83. Observations on Sudden Changes in the Rate of Acid Formation in Milk by Cultures of Lactic Streptococci

Abstract

1. Some cases of sudden failure in the growth of cheese starter cultures are described.2. It is shown that the cause lies in the method of pasteurisation of the milk in which the cultures are grown.3. Aeration of the milk immediately before the inoculation of the culture is shown with certain cultures to have an inhibitory efEect on the growth of the organisms. Normal starter cultures are not affected by aeration of the milk; they may, however, suddenly develop this sensitivity to aeration and may just as suddenly revert to the normal state.4. The mere absorption of atmospheric oxygen while milk is being cooled after pasteurisation in a wide-mouthed cylindrical vessel may constitute an aeration sufficient to inhibit sensitive cultures of lactic streptococci. Milk pasteurised and cooled in conical flasks plugged with cotton-wool does not reabsorb sufficient oxygen to lead to inhibition, provided that the flasks are not violently agitated.5. The possible mechanism of the inhibition of the sensitive streptococci is discussed. Evidence is brought forward which tends to show that the action is connected with the oxidation-reduction reactions by means of which the organisms normally obtain their growth energy.6. The practical significance of the phenomenon in the process of cheese manufacture is emphasised.