Amino Acid Catabolism

The starter lactococci and mesophilic lactobacilli, as discussed, are involved in the release of peptides and free amino acids from casein during the ripening of Cheddar cheese. Although these products of proteolysis contribute to the flavor, attempts to increase their formation by overexpression of enzyme activity or the addition of free amino acids to the curd at the manufacturing stage have not been successful in flavor enhancement. The implication, therefore, is that amino acid transformation rather than amino acid release is the key determinant in flavor formation. The catabolism of amino acids can result in the formation of many compounds that contribute to cheese flavor (16,109,110). Degradative mechanisms potentially include deamination, decarboxylation, desulfuration, oxidation, and reduction reactions resulting in the formation of amines, aldehydes, alcohols, indoles, carboxylic acids, and phenolic and sulfur-containing moieties (18,110).

Amino acid catabolism in starter lactococci occurs by two different pathways, depending on the nature of the amino acid. The degradation of branched-chain and aromatic amino acids is initiated by an a-ketoglutarate-dependent transamination mechanism. This appears to be the major pathway of amino acid breakdown in lactococci and the branched-chain and aromatic aminotransferase enzymes involved have been isolated and characterized both biochemically and genetically (18,105). The resultant a-ketoacids derived from the amino acids are subject to further enzymatic or chemical reactions. The end products of these reduction or decarboxylation reactions are hydroxyacids, aldehydes, and carboxylic acids (18). Nonenzymic degradations of ketoacids in cheese have also been reported (18). The degradation of sulfur-containing amino acid is also initiated by an elimination reaction catalyzed by the enzymes cystathionine-h-lyase and cystathionine-g-lyase (111). Methional and methanethiol formed by the breakdown of methionine may be oxidized subsequently to dimethyldisulfide and dimethyltrisulfide.

Although the lactobacilli are less well studied than the lactococci, there is sufficient evidence to conclude that mechanisms of amino acid degradation in both are generally similar. The range of amino acid-converting enzymes in cheese lactobacilli is restricted (18,111). Amino acids are metabolized in the presence of a-ketoglutarate and a range of volatile catabolites are formed (83,84). Branched-chain (84,112), aromatic (84,113-115), and methionine (84,115-117), aminotransferase, and cystathionine lyase activities (84,117) have been detected in nonstarter lactobacilli, and although some aminotransferase characteristics have been determined (116-118), only cystathionine-g-lyase from a cheese strain of Lb. fermentum has been purified and characterized (119). Amino acid decarbox-ylating lactobacilli have also been isolated from cheese (120).

Aminotransferases are intracellular enzymes, but cell lysates retain only limited activity under conditions prevailing in ripening Cheddar cheese (118). However, in growth studies, enzymes involved in amino release and turnover retained activity under cheese conditions provided that cell integrity was maintained (118). The aminotransferase-mediated conversion of methionine to thiols also proceeds more slowly under cheese conditions with extracts than with intact LAB cells (115). As the apparent stability and effectiveness of amino acid catabolizing enzymes released from lysed cells is less than in intact viable cells, the impact of lactobacilli on flavor formation during ripening may be diminished as a consequence of extensive cell lysis.

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