DETECTION OF PROTEOLYSIS IN RAW-MILK STORED AT LOW-TEMPERATURE BY AN INHIBITION ELISA

An inhibition ELISA (enzyme-linked immunosorbent assay) was developed for the determination of caseinomacropeptide (CMP) in order to estimat e the proteolysis of kappa-casein due to the enzymes of psychrotrophic bacteria in bulls raw milk. The CMP present in milk was quantified sp ecifically by an antibody. The limit of detection was similar to 0.1 m u g/ml and the CV was < 10%. This method was used to study the proteol ytic activity of three strains of psychrotrophic Pseudomonas fluoresce ns in raw milk and to analyse different raw milk samples supplied by f our dairy plants. The proteolytic activity for different strains of ps ychrotrophs and for different milk samples varied considerably, but no correlation was established between the level of microbial flora and kappa-casein proteolysis. It is thus not possible to determine the ext ent of proteolysis from the bacterial count alone. However, by CMP det ermination in bulk raw milk samples after 6 d storage at 4 degrees C, the mean kappa-casein proteolysis was similar to 4%. Among the milk sa mples analysed that contained < 10(7) cfu psychrotrophs/ml, 30% exhibi ted a proteolysis of kappa-casein < 0.5%, i.e. < 5 mu g CMP/ml.