(O2) is a gas that composes about 20% of air. It is needed by respiring barley during germination and is also required to support the synthesis of unsaturated fatty acids and sterols in yeast. See aeration. Oxygen can have a number of adverse impacts in wort, notably the production of color, lowering of rates of wort separation, and development of undesirable flavors; however, it also promotes the removal of complexes of oxidized polyphenols and polypeptides, thereby enhancing the colloidal stability of beer. Oxygen in the finished beer is highly undesirable because it promotes beer staling and haze formation.

In all cases it is oxygen in solution that is important rather than that which is present in the headspace above the liquid, whether it is wort, beer, or water.

The concentration of oxygen is dependent on the following:

• the partial pressure of oxygen above the liquid: higher pressures (and proportion of oxygen in the gas phase) give higher oxygen concentration in solution;

• the temperature: higher temperatures mean less oxygen in solution; and

• the concentration of other substances dissolved in the water: high levels of competing solutes make for less oxygen in solution.

The oxygen concentration in deionized water under a headspace of air is 0.34 mM (10.9 ppm) at 10oC (50oF) and 0.28 mM (8.9 ppm) at 20oC (68oF). In 15oPlato wort the equivalent values are 8.0 and 6.6 ppm, respectively. For worts at lower atmospheric pressures (higher altitudes) the concentration of oxygen in wort will be proportionately less. At a given atmospheric pressure the solubility of oxygen in beer is less than in pure water, but greater than in wort.

Ground-state oxygen can be dissociated into its constituent atoms by light energy, and in turn these can react with other oxygen molecules to form ozone, which is also more reactive than ground-state oxygen. Therefore, ozone is a good sterilant.

While oxygen is essential for aerobic life, it is toxic in excess. Some organisms are anaerobic: they have systems readily poisoned by even low levels of oxygen. The electrons used to “activate” oxygen come from metals such iron, copper, and manganese. Therefore, the aim is to minimize both oxygen levels and the amount of these metals if damage to the beer is not to occur.

See also oxidation.