Oxygen Little breaths Air is essential for the wine to mature but the quantity must be well judged; it can become one of the most dangerous enemies. Too much contact with air encourages development of vinegar bacteria and changes the colour, making red wine lose its colour and white wines go brown. At the start air is indispensable for fermentation; the yeasts can only start their work of transforming the sugars into alcohol in the presence of oxygen. However, little 'breaths' are required for the wine to mature; the small quantities of air present in the barrels and contact with oxygen during movement between containers and treatment helps the wine to develop. Some winemakers, contrary to normal technique, put the must 'under stress' from oxygen. This is called hyperoxygenating the must - a forced oxygenation which can cause colorant substances and extracts to become insoluble, improving the stability and colour.