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Age gelation of UHT whole milk as influenced by storage temperature

Authors: H.R. Kocak and J.G. Zadow

Abstract:

The onset of age gelation in ultra-high-temperature (UHT) sterilized milk was not directly related to the degree or extent of proteolysis in either the freshly prepared UHT milk or the gelled samples. In addition, the time of gelation (as defined by the product exceeding a viscosity of 10 mPa.s.) did not appear to be correlated with the microbiological quality of the raw milk. Age gelation did not occur in samples stored at 40°C or 50°C. Samples of UHT milk stored at ambient temperatures (20°, 25°C) gelled rapidly (typically 120-150 days), whereas the onset of age gelation was considerably retarded at low storage temperatures (2°C, 10°C). The order of resistance of UHT milk to the onset of age gelation at various temperatures was: 50°C, 40°C >2° > 10° > 15° > 20° > 25° >30°C. In general the rate of increase in proteolysis was storage-temperature dependent, except for samples stored at 40°C or 50°C. The initial rate of proteolysis in samples stored at 40°C was higher than any other. On further storage however, the extent of proteolysis in samples stored at 40°C did not increase appreciably. The level of proteolysis in samples stored over 200 days was highest in samples stored at 30°C. The rate and extent of proteolysis in samples stored at 50°C were low and comparable to levels observed in samples stored at 15°C. Gelled samples did not show a common level of proteolysis.

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