501. The effect of storage at different relative humidities on the survival of micro-organisms in milk powder and in pure cultures dried in milk

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Abstract
1. Two spray-dried milks, one separated and one whole milk, have been stored at 25°C. for 48 and 103 weeks respectively at relative humidities ranging from 0 to 100%. At high humidities (80–100% r.h.) a rapid fall in the numbers of bacteria was followed by rapid growth of bacteria and by overgrowth by moulds. At relative humidities below 80% the number of surviving bacteria increased with decreasing humidity to maximum survival at about 10% r.h. and then tended to fall again towards 0% r.h.2. The changes in the distribution of the different types of bacteria in the dried milks at different relative-humidities are discussed.3. Maximum survival at 5–15% r.h. has been confirmed with pure cultures (dried in milk) of a Streptococcus and a Micrococcus originally isolated from dried milks.4. The marked loss of viability of bacterial spores noted in dried milks stored over P2O5 or concentrated H2SO4 has been confirmed for a dried suspension of the spores of a typical strain of Bacillus subtilis.