it is perceived as a flaw. The sensory threshold for the compound can vary depending on the levels of certain wine components, such as sulfur dioxide. It can be produced as a metabolite of citric acid when all of the malic acid has been consumed. Diacetyl rarely taints wine to levels where it becomes undrinkable. Geranium taint Geranium taint, as the name suggests, is a flavour and aroma taint in wine reminiscent of geranium leaves. The compound responsible is 2-ethoxyhexa-3,5-diene, which has a low sensory threshold concentration of 1 ng/L. In wine it is formed during the metabolism of potassium sorbate by lactic acid bacteria. Potassium sorbate is sometimes added to wine as a preservative against yeast, however its use is generally kept to a minimum due to the possibility of the taint developing. The production of the taint begins with the conversion of sorbic acid to the alcohol sorbinol. The alcohol is then isomerised in the presence of acid to 3,5-hexadiene-2-ol, which is then esterified with ethanol to form 2-ethoxy-3,5-hexadiene. As ethanol is necessary for the conversion, the geranium taint is not usually found in must. Mannitol Mannitol is a sugar alcohol, and in wine it is produced by heterofermentative lactic acid bacteria, such as Lactobacillus brevis, by the reduction of fructose. Its perception is often complicated as it generally exists in wine alongside other faults, but it is usually described as viscous, ester-like combined with a sweet and irritating finish. Mannitol is usually produced in wines that undergo malolactic fermentation with a high level of residual sugars still present. Expert winemakers oftentimes add small amounts of sulfur dioxide during the crushing step to reduce early bacterial growth. Ropiness Ropiness is manifested as an increase in viscosity and a slimey or fatty mouthfeel of a wine. In France the fault is known as "graisse", which translates to fat. The problem stems from the production of dextrins and polysaccharides by certain lactic acid bacteria, particularly of the genera Leuconostoc and Pediococcus. Mousiness Mousiness is a wine fault most often attributed to Brettanomyces but can also originate from the lactic acid bacteria Lactobacillus brevis, Lactobacillus fermentum, and Lactobacillus hilgardii, and hence can occur in malolactic fermentation. The compounds responsible are lysine derivatives, mainly; 2-acetyl-3,4,5,6-tetrahydropyridine 2-acetyl-1,4,5,6-tetrahydropyridine 2-ethyltetrahydropyridine 2-acetyl-1-pyrrolene The taints are not volatile at the pH of wine, and therefore not obvious as an aroma. However, when mixed with the slightly basic