The chemistry of some microbially induced flavor defects in milk and dairy foods

Abstract

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

Esters and their biosynthesis in fermented dairy products: a review

“…In this two-step process, glycerides present in milk fat are first hydrolysed by lipases to liberate free fatty acids and glycerol, followed by the esterification of free fatty acids with an alcohol (in most cases, ethanol) to produce esters catalysed by esterases (Morgan, 1976;Kempler, 1983;Fox & Wallace, 1997;Fox, Guinee, Cogan, & McSweeney, 2000, Chapter 12).…”

2‐Methoxy‐3‐(1′‐methylpropyl)pyrazine, pea odour, from the marine bacterium Halomonas venusta

“…Although previous work by Morgan,19 McIver and Reineccius 20 and Cheng and Reineccius 21 revealed that the addition of exogenous amino acids does not improve the yield of 2-methoxy-3-isopropylpyrazine in P. perolens, we examined the respective microbial growth of H. venusta and 2-methoxy-3-(1 0 -methylpropyl)pyrazine production by variation of the culture medium.…”

Enhancement of fruity aroma production of Pseudomonas fragi grown on skim milk, whey and whey permeate supplemented with C3-C7 fatty acids

“…According to Morgan (1976), lipids are involved in the formation of fruity aromas by P. fragi. If the formation of the aroma was proportional to the lipid concentration of the culture media, higher concentrations of odour-active metabolites should have been detected in skim milk (0.3% w of lipids/v).…”