CategoryCCO410

Bacteria in wine

Bacteria are both the friend and foe of the winemaker and organisms that winemakers must control to prevent wine spoilage. While the bacterial malolactic fermentation is a friend of the winemaker, there are a range bacteria that can cause wine spoilage that winemakers must content with and control.

Acetobacter are responsible for converting acetaldehyde to vinegar in the presence of oxygen which can soil wine, ullage control and free SO2 are critical for control. Acetobacter can soil grapes that have been damaged by birds in the vineyard even before the grapes make it to the winery making bird control an important part of viticulture.

Brettanomyces is a spoilage yeast that can produce the classic band-aid, and barnyard characters in red wines by feeding on residual sugar while the wines are aging in the cellar. Molecular SO2, temperature, barrel sanitation and filtration are critical controls to limit Brettanomyces growth and wine spoilage.

Surface yeast, while rare can also spoil wine by production of acetaldehyde but are generally easily preventable with ullage control and topping of barrels or tanks. Of course in the rare case of sherry, the flor, which is a particular surface yeast, is a vital part in producing the wines of Jerez.

Dissolved Gases 1

Oxygen

Wine contains several gases in solution, O2, CO2 and N2, each has an impact on wine style or quality. Oxygen is the dissolved gas that can have the most negative impacts on wine via premature aging, browning and oxidized aromatic and flavour impacts.

That said, oxygen is important at various stages on the winemaking process and an important driver of style for some types of wines (sherry, maderia for example). Before and during ferment, oxygen is important for:

-good yeast health during the exponential growth phase

-for sparkling wine juices to bind phenolics reducing colour and phenolics in base wines

-aiding the volatilization of suphides during fermentation.

While CO2 protects the wine from oxidizing during fermentation. Once primary fermentation is complete, winemakers typically want to protect wines from excess O2 via SO2 additions, prevention of O2 pick up during wine movements and during wine aging. For barrel or oak aged wines, the slow pick up via O2 transfer through oak assists tannin polymerization on red wines and the development of texture and mouthfeel in white wines.

The management of oxygen in wine is critical during the bottling process is the final stage of O2 management where winemakers typically want to bottle with very low DO2 levels to prevent the wine from premature aging via in bottle oxidizing.

DO2 levels should be measured at many stages to ensure limited DO2 pickup, identify low SO2 levels and prevent oxidation using luminescence probes either in wine (in tank, barrel etc) or after bottling to determine O2 pick up using luminescence spot probes.