Oxygen is a necessary nutrient for proper yeast growth. Yeast requires oxygen for the synthesis of sterols. Sterols are membrane lipids which help maintain cellular fluidity and permeability enabling cells to grow and bud. Every time a cell buds the cellular sterol content is diluted. Once the level becomes too low (in approximately 3-4 doublings) the cells lose the ability to bud. Sterol content generally is the limiting factor of yeast growth. Synthesis of sterols occurs only when oxygen is present. Thus, it is critical to supply the yeast with appropriate levels of oxygen.
Yeast 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.