Aromatic amino acid catabolism by lactococci

Cold-tolerant bacteria, also known as psychrotrophic bacteria, are notorious contaminants of milk in the refrigerated dairy food chain. These organisms, especially the pseudomonads, may produce heat-resistant enzymes that are responsible for the breakdown of proteins and lipids in milk and dairy products. Such reactions result in a variety of defects in the raw or unprocessed milk that may affect the suitability of such milk for further processing. The enzymes produced may cause defects in long-life dairy products such as cheese, butter and long-life milk. In the present study, a range of 18 yellow pigmented psychrotrophic bacteria, collectively known as flavobacteria, were isolated from local dairy products. One aim of this study was to identify these bacteria to species level using molecular techniques. A second aim was to determine the spoilage potential of these organisms based on profiles generated by the BIOLOG system (that may relate to hydrolytic enzymes produced). Of the 18 isolates, 14 belonged to the genus Chryseobacterium while 4 were identified as Empedobacter isolates. The most active spoilage organisms in this group were shown to be C. bovis, C. shigense and E. brevis. These findings illustrate that enzymatically catalysed defects in dairy products should not be attributed solely to acknowledged psychrotrophic bacteria such as the pseudomonads, but that flavobacterial species may also be actively involved.

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“…These α-keto acids are converted to various metabolites, such as the aldehydes and sulfur compounds which may result in off-flavours. 46 …”

Section: Utilisation Of Carboxylic Acidsmentioning

confidence: 99%

Spoilage potential of a novel group of bacteria isolated from dairy products

Tsôeu¹,

Jooste²,

Charimba³

et al. 2016

S. Afr. J. Sci.

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“…One of the predominant amino acid conversion routes is initiated by transaminases. Transamination of amino acids yields the corresponding ␣-keto acids, which do not have strong flavor properties (12,13) but are the precursors of various aldehydes, alcohols, acids, and (thio-)esters. The latter compounds generally have much lower odor thresholds and strongly contribute to cheese flavor.…”

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confidence: 99%

Identification, Cloning, and Characterization of a Lactococcus lactis Branched-Chain α-Keto Acid Decarboxylase Involved in Flavor Formation

Smit

Vlieg

Engels

et al. 2005

Appl Environ Microbiol

122

139

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The biochemical pathway for formation of branched-chain aldehydes, which are important flavor compounds derived from proteins in fermented dairy products, consists of a protease, peptidases, a transaminase, and a branched-chain ␣-keto acid decarboxylase (KdcA). The activity of the latter enzyme has been found only in a limited number of Lactococcus lactis strains. By using a random mutagenesis approach, the gene encoding KdcA in L. lactis B1157 was identified. The gene for this enzyme is highly homologous to the gene annotated ipd, which encodes a putative indole pyruvate decarboxylase, in L. lactis IL1403. Strain IL1403 does not produce KdcA, which could be explained by a 270-nucleotide deletion at the 3 terminus of the ipd gene encoding a truncated nonfunctional decarboxylase. The kdcA gene was overexpressed in L. lactis for further characterization of the decarboxylase enzyme. Of all of the potential substrates tested, the highest activity was observed with branchedchain ␣-keto acids. Moreover, the enzyme activity was hardly affected by high salinity, and optimal activity was found at pH 6.3, indicating that the enzyme might be active under cheese ripening conditions.Flavor formation in cheeses is mainly the result of catabolism of milk proteins, sugar, and lipids. Degradation of proteins into amino acids followed by conversion of these amino acids by Lactococcus spp. is very important for flavor formation in semihard types of cheese, like Gouda (for reviews see references 12, 25, 38, and 43). One of the predominant amino acid conversion routes is initiated by transaminases. Transamination of amino acids yields the corresponding ␣-keto acids, which do not have strong flavor properties (12, 13) but are the precursors of various aldehydes, alcohols, acids, and (thio-)esters. The latter compounds generally have much lower odor thresholds and strongly contribute to cheese flavor. This is exemplified by the production of flavor compounds from leucine. Transamination of leucine results in the production of ␣-isocaproic acid (KICA). Interestingly, only a limited number of Lactococcus lactis strains are capable of decarboxylating this ␣-keto acid to 3-methylbutanal (36, 37). The decarboxylating activity is observed mainly in L. lactis strains with nondairy origins, whereas the industrial strains tested hardly showed this activity (36). Subsequently, (de)hydrogenation of 3-methylbutanal results in 3-methylbutanol or 3-methylbutyric acid, which also contributes to the flavor of cheese, but the odor thresholds for these compounds are higher than the odor threshold for 3-methylbutanal (24, 29). The rate-determining step in this route is the decarboxylation of KICA, and therefore, control of this enzyme offers good prospects for the design of starter cultures with tailored flavor production (37, 43).The enzyme performing this reaction has not been identified. A number of thiamine pyrophosphate (TPP)-dependent

