Effect of Heat on Mold Spores.

Because of their wide distribution and ease of dissemination, molds always present a problem in the preparation of foodstuffs that are not to be consumed immediately. The changes produced in foods by molds are in general very undesirable. As heat is the most common way of preserving foods, it was thought desirable to know something about the thermal death times of molds. Thorn and Avery 1 found that Rhizopus nigricans and Oidium lactis were killed in pasteurized milk at 54.5°C. (130.1°F.) in 30 minutes, but that most strains of Aspergillus and Penicillium resisted this temperature and were killed at 60°C. (140°F.) for 30 minutes.

Since 30 minutes is the customary pasteurization time, and since this experiment was carried out mainly to study the effect of pasteurization, no tests were made at intervals more frequent than 30 minutes. Streider and McClellan 2 studied the thermal relations of molds in bread. They submerged flasks containing mold suspensions in a water bath at 20° C. (68°F.). The temperature was slowly raised to 100°C. (212°F.) so that the elapsed time was 35 minutes. This was done to simulate the temperature obtained when baking bread in the ovens. No molds survived. Loaves of bread that had been exposed to molds were heated to 110°C. (230°F.) at 15 pounds pressure, for 30 seconds, and exposed to live steam for 4 minutes. In the first case, Rhizopus and Aspergillus species survived, while in the latter, Aspergillus species only survived. Complete destruction of the mold was almost attained, as there were very few mold colonies on the plates made from this last experiment. Using the same procedure, loaves were then exposed to dry heat at 120°C. (248°F.) for 10 minutes.