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SummaryThe "malty" flavor defect that commonly develops in raw milk produced in certain areas of North America and Europe has long been known to be due to the metabolic activity of Streptococcus lactis var. maltigenes. The identification of the aldehydes and alcohols responsible for this flavor defect and the mechanisms involved in their formation from amino acids are discussed.Pseudomonas jragi, a common psychrophilic recontaminant, is responsible for development of "fruity" flavors in processed dairy products by virtue of the organisms ability to hydrolyze milk fat and esterify certain of the lower fatty acids with ethanol. A similar esterase is present in certain lactic cultures used in the manufacture of cheddar cheese.The "musty potato" aroma first described in eggs and milk and other dairy products due to the growth of Pseudomonas graveolens (Pseudomonas taetrolens) continues to be reported as a defect in eggs and carcass meats. Pseudomonas perolens has been found to produce a similar aroma in spoiling fish. Vapors entrained from milk and fish tissue cultures of these organisms, collected on porous polymer traps and analyzed by GLC-alkali flame and GLC-MS systems, revealed both organisms prodilce 2-methoxy-3-alkylpyrazines. 2-Methyoxy-3-isopropylpyrazine was found to be responsible for the musty potato aroma. A possible mechanism for the formation of pyrazines is discussed. MALTY FLAVORAs early as 1899, McDonnell' described in a thesis completed at the University of Kiel an organism he named Bacterium lactis acidi var. maltigenum which produced a malt-like flavor and aroma in milk. In 1921 Hammer and Cordes2 reported a burnt or caramel off-flavor in milk and cream produced in Iowa and found that it was caused by an organism identical to Streptococcus lactis except for its ability to produce the off-flavor. Hammer and Baker3 later included S. lactis var. maltigenes in descriptions of several variants of S. lactis.The high incidence of malty flavor in the Connecticut raw milk supply in the late 1940's and the ease with which we at the University 953

Pseudomonas perolens ATCC 10757 in sterile fish muscle (Sebastes melanops) were identified by combined gas-liquid chromatography and mass spectrometry. Compounds positively identified included methyl mercaptan, dimethyl disulfide, dimethyl trisulfide, 3-methyl-1-butanol, butanone, and 2-methoxy-3-isopropylpyrazine. Compounds tentatively identified included 1-penten-3-ol and 2-methoxy-3-sec-butylpyrazine. The substituted pyrazine derivative 2-methoxy-3-isopropylpyrazine was primarily responsible for the musty, potato-like odor produced by P. perolens. Castell and Greenough (2, 3) and Castell et al. (4) described a musty, potato-like odor that commonly developed in chilled fish muscle during the early stages of spoilage. Although this distinctive odor was observed under commercial conditions on chilled fillets of cod, haddock, and flounder, it was not detected on fresh or spoiling round and eviscerated fish. The causative bacterial species was identified as Pseudomonas perolens, a psychrophilic organism which utilizes a variety of substrates for growth and odor production (4). This investigation was initiated to identify the volatile compounds produced in sterile fish muscle (Sebastes melanops) by P. perolens. Particular emphasis was placed on the identification of the compound or compounds responsible for the characteristic musty, potato-like odor. MATERIALS AND METHODS Sterile muscle tissue. Black rockfish (Sebastes melanops), line-caught off the coast of Newport, Ore., were immediately killed by a blow on the head, individually wrapped in clean towels to prevent puncture damage to the skin, and packed in ice. Sterile fish flesh was obtained essentially as described by Lobben and Lee (11). The whole fish was washed in water, thoroughly swabbed with ethyl alcohol (70%), and dried under the flow of sterile air in a sterility test cabinet (BioQuest). Portions of skin were aseptically removed, and sections of exposed flesh were excised in the sterile atmosphere of the 'Technical Paper No. 3457, Oregon Agricultural Experi

The characterization of several isolates of a new lactic acid bacterium which produces a malty aroma is presented. Strains MXSR, MXSS, 6748, and 6283 were asporogenous, catalase-negative, gram-positive, aciduric rods of varying length which did not reduce nitrate. The above strains also formed L(+)-lactic acid, produced 0.3 to 0.4% acidity in sterile milk, and grew at 7, IS, 25, and 32 C but not at 45 C . In addition, all strains produced limited amounts of CO2 from gluconate, did not produce COZ from glucose, required riboflavin and folic acid but not thiamine for growth, and demonstrated fructose-l,6-diphosphate aldolase (EC 4.1.2.13) activity. The guanine plus cytosine contents of deoxyribonucleic acid (DNA) extracted from representative strains MXSR and MXSS were 36.0 and 36.1 mol%, respectively. All isolates produced an extracellular nuclease which hydrolyzed ribonucleic acid as well as DNA. Like Streptococcus lactis subsp. maltigenes, all strains produced 2-methylpropanal, 2-methylpropanol, 3-methylbutanal, and 3-methylbutanol in skim milk and Trypticase soy broth. The characteristic malty aroma is most likely due to the production of the above aldehydes. On the basis of the data presented, the malty strains were placed in a new species, Lactobacillus maltaromicus, in the subgenus Streptobacterium. Strain MX5 (ATCO 27865) was designated the type strain of L. maltaromicus.