The Bacterial Flora of High-Grade Milk Before and After Pasteurization

IN THE PAST much of the high-grade milk has been sold raw, and little is known of the effect of pasteurization on the flora of such milk.Ayres and Johnson (2) in studying relatively high-count milks determined the proportions of physiologic types of bacteria in pasteurized milk as compared with those found in the raw milks.These investigators divided the types into four groups : i. e., acid- forming, inert, alkali-forming, and peptonizing, as shown on plates of litmus-lactose-agar and litmus-lactose-gelatin.They found that in raw milk the inert group was largest, followed in order by the acid-forming, peptonizing, and alkali-forming groups.Pasteurization at 145°F.(62.8°C.)resulted in an increase in the proportion of acid-forming bacteria and in decreases in the alkali-forming, peptonizing, and inert groups.At pasteurizing temperatures above 170°F.(76.7°C.) the acid group was largely destroyed and the rela- tive proportion of peptonizing organisms greatly increased.They found only a small percentage of spore-forming bacteria in milk pasteurized at 145°F.for 30 minutes.Later, Ayres and Johnson (3) found that by inoculating tubes of sterile litmus milk, each with a single colony grown on a plate, and incubating these tubes for 14 days at 30°C., they could divide the types of bacteria into their respective groups more accurately than by the use of litmus-lactose-gelatin or -agar.By this method they also distinguished two types of acid-producing bacteria : i. e., acid-forming and acid-coagulating.This method was found to show a higher percentage of acid-producing and peptonizing and fewer inert and alkali-producing bacteria than was shown on litmus-lactose- agar and litmus-lactose-gelatin.They studied three grades of raw milk, the best including 12 samples with an average count of 24,700, the second, 24 samples with an average count of 3,451,000, and the