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

[摘要] English: Milk flocculation/age gelation is regarded as a major problem for the dairy industry since it has anegative impact on milk quality. Flocculation can be observed as a physical change in fresh milkwhen milk is exposed to extreme destabilisation conditions such as low storage temperatures andheat exposure (milk added to boiling water during coffee preparation). The end result is decreasedfluidity and increased viscosity due to the formation of a three-dimensional protein network and theformation 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 thethree-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 theformation of a gel. During this interaction, a complex is formed between β-LG and К-casein. Theenzymatic mechanism involves proteases which are responsible for the release of this βК-complexwhich forms a protein network and eventually results in the formation of a gel. The two mainenzymes that play a role in milk flocculation are native plasmin and proteases from psychrotrophicbacteria.Detection methods for milk flocculation are needed in order to establish the cause and possiblycombat this problem. The already established milk flocculation detection techniques are the Alizaroltest and the protease assay. The techniques developed in this study to detect milk flocculation/agegelation included RP-HPLC (MILQC software) and the milk agar plate technique for proteasedetection. All the techniques can effectively detect high risk milk prone to flocculation/gelation andsome of the techniques can even distinguish between the proteolytic action of indigenous plasminand microbial proteases.

[发布机构] University of the Free State