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“…Specifically, the Phe catabolites phenylacetaldehyde and 2-phenethyl alcohol have been shown to impart floral, rose-like off-flavors, and the Tyr catabolite p-cresol imparts barny, medicinal, or utensil-like offflavors (15,22,32). Mechanisms for the production of these compounds in cheese have not been conclusively established, but AAA catabolism by lactococci and lactobacilli under simulated cheese-ripening conditions is initiated by aminotransferase (ATase) enzymes that convert AAAs into corresponding ␣-keto acids (17,20,21,39). Moreover, the aromatic ␣-keto acids produced by these reactions can be nonenzymatically converted to benzaldehyde, phenylacetaldehyde, 2-phenethyl alcohol, and other aroma compounds (17,19,20,21,38).…”

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confidence: 99%

Overexpression of Lactobacillus casei d -Hydroxyisocaproic Acid Dehydrogenase in Cheddar Cheese

Broadbent¹,

Gummalla²,

Hughes

et al. 2004

Appl Environ Microbiol

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Metabolism of aromatic amino acids by lactic acid bacteria is an important source of off-flavor compounds in Cheddar cheese. Previous work has shown that ␣-keto acids produced from Trp, Tyr, and Phe by aminotransferase enzymes are chemically labile and may degrade spontaneously into a variety of off-flavor compounds. However, dairy lactobacilli can convert unstable ␣-keto acids to more-stable ␣-hydroxy acids via the action of ␣-keto acid dehydrogenases such as D-hydroxyisocaproic acid dehydrogenase. To further characterize the role of this enzyme in cheese flavor, the Lactobacillus casei D-hydroxyisocaproic acid dehydrogenase gene was cloned into the high-copy-number vector pTRKH2 and transformed into L. casei ATCC 334. Enzyme assays confirmed that ␣-keto acid dehydrogenase activity was significantly higher in pTRKH2:dhic transformants than in wild-type cells. Reduced-fat Cheddar cheeses were made with Lactococcus lactis starter only, starter plus L. casei ATCC 334, and starter plus L. casei ATCC 334 transformed with pTRKH2:dhic. After 3 months of aging, the cheese chemistry and flavor attributes were evaluated instrumentally by gas chromatography-mass spectrometry and by descriptive sensory analysis. The culture system used significantly affected the concentrations of various ketones, aldehydes, alcohols, and esters and one sulfur compound in cheese. Results further indicated that enhanced expression of D-hydroxyisocaproic acid dehydrogenase suppressed spontaneous degradation of ␣-keto acids, but sensory work indicated that this effect retarded cheese flavor development.Microbial catabolism of amino acids generated from the degradation of milk proteins during cheese maturation is an essential and rate-limiting step in the development of cheese flavor and aroma properties (34, 38). Many of these reactions impact cheese flavor in beneficial ways. For example, the conversion of Met to methional, dimethyl sulfide, methanethiol, and other sulfur-containing compounds is thought to be essential for aroma development in many cheese varieties (35). On the other hand, compounds derived from the catabolism of aromatic amino acids (AAAs) have been implicated in the development of cheese off-flavors. Specifically, the Phe catabolites phenylacetaldehyde and 2-phenethyl alcohol have been shown to impart floral, rose-like off-flavors, and the Tyr catabolite p-cresol imparts barny, medicinal, or utensil-like offflavors (15,22,32). Mechanisms for the production of these compounds in cheese have not been conclusively established, but AAA catabolism by lactococci and lactobacilli under simulated cheese-ripening conditions is initiated by aminotransferase (ATase) enzymes that convert AAAs into corresponding ␣-keto acids (17,20,21,39). Moreover, the aromatic ␣-keto acids produced by these reactions can be nonenzymatically converted to benzaldehyde, phenylacetaldehyde, 2-phenethyl alcohol, and other aroma compounds (17,19,20,21,38). However, many lactic acid bacteria possess hydroxy acid dehydrogenases (HADH) such as D-hydroxyisocapr...

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Aromatic amino acid catabolism by lactococci

Cited by 100 publications

References 22 publications

Spoilage potential of a novel group of bacteria isolated from dairy products

Spoilage potential of a novel group of bacteria isolated from dairy products

Identification, Cloning, and Characterization of a Lactococcus lactis Branched-Chain α-Keto Acid Decarboxylase Involved in Flavor Formation

Overexpression of Lactobacillus casei d -Hydroxyisocaproic Acid Dehydrogenase in Cheddar Cheese

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