Oxidation, a process that is generally considered ruinous to beer. Exposure to oxygen can happen virtually anywhere in the brewing process, from the brewhouse, to the fermentation cellar, to the packaging line, and even within the bottle in storage after packaging. We perceive the evidence of oxidation as off-putting stale notes that are variously describes as leathery, papery, wet cardboard-like, catty, and ribes or black currant. See faults in beer. On rare occasions, however, oxidation can improve certain beers, when it is deliberately employed under controlled conditions—such as during the long vat storage and barrel aging of lambics prior to bottling or during gentle bottle aging of barley wines. Such slow and graceful oxidation, which can have similar effects to the oxidation of Madeira wines in an estufa, adds complex flavors, makes the brew robust, and allows it to be kept for years. See aging of beer.

The first chance for oxygen pickup in the brewing process occurs in the mash, where it is referred to as hot-side aeration. See hot-side aeration. It can be the result of excessive stirring of the mash or too much splashing of wort during recirculation. Although oxidation accelerates with a rise in temperature, the solubility of oxygen decreases simultaneously. Therefore, boiling wort, which would oxidize if cooler, is too hot to pick up oxygen. Conversely, cold wort, which needs to be saturated with oxygen at the beginning of fermentation to stimulate yeast action, is cold enough that damaging oxidative reactions are slowed; the yeast then strips the oxygen out of solution before any damage can be done. In the mash, however, there is just enough oxygen present at a high enough temperature to make damaging oxidation reactions possible.

A second major oxygen pickup location for beer is the filling line. The amount of oxygen pickup depends largely on the sophistication of the bottling and kegging equipment and the care of the operators, but zero oxygen pickup is virtually impossible to achieve. The more oxygen is in the packaged beer, the faster it will become stale. Importantly, the rate of oxidation accelerates significantly with an increase in the beer’s storage temperature. For instance, a particular beer that may have a shelf life of 4 months if kept at a temperature of 6°C (42°F) may have a shelf life of less than 3 months if kept at temperature of 30°C (86°F).

Oxygen picked up throughout the brewing and packaging process can react with many compounds in beer, and the effects are usually negative. Perhaps the most notable flavor-active compounds are trace elements of fatty acids (lipids), which, when combined with oxygen, make beer taste stale and give it a pronounced flavor of wet cardboard. See lipids. Oxygen can also react with melanoidins that are created during the malting process and the kettle boil. See boiling and malt. Oxidized melanoidins can give the beer a slight taste of sherry. In the cold brew, oxygen can turn alcohol into almond-tasting aldehydes, which is one reason why wort should never be aerated to alleviate a stuck fermentation.

Chemically speaking, oxidation is the uptake of oxygen on the molecular level by a compound in conjunction with a release of energy. The best proof of that point is perhaps the importance of oxygen for fire. Without oxygen, fire simply does not burn. A good example of the importance of oxygen in brewing is perhaps the oxidation of the carbohydrate glucose in the process of yeast respiration. This is represented by the following equation:$${\text{C}}_6{\text{H}}_{12}{\text{O}}_6{+\mathrm{6O}}_2\to {6\text{CO}}_2{+6\text{H}}_2\text{O}+\text{energy}$$

The reaction generates energy and is the primary means by which organisms derive energy from food. In this reaction, oxygen being incorporated into the sugar to generate carbon dioxide and hydrogen is removed to yield water. Another view of oxidation, therefore, is the removal of hydrogen from a molecule.

Hydrogen may be extracted from a substance by materials other than oxygen, and the molecule from which hydrogen is removed is still said to be oxidized, whereas the molecule that has accepted the hydrogen is said to be reduced. When yeast “burns” sugar during fermentation, for instance, there is a stage in which hydrogen is removed from an intermediate in the pathway. The hydrogen does not attach to oxygen, but rather to an enzyme called nicotinamide adenine dinucleotide (NAD) to produce NADH. Later the yeast replenishes the NAD using the NADH to reduce acetaldehyde, thereby making ethanol.

As well as being defined as the addition of oxygen or the removal of hydrogen, oxidation can also be understood as the loss of electrons from a substance, in which case reduction is defined as the opposite of these. Oxidation and reduction work in tandem: when one participant in a reaction is oxidized, another becomes reduced. The overall circumstance is called redox. Substances that oxidize other substances are known as oxidizing agents or electron acceptors. Conversely, substances that reduce other substances are called reducing agents or electron donors. There are numerous redox reactions in malting and brewing. Some examples are as follows:

• The reduction of acetaldehyde to ethanol takes place by way of alcohol dehydrogenase, a yeast enzyme that functions as a catalyst. The enzyme’s name, incidentally, highlights the reversible nature of the reaction.

Another substance that develops through this reaction of oxygen and metal ions is hydrogen peroxide, which can oxidize polyphenols. This converts them into deeper colored forms, which therefore darken wort and beer. These, in turn, can polymerize and adhere to proteins, which causes hazes. See colloidal haze and haze. When speaking of finished beer, outside of the pleasant effects of slow oxidation on a small class of age-worthy beer styles, the word “oxidation” is always pejorative. Unlike the world of wine, which has a number of deliberately oxidized types such as oloroso sherry, tawny port, and vin jaune, there are no beer styles that are deliberately oxidized before reaching the consumer.