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The Effect of Lactose on the Survival of Escherichia Coli when Heated to 145°F. for Thirty Minutes

Department of Bacteriology, University of Maryland, College Park, Maryland

ABSTRACT

In commercial pasteurization the evidence is conclusive that members of the Escherichia-Aerobacter group may survive in milk that has been heated to approximately 145°F. for a period of thirty minutes. This temperature has been officially accepted as being sufficiently high to insure the safety of pasteurized milk.

It is rather uncertain whether the survival of members of this group is due to defective pasteurizing equipment, to the physiology of the organism, or to certain biochemical influences of the milk itself. Considering the colloidal state and chemical constituents of milk, it would seem possible that a protective action may at times be established.

Of the chemical constituents in milk lactose offers the best possibility for investigation. This sugar is present in cow's milk in amounts varying between 4 and 5 per cent.

The available literature on this subject supports the evidence herewith presented.Robertson¹ states that,

Hypertonic solutions, as indicated by comparisons of heating cells in nutrient broth containing increasing concentrations of sucrose, have a protective action up to and including 50 per cent sucrose.

¹ Robertson, A. H. 1927. Thermophilic and thermoduric microorganisms with special reference to species isolated from milk. N. Y. State Agr. Exp. Sta. Buls. 130–131; Vt. Agr. Exp. Sta. Buls. 274–275.