Role of Yersinia intermedia and Pseudomonas putida in the development of a fruity off-flavour in pasteurized milk

Analysis by gas chromatography-mass spectrometry of pasteurized milk with a fruity (pineapple-like) off-odour and a sour, rancid and soapy taste indicted the presence of concentrations at mug/ml levels of ethyl butanoate, ethyl hexanoate, ethyl octanoate, ethyl deconate, octanoic acid, decanoic acid and dodecanoic acid. The off-odour and taste were attributed to the presence of these compounds in the milk. Microbiological examination confirmed that the milk was also contaminated with a series of psychrotrophic bacteria including Yersinia intermedia, Pseudomonas putida and Rahnella aquatilis. Growth of isolates of these bacteria in UHT milk at 23 degreesC for 7 d showed that Yer. intermedia produced significant quantities of the C-4-C-12 alkanoic acids; Ps. putida produced only small quantities of these acids and Rah. aquatilis produced none. In addition, Yer. intermedia and Ps. putida also produced small but significant quantities of the corresponding ethyl esters. In milk inoculated with both Yer. intermedia and Ps. putida, the quantity of ethyl esters produced was greater than that found in cultures containing only one of the isolates. These studies indicated that Yer. intermedia was the principal source of the alkanoic acids in the tainted milk and that the major producer of the corresponding ethyl esters was Ps. putida. This is the first report that Yer. intermedia and Ps. putida can cause an off-odour or taste in dairy products.