Summary-The influence of NaCl and reduced pH was determined for aminopeptidase, lipase/esterase and methanethiol-producing capability in selected lactic acid bacteria and brevibacteria in simulated cheese-like conditions. The observations on simulated cheese-like conditions were confirmed in 60% reduced-fat Cheddar cheese. The activity of each enzyme decreased with NaCI addition and when the pH was reduced to approximate Cheddar chee se conditions (5% NaCI, pH 5.2). Residual intracellular aminopeptidase activity was dominated by general aminopeptidase activity (aminopeptidase N and/or aminopeptidase C) in laboratory, simulated cheese-like conditions, and 60% reduced-fat Cheddar cheese curd. During chee se aging, totallipase/esterase activity peaked at 120 d then decreased, ev en though starter culture populations remained high. Methanethiol-producing capability occurred under cheese-like conditions in whole cells, but not in cell-free extracts. Met and Met-containing peptides induced methanethiol-producing capability for 2-3 generations and could be re-induced later in the growth cycle of Brevibacterium linens BL2. Aminopeptidase and lipase/esterase activity in reduced fat cheese were not correlated to an increase in Cheddar-type flavar, but a culture's methanethiol-producing capability was associated with higher chee se consumer preference scores. Results suggest that use of cheese-like conditions may aid in selecting cultures to cheddar cheese / ripenlng / flavor /Iactococcus /Iactobacillus / brevibacterlum / methanethiol production Résumé -Influence du NaCI et du pH sur les enzymes intracellulaires qui affectent l'affinage du cheddar. L'influence du NaCI et d'une réduction du pH a été déterminée sur les ami nopeptidases, les lipases/esterases, ainsi que l'aptitude à produire du méthanéthiol de bactéries lactiques et de brevibactéries sélectionnées dans des conditions de fabrications fromagères simulées. Les observations, dans ces conditions, étaient confirmées, dans le cas du cheddar à teneur en matières grasses réduites de 60 %. Les activités de chaque enzyme diminuaient avec l'addition de NaCI et lorsque le pH était abaissé à des niveaux proches des conditions de fabrication du cheddar (NaCI5 % et pH 5,2). L'activité aminopeptidasique intracellulaire résiduelle était dominée par l'activité aminopeptidasique générale (aminopeptidase Net/ou aminopeptidase C) au laboratoire, en conditions fromagères simulées, et dans un caillé de cheddar à teneur en MG réduite de 60 %. Pendant le vieillissement des fromages, l'activité totale lipase/esterase atteignait un pic à 120 jours puis diminuait, même si les populations de levains demeuraient élevées. L'aptitude à produire du métha-néthiol se manifestait en conditions fromagères simulées dans les cellules entières, mais non dans les extraits dépourvus de cellules. Les peptides Met et Met-contenant induisaient une aptitude à produire du méthanéthiol sur deux ou trois générations, et pouvaient être réinduits ultérieurement dans le cycle de croissance de Brevibactérium lin...

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Attempts to develop a desirable reduced fat Cheddar cheese are impeded by a propensity for flavor defects such as meaty-brothy, putrid, fecal, and unclean off-flavors in these products. Recent studies suggest aromatic amino acid catabolism of starter, adjunct, and nonstarter lactic acid bacteria significantly impact off-flavor development. The objective of this study was to delineate pathways for catabolism of tryptophan (Trp) in Brevibacterium linens, a cheese flavor adjunct, and to determine the potential for this organism to contribute to this defect. Growth and production of aromatic compounds from Trp by B. linens BL2 were compared in two incubated conditions (laboratory and a cheese-like environment). A chemically defined medium was used to determine the cellular enzymes and metabolites involved in Trp catabolism. Trp was converted to kynurenine, anthranilic acid, and three unknown compounds in laboratory conditions. The accumulation of other unknown compounds in the culture supernatant in laboratory conditions indicated that B. linens BL2 degraded Trp by various routes. Up to 65% of Trp was converted to anthranilic acid via the anthranilic acid pathway. To assess this potential before cheese making, the cells were incubated in cheese-like conditions (15 degrees C, pH 5.2, no sugar source, 4% NaCl). Trp was not utilized by BL2 incubated in this condition. Enzyme studies using cell-free extracts of cells incubated in laboratory conditions and assayed at optimal and nonoptimal enzyme assay conditions revealed Trp transaminase (EC 2.6.1.27) was active before enzymes of the anthranilic acid pathway were detected. The products of Trp transaminase activity were not, however, found in the culture supernatant, indicating these intermediates were not exported nor accumulated by the cells. Enzymes assayed in nonoptimal conditions had considerably lower enzyme activities than found in laboratory incubation conditions. Based on these results, we hypothesize that these enzymes are not likely to be involved in the formation of compounds associated with off-flavors in Cheddar cheese.

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