Background: This study aimed at investigating the activity of porcine pepsin on egg white proteins (EWP) and casein micelles (CA) over a broad range of pH (from 1 to 7) for short (3 min) and long (2 h) digestion times. Methods: Static in vitro gastric digestions were conducted at different pH on both substrates. Degrees of hydrolysis (DH) were determined using the OPA and pH-stat methods. Results: At short time, different pH activity profiles were obtained for both substrates. Remarkably, the DH of CA after 2 h was constant from pH 1 to pH 5, and only reduced by half at pH 6. This demonstrates that pepsin can hydrolyse caseins from the very beginning of gastric digestion. The shape of the reaction kinetics over 2 h also appeared rather characteristic of the substrate and largely independent on pH. Hydrolysis profiles could be accurately fitted by a power law, an empirical model that also proved very well adapted to other data of ours. Conclusions: Our findings suggest that pepsin activity under weakly acidic conditions (pH ≥ 4) should not always be neglected, in particular for milk caseins, and that pepsin reaction kinetics are proportional to the power of the digestion time.