Synthesis of lipids (sterols and unsaturated fatty acids) by yeast requires oxygen. Wort lipids provide some unsaturated fatty acid, but insufficient sterol. Oxygen is therefore an essential nutrient for yeast growth.
Too low a level of dissolved oxygen can lead to insufficient yeast growth and, therefore, sticking fermentations. However, too much oxygen can lead to excessive production of yeast. This can give an apparently low OG determination, low beer pH and in some cases a long lag phase at the start of the fermentation.
Variable oxygen concentration at pitching can lead to variable beer flavour.
Range of Values
Many breweries find that an oxygen level of 8 mgs/litre (i.e. saturation against air) is optimal. Others find that a level as high as 20 mgs/litre (i.e. achieved using oxygen) is required. The exact requirements of yeast for oxygen should be established in each case and maintained on a brew to brew basis.
The pressure differential at the injection point should be about 0·7 bar (10 p.s.i.) and gas bubbles should be small enough to ensure good dissolution of oxygen.
Compressed gases (air or oxygen) are used to intoduce oxygen into wort. Even where only 8 mgs/ltr oxygen or less is required, oxygen is often preferred to reduce fobbing. Injection on the hot side of the paraflow cooler is preferred to reduce risks of microbial contamination. However less oxygen dissolves in hot wort than in cold wort.
The amount of dissolved oxygen in the wort can be increased if the previous fermentation has been sluggish.
The concentration of oxygen in wort is measured in-line using a dissolved oxygen meter.