Susceptibility to trypsinolysis of esterified milk proteins

Methyl-, ethyl- and propyl-esters of beta-lactoglobulin, alpha-lactalbumin and beta-casein were prepared and then hydrolyzed with trypsin in various conditions. Resulting hydrolysates were analysed by SDS electrophoresis and RP-HPLC. The degree of hydrolysis of esterified samples was generally lower than those of the non-modified proteins. The highest degrees of hydrolysis were obtained at pH 7--8 with native and esterified protein samples. beta-Lactoglobulin propyl ester and beta-casein methyl ester yielded the lowest degrees of hydrolysis. Ethyl- and propyl-esters of beta-casein showed high resistance towards tryptic attack, even after 20 h of hydrolysis. SDS electrophoretic patterns of tryptic hydrolysates of native proteins showed bands corresponding to low molecular weights. Tryptic hydrolysates of esterified proteins showed bands with higher sizes. RP-HPLC profiles of tryptic hydrolysates of esterified samples showed peaks with longer elution times than those obtained with native proteins, indicating the presence of more hydrophobic peptide populations. A peptic pre-treatment improved tryptic action on esterified proteins. It resulted in a better resolution of RP-HPLC profiles and in a complete disappearance of the protein after 20 h tryptic hydrolysis.