Ester Production by Pseudomonas fragi. I. Identification and Quantification of Some Esters Produced in Milk Cultures

Abstract

Cultures of P. fragi isolated from fruity flavored cottage cheese and pasteurized milk were grown in sterile homogenized milk at 21 and 7 C. After a typical fruity aroma developed, the volatile constituents in the cultures were examined by gas-liquid chromatography. Ethyl acetate, ethyl propionate, ethyl butyrate, ethyl isovalerate, and ethyl hexanoate were tentatively identified on the basis of retention time. Inclusion of 0.2% ethanol in the milk medium markedly stimulated ester production by the organism at 21 C. Coupled gas-liquid chromatography-mass spectrometry yielded positive identifications for ethyl acetate, ethyl butyrate, and ethyl hexanoate in an ether extract of a culture distillate. A tentative mass spectrum also was obtained for ethyl formate. The most abundant esters with fruity odors were ethyl butyrate and ethyl hexanoate. In a culture with a strong fruity aroma, the concentration of these 2 esters was 0.35 and 0.50 ppm, respectively.