Oxygenation/Aeration – Why?

oxygenate or aerate your must or yeast starter is a common sense in mead making.

Yeast only multiply in aerobic conditions because oxygen is needed for the respiration process.

But oxygen is also a necessary component for the production of Sterols like Ergosterol.

Ergosterol (ergosta-5,7,22-trien-3ß-ol) is a sterol found in fungi and does not occur in plant or animal cells. It is a component of yeast and fungal cell membranes, serving the same function that cholesterol serves in animal cells.

Ergosterol is a component of yeast and fungal cell membrane lipids which help maintain cellular fluidity and permeability enabling cells to grow and bud. Every time a cell buds the Ergosterol content is diluted. Once the level becomes too low (in approximately 3-4 doublings) the cells lose the ability to bud.
Ergosterol content generally is the limiting factor of yeast growth. Synthesis of Ergosterol occurs only when oxygen is present. Thus, it is critical to supply the yeast with appropriate levels of oxygen.

Yeast require 7 to 16 ppm of oxygen. The most that atmospheric oxygen can provide to must is about 8 ppm. For this reason, it may be necessary to oxygenate or aerate shortly after inoculation.
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 or aeration may be necessary.
Pure oxygen can also be used to provide adequate oxygen in the worst. 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.

Remember that if very high pitch rates are used, in a yeast starter, the culture can exhaust the sugar source prior to depleting the Ergosterol contents. The depletion of sugar will cause the culture to stop growing.