PROGRESS WITH STUDIES TO ADDRESS THE PROBLEM OF MILK FLOCCULATION

The investigation to the causes of milk flocculation is an important R & D Programme of Milk SA. One of possible causes earlier discussed and which is further investigated, is the high correlation between potassium in the diet of the cow and the alizarol test. This points to K⁺ fertilization of pastures. In this contribution the in-milk relationship of various compositional or added substances with protease activity is taken under the loop. The protease activity results in protein breakdown and depending on severity and time, may end in flocculation. These studies are done at the UFS with Dr Koos Myburgh as project leader.

Objective 1: To investigate the effect of the fat content on milk flocculation: The question was whether the fat could act as a hindrance for the proteolytic enzymes. To vary the fat content fat-free, low fat and full cream UHT milk were used. Bacillus and plasmin proteolytic enzymes were added to the milk. They represent respectively the protease activity from bacterial contamination and the inherent activation of plasminogen in the milk itself. The results showedthat when the fat-free milk was proteolysed by the Bacillus enzyme, it was more susceptible to proteolytic attack than low fat and full cream milk. However, when fat-free milk is proteolysed by the plasmin enzyme, it was less susceptible to proteolytic attack, whereas full cream milk was more susceptible to proteolytic attack. This appears contradictory and will be followed up.

Objective 2: To stabilize milk with high proteolytic activity but does not flocculate with the alizarol test. Thus, the objective was to lower the susceptibility of milk protein towards proteolytic attack. The first section of the work included the addition of the chemicals Sodium hexametaphosphate (SHMP), Ethylenediaminetetra-acetic acid (EDTA), calcium chloride and sodium citrate in different concentrations to fat-free UHT milk before adding the Bacillus and plasmin proteolytic enzymes. The results indicated a significant reduction in enzyme activity when the chemicals (EDTA, CaCl, SHMP and Sodium Citrate) were added.  The reduction in activity occurred in the HPLC response curve regions where the Bacillus enzyme is most active. The addition of SHMP reduced the enzyme activity the most, followed by sodium citrate, calcium chloride and finally EDTA.  When the milk was treated with plasmin enzyme, there was a more pronounced reduction in the protease activity of the regions where plasmin is active.  Sodium citrate reduced the enzyme activity the most followed by CaCl, SHMP and lastly EDTA.

Due to the fact that no chemicals may be added to milk, the second section of the investigation was to establish whether bubbling gasses through the milk have any impact on the activity of the proteolytic enzymes. It was shown that the activity of the Bacillus enzyme in full-cream milk was reduced when the milk was bubbled with nitrogen gas. Similarly, the activity of Bacillus enzyme in fat-free milk was also reduced when the milk was bubbled with nitrogen gas. Accordingly, the activity of plasmin on both full cream and fat-free milk was reduced when bubbled with nitrogen gas. The primary reason for the inhibition is because the milk becomes less oxygenated when nitrogen is bubbled through. Oxygen is required for protease activity.