Age gelation within UHT milk
Dairy industries lose millions of rand per annum due to the age gelation of milk that reduces its shelf life. This can be induced by anything that has an impact on the suspension of the negative charge on the casein micelle during storage of the UHT milk. This includes the enzymatic or chemical processes that occur during storage of milk. The enzymatic processes involve the action of the plasmin (native enzyme) or the heat stable microbial protease. During storage at the processing plant or on the farms, psychrotolerant bacteria predominantly the Pseudomonas spp. or Bacillus spp. start to grow in the milk and produce enzymes called protease. Additionally, during storage of milk at the processing plant, inactive plasminogen in milk can be activated to plasmin protease during storage at 5°C. The heat stable proteases are not destroyed during UHT treatment and remain active in UHT milk. This can result in gelation of milk being observed within few weeks after storage. In order to implement means to combat the age gelation problems, knowledge of plasminogen activators that mediate the conversion of plasminogen to plasmin after heat treatment is required. In this study, it was found that free-fatty acids were able to act as activators in this conversion. Furthermore, once the mechanisms are in place, stopping the age gelation of milk is nearly impossible. Retarding the flocculation processes however may be possible by making use of chemicals such as EDTA, SHMP, CaCl2 and Sodium citrate, or by bubbling gases such as CO2, He and N2 through milk.