10 – 50 mg/l of sulphur dioxide are usually added to the vintage as it is transferred from the crusher to the fermentation tank in order to control the activity of wild yeasts, bacteria and oxidative enzymes and increase phenolic extraction. Red wine fermentation tanks should only be filled up to ¾ of their capacity, as the carbon dioxide generated by the alcoholic fermentation gets trapped in the ‘cap’ of skins (and stems?) that floats to the top of the tank and makes it swell.

Fermentation tanks may be made of wood, concrete, glass fibre or stainless steel. Stainless steel is generally preferred as it is easier to clean and its conductivity makes temperature control easier. They can be open at the top or sealed.

Once filled, the tank is inoculated with the selected yeast strain. This will vary dependent on the required wine style. For example, a wine requiring high alcohol and increased glycerol may utilise S. cerevisiae var bayanus.

After inoculation, a short pumping-over will suffice to mix the yeast and aerate the must sufficiently for the fermentation to start. Aerating the must at the beginning of fermentation will help to prevent stuck ferments. At this time, the yeasts are in their growth phase and oxygen is used to improve their rate of reproduction. Aeration is achieved by pumping over. This involves drawing the must from the racking valve, allowing it to ‘tumble’ into a container, then pumping it back to the top of the tank and over the floating cap of skins (pomace) after. The tank must be equipped with a sieve, inside the exit valve, to stop skins and pips blocking the valve orifice.

Phenolic extraction from the skins is dependent on three factors listed below: