Identity of Esters Produced in Milk Cultures of Pseudomonas Fragi

Isovalerate and acetate esters were the principal esters detected in the steam distillate of a milk culture of Pse~domonas 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.