Moderate esterification induces slight secondary structure changes in two major milk proteins, beta-lactoglobulin and beta-casein. Esterification of beta-lactoglobulin prompts its tertiary structure 'melting', opening it to peptic cleavage. Twenty-two new cleavage sites were characterised in beta-lactoglobulin and five in beta-casein. Some of them are due to esterification-improved peptide bond accessibility, some to the bias of pepsin specificity by glutamate and aspartate esters. The resulting fragmentation yields original and partially amphiphilic peptide populations.