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Identity of Esters Produced in Milk Cultures of Pseudomonas Fragi¹

Department of Animal Industries, Storrs Agricultural Experiment Station, Storrs, Connecticut

ABSTRACT

Isovalerate and acetate esters were the principal esters detected in the steam distillate of a milk culture of Pseudomonas fragi. In an analysis of the acids present in similar culture distillates, isovaleric acid was the most prominent acid detected which was not present in the uninoculated control milk. Ethanol was the only alcohol detected in the cultured milk and was not found in the control milk. Supplementation of sterile milk with ethanol, prior to culturing, stimulated rapid production of esters by the organism. In such cases, the isovalerate ester was most prominent. Four of a panel of six judges were able to detect as little as 0.01 p.p.m. of ethyl isovalerate in a series of milk samples, and all described the aroma of milk dilutions of this ester as very similar to that of P. fragi milk cultures. Evidence is presented which indicates that the production of esters responsible for the fruity aroma in milk cultures is markedly affected by the previous nutrition and/or incubation temperature of P. fragi cells used as inoculum. The role of leucine as a probable substrate in the production of the isovalerate ester by P. fragi is discussed.

¹ This paper is, in part, a report of research presented in a thesis by the senior author in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Connecticut.

² Present address: Chas. Pfizer and Company, Groton, Connecticut.

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