WINETECH Technical Yearbook 2019

Zygosaccharomyces , a fermentation yeast, utilises residual glucose and fructose causing turbidity, a 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. However, Zygosaccharomyces is also capable of growth without fermenting the wine, resulting in no alcohol or CO 2 production. In these cases, Zygosaccharomyces utilises sorbic acid as a growth substrate resulting in the formation of a large granular sediment. Zygosaccharomyces i s par t i cu l ar l y a problem in wineries that sweeten wines with grape concentrate at bottling and once contamination of the bottling line has occurred, it is very difficult to eliminate. Zygosaccharomyces is resistant to free molecular SO 2 levels of up to 3 mg/L. Dry wines (wines with no residual glucose and fructose) are microbiologically stable with respect to Zygosaccharomyces growth.

Viable Brettanomyces populations may produce 4-EP and 4-EG, the compounds associated with Brettanomyces spoilage. Brettanomyces growth may continue in wines with glucose + fructose levels of as little as 0.2 g/L. Various other wine components can be utilised by Brettanomyces as a carbon source, such as non-fermentable sugars, glycerol, ethanol, malic acid and cellobiose from barrels. When present in higher populations, Brettanomyces growth may cause an increased turbidity and a slight granular sediment. Brettanomyces growth is inhibited by free molecular SO 2 levels of 0.4-0.8 mg/L and suppressed by higher alcohol levels (>15%). Brettanomyces populations can be reduced by egg white fining followed by racking, or completely removed by filtration (0.65-micron). Common surface film yeasts in wine include yeasts of the genera Candida ,

Pichia and Hansenula. Saccharomyces and Brettanomyce s are also capable of forming surface films. Alcohol in the wine is utilised by surface film yeasts to produce acetaldehyde and other off-flavours. The presence of acetaldehyde in wine will result in a “sherry-like” aroma, described as bruised apple, nutty and straw-like. Surface film yeasts are aerobic and can be observed as a film on the wine. Growth is, therefore, only possible in bottled wines until all oxygen is depleted and can either be observed as a film at the surface of the wine or cling to the sides of the bottle near the surface. When this film is disturbed, a granular sediment may result. Good winery sanitation is critical in preventing the growth of surface film yeasts. NON-WINE MICROBES AND MOULDS Non-wine microbes, both yeast and bacteria, are incapable of growing in wine due to the

low pH, alcoholic environment present. Non- wine microbes may be found in the cellar, however, will not continue to grow in bottled wines and will, therefore, not cause the wine to deteriorate. Moulds are alcohol-intolerant, aerobic microbes and will not grow in wine. However, mould growth in the vineyard or cellar may indirectly affect the wine through the production of off-flavours and aromas. SUMMARY It is said that wine is the microbial transformation of grape juice. However, as pleasant as the end product might be if things go right, the microbial transformation can occasionally go very wrong. REFERENCE Smit, H. & Wilton, N., 2018. The Vinlab Manual (3 ed.), Stellenbosch.

– For more information, contact Caitlyn McCartney at micro@vinlab.com.