Oxygen is a necessary nutrient for proper yeast growth. Yeast requires oxygen for unsaturated fatty acid (sterol) synthesis in the cell membrane. Sterols are lipids that help yeast membranes maintain fluidity and functionality as well as to deal with ethanol stress. Sterols are only synthesized while oxygen is present during the early stages of fermentation. Once fermentation commences, the initial level of sterols produced must be adequate for all subsequent yeast growth. Sterol content becomes a limiting factor to yeast growth.
Yeast typically require 8 to 16 ppm of oxygen. Oxygen is typically dissolved into the must during crushing/destemming and then during pump-over/punching down for red wines fermented on skins. The most that atmospheric oxygen can provide to must is about 8 ppm. For this reason, it may be necessary to rack the wine or pump-over shortly after inoculation.
Pure oxygen can also be used to provide adequate oxygen in the must. Oxygen can be injected using a scintered stone. It is important to note that too much oxygen can be added using pure oxygen. Up to 40 ppm can be achieved with pure oxygen, but it is best to keep the dissolved oxygen levels below 20 ppm.
High sugar levels and high must temperatures both make it more difficult to dissolve oxygen into must. For high sugar musts or musts that have high temperatures, additional oxygenation/aeration may be necessary.