Hydrolysis of milk proteins by lactic acid bacteria leads to the formation of a large number of peptides. In this work, the hydrolysis of beta-casein by the protease PrtP(1) of Lactococcus lactis was studied. Experiments were carried out at different initial enzyme/substrate ratios. Identification and quantification of peptides were performed by MS and RP-UHPLC analyses. Nine low molecular weight (LMW) peptides were quantified absolutely. Additionally, semi-quantification of six high molecular weight peptides (HMW) was provided. To describe the dynamics of peptides concentrations, an aggregated model was developed. This model links peptide formation to the breakdown of intact protein by introducing the concept of virtual intermediate peptides (VIP). The model represented the experimental data with an average error of 14% (comparable with the experimental error). By using the model, three dynamic pools of peptides were identified. The model suggests that LMW peptides have similar dynamic characteristics as their counterpart HMW peptides in the beta-casein sequence. This study indicates that the presence and structure of micelles affect the hydrolysis dynamics and that, for some peptides, the enzyme/substrate ratio appears to affect the hydrolysis stoichiometry. The model developed is parsimonious and has a basic mechanistic component. It allows for a rational study of protein hydrolysis. (C) 2011 Elsevier Ltd. All rights reserved.