Analysing para-κ-casein and related peptides as indicators of milk proteolysis

Abstract

A new capillary electrophoresis (CE) method was used to analyse raw, pasteurised and UHT milk samples. Some para-κ-casein-related peaks were observed in raw and pasteurised milks stored at 6°C for 5 or more days, and in UHT milks stored at 20°C for 30, 60 or 90 d. The presence of these peaks was attributed to the action of proteases from psychrotrophic bacteria on κ-casein. In the CE analysis, the area of pure κ-casein digested with a proteinase-containing cell-free extract from Pseudomonas fluorescens B52 was gradually reduced and several peaks with migration times close to para-κ-casein like those found in the stored milks, appeared. On the other hand, in the sample of κ-casein treated with chymosin, only one peak corresponding to para-κ-casein was formed. The analysis of the tryptic digests of these samples by high performance liquid chromatography (HPLC) coupled to electrospray mass spectrometry (ES-MS) revealed the presence of fragments 1-105, 1-103, 1-104, 1-106 and 1-107 of κ-casein (para-κ-casein and other related peptides) in the sample of this protein treated with the P. fluorescens B52 proteinase. Their separation by CE permits them to be used as indicators of proteolysis in the casein fraction of milks.