In order to explore the inhibitory effect of bioactive substances on the degradation of milk fat during simulated gastrointestinal digestion, the impact of bioactive substances on the in vitro digestibility of milk proteins was examined as well as the inhibitory effect on pancreatic lipase activity and the micellar solubility of cholesterol. The results showed that the in vitro digestibility of milk proteins was inversely proportional to the concentration of the tested bioactive compounds, which decreased from 90.94% to 68.37% and 86.00% with increasing concentration of epigallocatechin (EGC) and β-glucan, respectively. The inhibitory effect of EGC on pancreatic lipase activity decreased to different extents with increased amount of added milk proteins, and so did the inhibitory effect of β-glucan and a mixed solution of 2.5 g/L β-glucan, 3.0 g/L epigallocatechin gallate (EGCG) and 2.0 g/L EGC on the micellar solubility of cholesterol. The inhibitory effect of this mixture on the degradation of milk fat was stronger than that of β-glucan.

A gas chromatography-mass spectrometry (GC-MS) method was developed for determining β-sitosterol in milk by optimizing important sample pretreatment parameters such as saponification temperature, extraction solvent, extraction number, shaking method during extraction and rotary evaporation temperature. The feasibility of the developed method was examined. This method exhibited a wide linear range and could meet the requirements for routine quantitative analysis of β-sitosterol in milk samples with high precision, good repeatability, accurate results and high reliability.