Ability of lactic acid bacteria to produce N-heterocycles causing mousy off-flavour in wine

Thirty-four strains of the genera and species representative of the wine lactic acid bacteria (LAB) were screened in an ethanolic grape juice medium for the production of mousy off-flavour as determined by an alkaline test strip sensory method. Most Lactobacillus spp., Oenococcus oeni and Leuconostoc mesenteroides produced mousy off-flavour, whereas Pediococcus spp. were generally incapable of detectable off-flavour formation. To verify these results, selected LAB were tested for the ability to produce the three sensorially potent N-heterocycles, 2-ethyltetrahydropyridine (ETPY), 2-acetyltetrahydropyridine (ACTPY) and 2-acetyl-1pyrroline (ACPY), which are associated with the mousy off-flavour in wine. N-heterocycle formation was determined by incubating a high density of bacterial cells in a chemically defined N-heterocycle assay medium, and quantifying the 'mousy' compounds by gas chromatography-mass spectrometry. Most strains produced each of the three N-heterocycles, with the general order for this capability being Lactobacillus (heterofermentative) > Oenococcus > Pediococcus. Strains of Lactobacillus hilgardii and Lactobacillus brevis were characterised as producing the highest concentration of ACTPY (328-580 mug/L, sensory threshold in water = 1.6 mug/L), whereas ACPY and ETPY were produced at a moderate concentration (<50 and <10 mug/L, respectively). In addition to the formation of ACPY and ACTPY, three of five strains of the commercially important wine malolactic bacterium, O. oeni, produced the highest concentration of ETPY (87-162 mug/L). Strains of the homofermentative LAB, Pediococcus sp. and Lactobacillus plantarum L11a, however, produced only a relatively low concentration of each mousy N-heterocycle (less than or equal to 37 mug/L), suggesting that the ability of LAB to produce mousy N-heterocycles is linked to the pathway of sugar catabolism. The importance of selecting LAB for induction of malolactic fermentation which have limited ability to produce mousy N-heterocycles is emphasised.