Chapter 6

Wine Fermentation

Managing Oxygen in Wine Fermentation

Oxygen is essential during the initial stages of alcoholic fermentation for healthy yeast propagation and alcoholic fermentation. Oxygen exposure occurs to varying degrees during production of grape juice. Practices such as mechanical harvesting, crushing, and pressing all contribute to oxygen contact with grape components, which can approach the saturation point at 6 to 9 mg/L depending on temperature, equipment, and management practices. The lack of oxygen during fermentation can cause stuck or sluggish fermentations, resulting in typically dry wines being sweet on the palate in addition to potential yeast stress producing off by-products such as hydrogen sulfide (Section 6.12).

Timing of Oxygen Additions

Aeration is most efficient if performed during rehydration and acclimatization of the yeast and during the initial stages of fermentation, corresponding with the growth phase of the yeasts. During fermentation there are two optimal times to aerate or add oxygen to the fermenting must and that is during the initial stages of fermentation (the first 36 to 48 hours) and later during cap management (Butzke, 2010a).

Oxygen's Influence on Wine Style

Effective management of oxygen during winemaking can help create diverse wine styles. Although the fermentation takes place under oxygen-free conditions, the controlled addition of oxygen results in the desired sensory characteristics, such as color and aroma of the wines, and also reduces the formation of reducing aromas - hydrogen sulfide and other volatile sulfur-containing compounds. Therefore, the practice of winemaking is more of a balancing act between reductive and oxidative environments (Chapter 17). In reductive winemaking, oxygen is the enemy, and winemakers aim to protect the grapes and pre-fermented juice from air. If successful, they preserve the fresh fruit character of their wines, resulting in a lighter, fresher, fruitier style that's typically paler in color.

Post-Fermentation

Generally, oxygen is detrimental to wine quality especially from the end of fermentation through wine storage and bottling. The presence of oxygen during the latter stages of wine production can increase browning reactions, chemical and microbiological instability and the production of off aromas such as acetaldehyde. Following fermentation, limited slow oxygenation (approx. 40 mg O2/L) may benefit the maturation of red wines (Jackson, 2008).

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