5. Pepsin–trypsin simulated gastro-intestinal digestion A pepsin–trypsin in vitro system simulating gastro-intestinal digestion was carried out essentially according to the method of Zhu, Chen, Tang, and Xiong (2008). The peptides extracted from Jinhua hams were adjusted to pH 2.0 with 1 M HCl. Pepsin was then added to a level of 4% of the peptide contents. The reactionmixture was incubated at 37 °C for 2 h and then the tubes were placed in boiling water for 10 min to terminate the digestion. Then the samples were cooled to room temperature and centrifuged at 5000 g for 15 min. The supernatant was divided into two parts. One was lyophilized, and the other was adjusted to pH 5.3 with 0.9 M NaHCO3 solution and then to pH 7.5 with 1.0 M NaOH. Trypsin was added (4% of the peptide contents), and repeated the above performance. The supernatant was lyophilized, sealed in plastic bags and stored at 4 °C until use.
2. Amino acid analysis