WINETECH Technical Yearbook 2019

WHEN MICROBES GET IT WRONG

NOVEMBER 2019

CAITLYN MCCARTNEY: Vinlab, Stellenbosch KEYWORDS: Acetobacter , Brettanomyces , Lactobacillus , non-wine microbes, Oenococcus , Pediococcus , Saccharomyces , surface film yeasts, wine microbes, Zygosaccharomyces . This article summarises the “dark side” of microbial transformation that can only be prevented by the watchful eyes, and if needed, interventionist actions of winemakers. BACTERIAL SPOILAGE Oenococcus utilises malic acid to produce lactic acid, making it the preferred bacteria for malolactic fermentation. However, it can also utilise residual glucose and fructose resulting in the production of alcohol, lactic acid and other metabolites. Oenococcus is, therefore, only considered a microbiological risk in bottled wines containing a residual malic acid of >0.2 g/L or a combined glucose fructose level of >1 g/L. Re-fermentation will result in the production of small amounts of CO 2 and a granular sediment. Oenococcus is sensitive to free molecular SO 2