Flavour Stability - Oxidation
By Moritz Kallmeyer
Chief Brewer of Drayman’s Microbrewery, Silverton Pretoria, February 2006

The ability of the beer to retain its fresh flavour is called flavour stability. In almost all cases flavour stability depends on the prevention of oxidation.
The initial stage of staling is the papery flavors (stale bread crumbs and cardboard) which are most frequently encountered and are caused by a large number of aldehydes and ketones (also called carbonyl compounds) The aldehydes which are formed during malting and mashing are mostly evaporated during wort boiling.

Other aldehydes are gradually formed in final package by oxidation of higher alcohols, produced by yeast.
Probably the most important stale flavour compounds are unsaturated aldehydes formed from the breakdown of UFA (unsaturated long chain fatty acids) in barley and hops. One such aldehyde, trans-2- nonenal gives stale tastes at concentrations as low as 0.1ppb. There are more than 50 such oxidation products. Oxidized UFA’s that are washed out of the mashtun into the wortboiler are not removed by boiling and give stale flavours later on. How is this prevented?
  1. Utmost care must be taken not to collect any cloudy mash runoff (which contains the UFA’s) which means less UFA’s in the wort boil and thus less potential oxidation.
  2. Limitation of the aircontact called hot side aeration (HSA) with the mash and wort, limit the extent of oxidation of the acids and also prevents the oxidation of reducing compounds (SO2) which could protect beer later. It is of great help to add a reducing agent KMS (potassium metabisulphite) to the mash.
  3. Certain polyphenols acts as antoxidants and should not be removed by filtration – others (protein and polyphenol complexes) which cause colloidal instability (hazes and harsh bitter astringent taste) should be filtered out.
Aldehydes can be oxidized to the corresponding carboxylic acids or be reduced to the corresponding alcohol. These are fermentation reactions. For example acetaldehyde by the action of the enzyme alcohol dehydrogenase forms ethanol. Carboxylic acids in turn react with alcohols to form esters. Acetate plus ethanol forms ethyl acetate.
Prevention: there is no cure!
  • Keep it cold.
  • Closed transfers!
  • No contact with direct sunlight – even when in a keg.
  • Avoid shaking.
  • Purge all pipes, tanks with D-water or CO2.
  • Foam before capping
  • Properly sealed containers.
 
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