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. [Illustration: FIG. 28. Diagram showing temperature changes in pasteurizing, and the relation of same to bacterial growth. Shaded zone represents limits of bacterial growth, 50 deg.-109 deg. F. (10 deg.-43 deg. 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.] Emphasis has already been laid on the selection of a proper limit of pasteurizing (p. 114). It should be kept constantly in mind that the thermal death-point of any organism depends not alone on the temperature used, but on the period of exposure. With the lower limits given, 140 deg. F., it is necessary to expose the milk for not less than fifteen minutes. If a higher heat is employed (and the cooked flavor disregarded) the period of exposure may be curtailed. ~Chilling the milk.~ It is very essential in pasteurizing that the heated milk be immediately chilled in order to prevent the germination of the resistant spores, for if germination once occurs, growth can go on at relatively low temperatures. The following experiments by Marshall[150] are of interest as showing the influence of refrigeration on germination of spores: Cultures of organisms that had been isolated from pasteurized milk were inoculated into bouillon. One set was left to grow at room temperature, another was pasteurized and allowed to stand at same temperature, while another heated set was kept in a refrigerator. The unheated cultures at room temperature showed evidence of growth in thirty trials in an average of 26 hours; 29 heated cultures at room temperature all developed in an average of 50 hours, while the heated cultures kept in refrigerator showed no growth in 45 days with but four exceptions.