Abstract:The demand for ultra-high-temperature (UHT) processed and aseptically packaged milk is increasing worldwide. A rise of 47% from 187 billion in 2008 to 265 billon in 2013 in pack numbers is expected. Selection of UHT and aseptic packaging systems reflect customer preferences and the processes are designed to ensure commercial sterility and acceptable sensory attributes throughout shelf life. Advantages of UHT processing include extended shelf life, lower energy costs, and the elimination of required refrigeration during storage and distribution. Desirable changes taking place during UHT processing of milk such as destruction of microorganisms and inactivation of enzymes occur, while undesirable effects such as browning, loss of nutrients, sedimentation, fat separation, cooked flavor also take place. Gelation of UHT milk during storage (age gelation) is a major factor limiting its shelf life. Significant factors that influence the onset of gelation include the nature of the heat treatment, proteolysis during storage, milk composition and quality, seasonal milk production factors, and storage temperature. This review is focused on the types of age gelation and the effect of plasmin activity on enzymatic gelation in UHT milk during a prolonged storage period. Measuring enzyme activity is a major concern to commercial producers, and many techniques, such as enzyme-linked immunosorbent assay, spectrophotometery, high-performance liquid chromatography, and so on, are available. Extension of shelf life of UHT milk can be achieved by deactivation of enzymes, by deploying low-temperature inactivation at 55• C for 60 min, innovative steam injection heating, membrane processing, and high-pressure treatments.

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