WINETECH Technical Yearbook 2019

levels of 0.8 mg/L and is inactivated by the addition of lysozyme. Lactobacillus in wines with residual sugars can lead to the production of acetic acid (VA) and other off-odours and flavours. These off- odours and flavours include: • Acrolein: Wines containing acrolein are known to be excessively bitter. Degradation of glycerol results in the formation of acrolein, which further reacts with the phenolics in wine producing extremely bitter compounds. • Di acet y l : Di acet y l at the cor rec t concentrations is not considered a fault, but contributes positively to the buttery, nutty character of the wine. However, at high concentrations wines have an excessive buttery and lactic aroma. Diacetyl is produced from citric acid by the Lactobacillus populations present. • Mannitol: Certain strains of Lactobacillus may produce mannitol from fructose resulting in viscous, slimy wines having aromas of acetate, vinegar, excess lactic, buttery, or diacetyl aromas. • Mou s i ne s s : I n t he p r e s en c e o f ethanol, Lactobacillus can produce the compounds 2- ethyltetrahydropyridine, 2 - a c e t y l - 1 - p y r r o l i n e a n d 2 - a c e t y l t e t r a hyd r op y r i d i n e , t h e compounds associated with strong mouse-cage, popcorn and basmati/

j asmine- r i ce characters . As these compounds are not very volatile, they will be more prominent on the palate than the nose. • “Geranium” taint: Certain strains of Lactobacillus can reduce sorbic acid, a preservative added to wine to inhibit the fermentation activity of Saccharomyces , to the compound 2- ethoxyhexa-3,5- diene. The production of this compound results in a wine with a strong aroma of crushed geranium leaves. Certain strains of Lactobacillus are known to utilise tartaric acid (generally regarded as microbiologically stable) contributing to the production of acetic acid, lactic acid and CO 2 . Furthermore, Lactobacillus growth in bottles may cause turbidity and a granular sediment. Residual malic acid can be utilised by Lactobacillus to produce lactic acid, CO 2 and in a few cases have even led to crystal formation. Crystal formation may occur within bottles as the Lactobacillus ferment malic acid, resulting in an increase in pH impacting the tartrate stability of the wine. Growth is encouraged in wines with high pH levels (pH >3.60). Lactobacillus is sensitive to free molecular SO 2 levels of 0.8 mg/L and above, and most strains are inactivated by the addition of lysozyme. Lactobacillus is removed by sterile filtration (0.45-micron). Pediococcus can use any of the sugars (including the un-fermentable pentoses)

and malic acid to produce CO 2 , a granular sediment and adverse flavours and aromas; such as excessive amounts of diacetyl and acrolein. Diacetyl is produced from citric acid and depending on the concentrations, can either positively alter the wines by giving them a buttery, nutty character or when present in high concentrations, results in an excessive buttery and lactic aroma. Acrolein, on the other hand, is formed when Pediococcus metabolises glycerol and further reacts with the phenolics in wine, forming an extremely bitter compound. Acrolein formation may occur after fermentation has completed. Pediococcus growth is encouraged in wines having a high pH (pH >3.60). Pediococcus is sensitive to free molecular SO 2 levels of 0.8 mg/L and above, and most strains will be inactivated by the addition of lysozyme. Certain strains of Pediococcus , usually Pediococcus strains resistant to SO 2 , can cause ropiness, resulting in highly viscous, oily wines that become increasingly viscous when agitated or moved. Ropiness may be developed either during fermentation, maturation and may even be formed in the bottle. Pediococcus is removed by sterile filtration (0.45-micron). Acetobacter requires oxygen for growth and may continue to grow in even very low oxygen environments, such as oak barrels. Acetobacter utilises sugar to produce acetic acid (VA) and ethanol to produce acetaldehyde and ethyl acetate.

Acetaldehyde will give the wine a typical “sherry” aroma (bruised apple, nutty and straw-like), while ethyl acetate results in an acetone aroma associated with paint thinners or nail varnish remover. Acetobacter cannot grow in the oxygen- poor environments of full storage vessels and is only capable of growth in bottled wines until oxygen levels are depleted. In half-full tanks Acetobacter can grow on the wine surface resulting in the production of volatile aromas capable of contaminating the wine. Like in half-full tanks, in bottled wine Acetobacter can grow on the surface, or cling to the sides of the bottle near the surface. Once the microbes have died off and the film is disturbed, a granular sediment may form. Growth of Acetobacter can be prevented by keeping wine storage containers full and following strict barrel topping regimes. YEAST SPOILAGE Saccharomyces utilises glucose and fructose to produce alcohol and CO 2 . Wines having residual glucose/fructose levels of more than 1 g/L are at risk of re-fermentation by Saccharomyces. Re-fermentation will result in turbidity, sediment in the form of a yeast lees deposit and the production of CO 2 , sometimes to the point of the cork being expelled from the bottle. Dry wines (wines with no residual glucose and fructose) are microbiologically stable with respect to Saccharomyces growth.