The writer tested Reid's pasteurizer at 155° to 165° F. with the following results: in some cases as many as 40 per cent. of the bacteria survived, which number in some cases exceeded 2,000,000 bacteria per cc.
Pasteurizing details. While the pasteurizing process is exceedingly simple, yet, in order to secure the best results, certain conditions must be rigidly observed in the treatment before and after the heating process.
It is important to select the best possible milk for pasteurizing, for if the milk has not been milked under clean conditions, it is likely to be rich in the spore-bearing bacteria. Old milk, or milk that has not been kept at a low temperature, is much richer in germ-life than perfectly fresh or thoroughly chilled milk.
The true standard for selecting milk for pasteurization should be to determine the actual number of bacterial spores that are able to resist the heating process, but this method is impracticable under commercial conditions.
The following method, while only approximate in its results, will be found helpful: Assuming that the age or treatment of the milk bears a certain relation to the presence of spores, and that the acid increases in a general way with an increase in age or temperature, the amount of acid present may be taken as an approximate index of the suitability of the milk for pasteurizing purposes. Biological tests were carried out in the author's laboratory[149] on milks having a high and low acid content, and it was shown that the milk with the least acid was, as a rule, the freest from spore-bearing bacteria.
This acid determination can be made at the weigh-can by employing the Farrington alkaline tablet which is used in cream-ripening. Where milk is pasteurized under general creamery conditions, none should be used containing more than 0.2 per cent acidity. If only perfectly fresh milk is used, the amount of acid will generally be about 0.15 per cent with phenolphthalein as indicator.
Fig. 28.
Diagram showing temperature changes in pasteurizing, and the relation of same to bacterial growth.
Shaded zone represents limits of bacterial growth, 50°-109° F. (10°-43° C.), the intensity of shading indicating rapidity of development. The solid black line shows temperature of milk during the process. The necessity for rapid cooling is evident as the milk falls in temperature to that of growing zone.]