Pepsin activity as a function of pH and digestion time on caseins and egg white proteins under static in vitro conditions

The activity of pepsin, the gastric protease, is generally considered to be negligible for pH >= 4, based on the results obtained with a few purified globular proteins. The present study aimed at studying the activity of porcine pepsin on egg white proteins (EWP) and casein micelle micro-aggregates (CA) over a broad range of pH (from 1 to 7) for short (3 min) and long (2 h) digestion times. For a short time, the results confirmed a tendency for a higher rate of hydrolysis with decreasing pH, but with different pH activity profiles for both the substrates. More remarkably, the degree of hydrolysis of CA after 2 h of digestion was constant from pH 1 to pH 5, and was only reduced by half at pH 6. This finding demonstrates that pepsin can hydrolyse caseins from the very beginning of gastric digestion. Interestingly, the trend of the reaction kinetics over 2 h appeared to be rather characteristic of the type of the substrate and was largely independent in terms of pH. Most hydrolysis profiles could be accurately fitted by a power law, an empirical model that was then successfully applied to the static in vitro gastric proteolysis of 6 other food matrices. Overall, our results support the idea that pepsin activity under weakly acidic conditions (pH >= 4) should not always be neglected, in particular, for milk caseins, and that pepsin reaction kinetics during static in vitro gastric digestion seems to evolve proportionally to the power of the digestion time.