when growing in wine, some of which are volatile phenolic compounds. Together these compounds are often referred to as phenolic taint, "Brettanomyces character", or simply "Brett". The main constituents are listed below, with their sensory threshold and common sensory descriptors: 4-ethylphenol (>140 μg/L): Band-aids, barnyard, horse stable, antiseptic 4-ethylguaiacol (>600 μg/L): Bacon, spice, cloves, smoky isovaleric acid: Sweaty, cheese, rancidity Geosmin Geosmin is a compound with a very distinct earthy, musty, beetroot, even turnip flavour and aroma and has an extremely low sensory threshold of down to 10 parts per trillion. Its presence in wine is usually derived as metabolite from the growth of filamentous actinomycetes such as Streptomyces, and moulds such as Botrytis cinerea and Penicillium expansum, on grapes. Wines affected by but not attributed to geosmins are often thought to have earthy properties due to terroir. The geosmin fault occurs worldwide and has been found in recent vintages of red wines from Beaujolais, Bordeaux, Burgundy and the Loire in France. Geosmin is also thought to be a contributing factor in cork taint. Lactic acid bacteria Lactic acid bacteria have a useful role in winemaking converting malic acid to lactic acid in malolactic fermentation. However, after this function has completed, the bacteria may still be present within the wine, where they can metabolise other compounds and produce wine faults. Wines that have not undergone malolactic fermentation may be contaminated with lactic acid bacteria, leading to refermentation of the wine with it becoming turbid, swampy, and slightly effervescent or spritzy. This can be avoided by sterile filtering wine directly before bottling. Lactic acid bacteria can also be responsible for other wine faults such as those below. Bitterness taint Bitterness taint or amertume is rather uncommon and is produced by certain strains of bacteria from the genera Pediococcus, Lactobacillus, and Oenococcus. It begins by the degradation of glycerol, a compound naturally found in wine at levels of 5-8 g/L, via a dehydratase enzyme to 3-hydroxypropionaldehyde. During ageing this is further dehydrated to acrolein which reacts with the anthocyanins and other phenols present within the wine to form the taint. As red wines contain high levels of anthocyanins they are generally more susceptible. Diacetyl Diacetyl in wine is produced by lactic acid bacteria, mainly Oenococcus oeni. In low levels it can impart positive nutty or caramel characters, however at levels above 5 mg/L it creates an intense buttery or butterscotch flavour, where