The proteolytic activity in raw milk and the effect of such activity on the stability of milk proteins

Milk flocculation/age gelation is regarded as a major problem for the dairy industry since it has a negative impact on milk quality. Flocculation can be observed as a physical change in fresh milk when milk is exposed to extreme destabilisation conditions such as low storage temperatures and heat exposure (milk added to boiling water during coffee preparation). The end result is decreased fluidity and increased viscosity due to the formation of a three-dimensional protein network and the formation of visible flakes. This type of milk is totally rejected by the consumers. Flocculation can occur through chemical or enzymatic action. The chemical mechanism is when the three-dimensional protein network is formed during the storage of milk by the interaction between β- LG and К-casein within the casein micelle due to heat treatment which eventually results in the formation of a gel. During this interaction, a complex is formed between β-LG and К-casein. The enzymatic mechanism involves proteases which are responsible for the release of this βК-complex which forms a protein network and eventually results in the formation of a gel. The two main enzymes that play a role in milk flocculation are native plasmin and proteases from psychrotrophic bacteria. Detection methods for milk flocculation are needed in order to establish the cause and possibly combat this problem. The already established milk flocculation detection techniques are the Alizarol test and the protease assay. The techniques developed in this study to detect milk flocculation/age gelation included RP-HPLC (MILQC software) and the milk agar plate technique for protease detection. All the techniques can effectively detect high risk milk prone to flocculation/gelation and some of the techniques can even distinguish between the proteolytic action of indigenous plasmin and microbial proteases.