Feasibility of using a realistic food bolus for semi-dynamic in vitro gastric digestion of hard cheese with pH-stat monitoring of protein hydrolysis.

Oral processing of solid foods leads to boluses made of a human saliva and particles distributed in the size range ∼ 0 to 5 mm. However, studies on the release of nutrients from realistic solid food boluses during digestion are scarce because such mechanisms are difficult to investigate in vivo, and in vitro experiments generally recommend to extensively mince solid foods during the oral stage. Similarly, it has previously been shown that the peptic hydrolysis of protein solutions during in vitro gastric digestion can be monitored by acid titration in both static and dynamic pH conditions, but such approach has never been evaluated in the presence of particles of several millimetres in size. The first objective of the study was therefore to test the feasibility of using a realistic food bolus for gastric digestion studies with a pH-stat monitoring of proteolysis, using Emmental cheese as a solid food and with consideration of gastric acidifying kinetics. Degree of hydrolysis (DH) of proteins was monitored from two series of experiments performed in the presence and absence of pepsin. Other DH measurements, estimated from an independent approach based on the amount of free NH groups (OPA method) contained by peptides released in the supernatant (UV absorbance) validated the pH-stat results. A second objective of this work was to test the possible influence of human saliva on gastric proteolysis (in comparison with a water-based bolus). Results showed that saliva slightly delayed initiation of proteolysis, which could be explained by the slightly higher initial pH of the saliva-based bolus, but had no statistical effects on pepsin activity. We conclude that acid titration with a pH-stat system can be a valuable approach to monitor the gastric in vitro proteolysis of realistic solid food boluses in dynamic pH conditions.