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.