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Thermoduric Bacteria in Pasteurized Milk. II. Studies on the Bacteria Surviving Pasteurization, with Special Reference to High-Temperature, Short-Time Pasteurization

ABSTRACT

The higher bacteria counts occurring in milk pasteurized by the high-temperature short-hold method as compared with the low-temperature long-hold method are largely due to the ability of certain species of micrococci to survive the former method of pasteurization in greater numbers. The most common species of micrococci among those found in the milk pasteurized at high temperature are M. candidus, M. epidermidis, M. luteus and M. varians, although five other species were encountered less frequently. These micrococci make up the predominant thermoduric flora of dirty milking machines, strainers and pails on farms, and about half of the thermoduric organisms isolated from milk cans. However, milk cans at the plant where this work was done were not an important source of thermoduric bacteria.

The work of Harding and Wilson (10) and of Alice Breed (11) indicates that micrococci make up about 75 per cent of the flora of the normal cow's udder. They studied in all 226 cultures and found that six of the seven species encountered in commercially pasteurized milk in the work reported here made up about 60 per cent of the micrococci of the udder, or over 45 per cent of the total flora of normal udders. The only species encountered in this work not found in the udder by these previous workers was M. caseolyticus, and only 3 cultures of this organism were encountered among a total of 153 cultures of micrococci identified. The principal source of bacterial contamination of the rubber tubes of a milking machine is probably the milk itself. Moreover, many of these species of micrococci can survive inefficient hot-water sterilization just as they can survive pasteurization. Robertson (12) reports that many of them apparently also can survive sterilization by chlorine sterilizers and by salt brine. All this explains very well the source of the thermoduric micrococci in the milk. They originate in the udder and grow in improperly cleaned dairy farm utensils.

Workman has reported (13) that micrococci occur in the soil, in the stable air, and on the skin of cows. All of these sources doubtless contribute somewhat to the original contamination of dairy farm utensils and of the milk itself. However, it must be emphasized that dust, dirt from the skin of the cow, and the udder itself contribute relatively very small numbers of organisms to the milk (14, 15). Only when dairy farm utensils are improperly cleaned so as to offer a place where the micrococci can multiply does the problem of thermoduric bacteria become a serious one.

The data presented here were obtained at a single large milk plant, with about 400 producers. Whether similar results would be obtained with other milk supplies can only be determined by further investigation, now under way.

¹ Dairymen's League Cooperative Assn., Inc., Syracuse, N. Y.

² Dairymen's League Cooperative Assn., Inc., Poughkeepsie, N. Y.

³ Dairymen's League Cooperative Assn., Inc., Poughkeepsie, N. Y.—Now at Department of Bacteriology, University of Maryland, College Park, Md.