Ultra-heat treatment (UHT Milk Machine) gelation during storage of milk is one of the main factors limiting its shelf life. A large amount of research work on gelation can be used to explain possible mechanisms and methods of controlling gelation. Although the exact mechanism is not fully understood, it is generally accepted that gels are three-dimensional protein matrices initiated by proteolysis of milk proteins. First, β-lactoglobulin and κ-casein complexes form (βκ-complex) on casein micelles. This complex is then dissociated from the casein micelles and aggregated into a three-dimensional matrix, resulting in increased viscosity and gel formation. At this point, the milk is commercially sterile when packaged and will not become sour, but the gel will form and the milk will not be unacceptable to human consumption.
What is the cause of protein hydrolysis in sterile milk? This is due to the enzymes naturally present in milk and enzymes derived from bacteria. Plasmin is a protease that is usually present in small amounts in milk together with the precursor plasminogen. These precursors are converted to plasmin in the presence of an activating agent. Bacteria, especially psychrophiles, are also grown in raw milk to produce extracellular proteases. These psychrophilic bacteria can grow in raw milk before processing. Both enzymes are very thermostable and have withstood to varying degrees of ultra-high temperature treatment and therefore remain in milk for proteolysis during storage.
Plasmin is associated with casein micelles and is also present in the fat globule membrane. It is very stable at high temperatures, and it turns out that 90% can be inactivated by heating at 288 °F for 18 seconds. Plasminogen and plasminogen activators are more thermostable than plasmin, so proteolysis of plasmin is possible during storage.
Thermostable proteases produced by discoloring bacteria such as pseudofluorescence are the main cause of gel formation during storage. The protease of Pseudomonas fluorescens is very thermostable and needs to be inactivated by heating at 284 °F for 2 seconds to 300 seconds. These proteases show less than 10% damage during 4 seconds of ultra-high temperature sterilization of milk at 300 °F.
To minimize proteolytic activity in UHT processed milk:
Store at low temperatures, such as 35°F
Preheat the milk to 194 degrees Fahrenheit
Increase the sterilization temperature from 288 to 305 °F and increase the retention time from 6 seconds to 12 seconds to extend the shelf life.
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