

Key words: heat treatment, in vitro gastrointestinal digestion, digestibility, peptide, Maillard reaction