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

TLDR

Microbial spoilage by Streptococcus lactis var. maltigenes, Pseudomonas fragi, Pseudomonas graveolens, and related species produces characteristic flavor defects such as malty, fruity, and musty‑potato aromas in milk and dairy products. The authors aim to identify the aldehydes and alcohols responsible for the malty flavor defect and to elucidate their formation mechanisms from amino acids. Using polymer trap collection and GLC‑MS analysis, the authors detected 2‑methoxy‑3‑alkylpyrazines produced by these bacteria and propose a biosynthetic pathway for pyrazine formation.

Abstract

Abstract The “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 fragi , 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 produce 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.