Effect of low-temperature storage on the bacteriological quality of raw milk

Abstract

The effect of storage of raw milk from a farm bulk tank and creamery silo has been investigated during storage under three different regimes. The first storage treatment involved keeping milk under active refrigeration at 2°C i.e. deep cooling. In the second treatment, milk was stored at a constant temperature of 6°C which corresponds to normal conditions in a commercial silo. The final (intermediate temperature) treatment was an attempt to simulate the effect of chilling milk to low temperature followed by storage in an insulated silo. In this treatment, the initial temperature was 2°C and temperature was increased by 1°C every 24 h. A 48 h increase in storage life was achieved when farm bulk milk was stored at 2°C rather than 6°C. The efficacy of the intermediate temperature treatment, or of both low temperature (2°C) methods on creamery silo milk, was comparatively poorer on average. The initial count of psychrotrophic bacteria in the milk before the temperature was lowered was the critical determinant of storage life both at 2°C and 6°C. Some organisms, especially some pseudomonads, were capable of growth at all temperature regimes which were tested but members of the Enterobacteriaceae were better adapted to growth at the higher temperature. A limited number of experiments were carried out in which raw milk was first thermised (65°C for 15 s) then stored at 2°C. This combined treatment was very effective in assuring the quality of stored milk for several days.