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

Abstract

English: 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.

Afrikaans: Melk flokkulasie/ouderdomsgelering is huidiglik 'n groot probleem vir die suiwel industrie aangesien dit 'n negatiewe impak het op die kwaliteit van melk. Flokkulasie is die fisiese verandering wat plaasvind in melk as gevolg van blootstelling aan ekstreme destabilisasie toestande soos lae opbergings temperature en blootstelling aan hitte (melk wat by kookwater gevoeg word tydens bereiding van koffie). Die eind resultaat is die afname in vloeistof en terselfde tyd „n toename in viskositeit wat gevolglik lei tot die vorming van 'n drie-dimensionele proteïen netwerk. Dit lei daartoe dat melk flokkies bevat wat verbruikers onaanvaarbaar vind en verwerp. Flokkulasie kan deur middel van chemiese- of ensiematiese aksie plaasvind. Die chemiese aksie word toegeskryf daartoe dat die drie-dimensionele proteïen netwerk gevorm word as gevolg van interaksies wat plaasvind (tussen β-LG en К-kaseïen) tydens die opberging van melk. Die interaksies vind binne die kaseïen missel plaas as gevolg van hitte behandelings wat op melk uitgevoer word. Tydens hierdie interakies word 'n kompleks gevorm tussen β-LG en К-kaseïen. Die ensiematiese aksie behels die vrystelling van hierdie kompleks wat gevolglik lei tot die vorming van 'n gel. Proteases van mikrobiese oorsprong is hiervoor verantwoordelik. Daar is hoofsaaklik twee ensieme wat 'n rol speel by melk flokkulasie, naamlik plasmien en bakterieë van 'n psigotrofiese oorsprong. Opsporings metodes vir melk flokkulasie is noodsaaklik om ten einde die oorsaak te kan bepaal en moontlik 'n oplossing te vind vir die probleem. Tegnieke wat reeds ontwikkel is vir die opsporing van melk flokkulasie is die Alizarol toets en die protease toets. Die tegnieke wat ontwikkel is met die doel om flokkulasie/gelering te bepaal sluit die RP-HPLC (MILQC sagteware) en die melk agar plaat tegniek vir protease bepaling in. Al die bogenoemde tegnieke kan hoë risiko melk wat geneig is tot flokkulasie/gelering effektief aandui en sommige van die tegnieke kan selfs 'n onderskeid tref tussen plasmien en mikrobiese protease.