Volatile sulfur compounds such as methanethiol, dimethyl disulfide, dimethyl trisulfide, and hydrogen sulfide constitute an important fraction of Cheddar cheese flavor. These compounds are products of the catabolism of methionine and cysteine by bacteria in the cheese matrix. The objectives of this study were to examine the levels and types of volatile sulfur compounds produced from methionine by lactic acid bacteria frequently used in cheese making and to investigate cystathionine degrading activity, which may be responsible for the liberation of these compounds. Gas chromatography with headspace sampling was used to determine volatile sulfur compounds (VSC) produced by whole cells of 24 strains of lactobacilli and 13 strains of lactococci incubated with methionine. Total VSC production varied widely in the species and subspecies tested. Nearly all strains produced VSC from methionine, but the enzyme responsible for this activity remains unclear. Cystathionine-degrading ability and the effect of methionine concentration on this ability of some of the strains was investigated. Increasing the concentrations of methionine inhibited the cystathionine-degrading ability of lactococci, but not of lactobacilli. Lactococci were found to require methionine for growth, while lactobacilli required both methionine and cysteine. Because of the low level of cystathionine-degrading activity in lactobacilli and the inhibition of this activity by methionine in lactococci, VSC production is likely due to enzymes other than cystathionine beta- and gamma-lyase in whole cells.
Metabolism of sulfur in bacteria associated with cheese has long been a topic of interest. Volatile sulfur compounds, specifically methanethiol, are correlated to desirable flavor in Cheddar cheese, but their definitive role remains elusive. Only recently have enzymes been found that produce this compound in bacteria associated with cheese making. Cystathionine beta- and gamma-lyase are found in lactic acid bacteria and are capable of producing methanethiol from methionine. Their primary function is in the metabolism of cysteine. Methionine gamma-lyase produces methanethiol from methionine at a higher efficiency than the cystathionine enzymes. This enzyme is found in brevibacteria, bacilli, and pseudomonads. Addition of brevibacteria containing this enzyme improves Cheddar cheese flavor. Despite recent progress in sulfur metabolism more information is needed before cheese flavor associated with sulfur can be predicted or controlled.
Low concentrations of branched-chain fatty acids, such as isobutyric and isovaleric acids, develop during the ripening of hard cheeses and contribute to the beneficial flavor profile. Catabolism of amino acids, such as branched-chain amino acids, by bacteria via aminotransferase reactions and ␣-keto acids is one mechanism to generate these flavorful compounds; however, metabolism of ␣-keto acids to flavor-associated compounds is controversial. The objective of this study was to determine the ability of Brevibacterium linens BL2 to produce fatty acids from amino acids and ␣-keto acids and determine the occurrence of the likely genes in the draft genome sequence. BL2 catabolized amino acids to fatty acids only under carbohydrate starvation conditions. The primary fatty acid end products from leucine were isovaleric acid, acetic acid, and propionic acid. In contrast, logarithmic-phase cells of BL2 produced fatty acids from ␣-keto acids only. BL2 also converted ␣-keto acids to branched-chain fatty acids after carbohydrate starvation was achieved. At least 100 genes are potentially involved in five different metabolic pathways. The genome of B. linens ATCC 9174 contained these genes for production and degradation of fatty acids. These data indicate that brevibacteria have the ability to produce fatty acids from amino and ␣-keto acids and that carbon metabolism is important in regulating this event.
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