The survival of sporeforming bacteria in the heat processing of milk and the various mechanisms by which spores withstand severe heat treatment are discussed. Data are given concerning the spoilsage potential and heat resistance of psychrotropic sporeforming bacteria in milk. The types, numbers and heat resistance of spores in milk from countries with high average temperatures are considered in relation to processing. Laboratory experiments predicting thermal death of spores at temperatures within the UHT range are described and the many factors influencing the heat resistance of endospores, the most significan,t of which are the environmental conditions during sporulation, heating and recovery, are summarized.
The first study of this series (Magoon (1926a)) reported the effect of age, temperature and humidity on the thermal resistance of spores of B. mycoides. These spores, washed free of metabolic products, were stored at 10°C., 200C., and 300C., under three conditions of humidity-over calcium oxide, over 50 per cent H2SO4, and over water. Samples from each of the nine sets of storage conditions were tested for thermal resistance (as measured by the minutes of heating at 1000C. required for 100 per cent destruction of spores) after one, thirty, sixty, ninety, one hundred and twenty, one hundred and fifty and one hundred and eighty days of storage, respectively. It was found that the thermal resistance of spores had increased during thirty days under all sets of storage conditions. Though changes after thirty days were rather irregular, it was outstanding that the slowest change in resistance took place at 10°C. over CaO. Considering the storage period as a whole, it was found that at 100C. the thermal resistance of the spores increased regularly with the humidity. At 200C. it was about the same over CaO and 50 per cent H2SO4 and was slightly greater over water, while at 300C. it was maximum over 50 per cent H2SO4 and least over water. Under the three conditions of humidity the maximum thermal resistance was found at 200C. In the second report (Magoon (1926b)) experiments were described in which by cultivation and selection of survivors from successive thermal death-time tests, a strain of B. mycoides was 85
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