Abstract:

In order to provide scientific reference for the safety assurance of storage and consumption of milk powder, the factors affecting survival of Citrobacter freundii in milk powder contaminated by it artificially were explored including cool storage at 4 ℃, freezing storage at －18 ℃, temperature of water for dissolving milk powder, D value of standard culture, water activity (aw) and pH. The results showed that at the 15th week of cool storage, the CFU of Citrobacter freundii declined to one tenth of the value before storage, that is, D4℃ was 15 w, and mould was observed to grow at the 18th week. culture at 4 ℃. Freezing storage caused a slight decline in the colony of C.freundii. However, a significant reduction in colony of C.freundii. was found when the water temperature was higher than 55 ℃. The D55 ℃ and D60 ℃ were 70 and 7 s, respectively. At aw≤ 0.76, the higher the aw was, the bigger the reduction in CFU of this bacterium was within the extended storage and a converse pattern was observed at aw ≥ 0.75. The pH range of 3.5－9.5 was optimum for the growth of C.freundii. In summary, newly packaged milk powder is suitable for preservation at room temperature for a short period of time and the milk powder should not be preserved for more than 18 w at refrigerate temperature after being opened. The safe central temperature for dissolving milk powder is 60 ℃.

Key words: milk powder, Citrobacter freundii, affecting factors