Yeast about to begin a brewery fermentation requires a certain amount of oxygen to fuel some of its biochemical pathways, notably the production of unsaturated fatty acids and esters. It is normal practice to add oxygen to wort prior to fermentation; absence of sufficient oxygen will hamper yeast reproduction and can lead to poor fermentations. Historically aeration may have simply meant allowing the wort to cool in the presence of air or by dropping the wort into a fermentation vessel from above. Modern brewing practice dictates that oxygen be added directly to the wort after it has been cooled. Either filtered compressed air or oxygen from a cylinder is used and introduced into the flowing wort via a sintered metal device, a pumice stone, or Venturi device. The solubility of the oxygen in wort is lower than in water because of the dissolved solids already in the wort. The stronger the wort, the lower the solubility of the oxygen. For wort at 120Plato and using air as the oxygen source, the maximum amount of oxygen that can dissolve in wort at normal wort collection temperatures is between 7–8 ppm. Using pure oxygen, figures in the 35–40 ppm range can be achieved depending on the temperature. Calculating the amount of oxygen dissolved in the wort is possible but depends on the temperature of the wort and the oxygen as well as their relative flow rates. Brewers will generally place a dissolved oxygen measuring device downstream to achieve consistent dissolved oxygen concentrations. Some brewers have experimented with direct oxygenation of yeast slurries, arguing that this reduces the extent of undesirable oxidation of wort components.

See also oxidation, oxygen.