Problems with detection of proteolytic microorganisms and their undesirable activities in milk

Occurrence of proteolytic enzymes in milk is often associated with technological problems and sensory, rheological and functional defects of final dairy products. Thus, the simple, cost-effective and available laboratory method for evaluation of undesirable proteolysis risk is needed. In our work we have tested cultivation plate methods and chemical methods (formol titration, ammonium reflectometric determination, the Kjeldahl method, the agar-well diffusion assay and spectrofotometry after cleavage of azo-casein) to choose the proper ones which can provide information on undesirable proteolytic changes especially in raw milk. Although the microbiological analyses cannot detect enzymes indigenous to milk, but only the quantity of producers of microbial enzymes, they seem to be the most acceptable, particularly usage of the Glucose-Trypton-Yeast Extract agar with 10% vol. of sterile milk added before pouring onto plates (incubation at 30°C for 72 h). The chemical methods are not sensitive enough to analyse the real milk samples.