Hot-Side Aeration (HSA) is the introduction of oxygen in the form of air into hot wort anywhere in the brewing process prior to fermentation. Oxygen is not desired in hot wort because it can combine with lipids, melanoidins, tannins, and other elements to produce undesirable compounds, which can stay in the wort through fermentation, ending up in the finished beer and promoting staling. See lipids, melanoidins, staling, and tannins. Whereas oxidized lipids can make beer taste slightly rancid and give it a pronounced flavor of wet cardboard, oxidized melanoidins can cause finished beer to take on sherry-like flavors (the latter can be desirable in deliberately aged beer, but not in fresh beer). Other oxidation products may slowly break down in the beer, freeing oxygen to transform alcohol into sweetish-tasting aldehydes, giving flavors of toffee and almond. Golden worts will tend to be darkened. Paradoxically, although oxygen in hot wort is detrimental to beer quality, oxygen in cold wort at the beginning of fermentation is essential for the yeast’s aerobic growth phase. See aeration. Fortunately, there is no “cold-side” aeration because of two opposing correlations: The oxidation rate increases with temperature, but the ability of a solution to dissolve oxygen decreases with temperature. This means that very hot wort does not contain enough oxygen for oxidation, whereas oxygen-rich wort at the beginning of fermentation is too cold to permit oxidation. Thus, HSA is a problem only in a temperature band somewhere below the kettle boil and above the heat-exchange temperature. The exact temperature interval in which HSA is a potential problem, however, is a topic of much debate among brewers. But most experts agree that the mash temperature is within the danger zone. Excessive stirring of the mash, therefore, too much splashing during recirculation, and any air pickup during the mash transfer from the mash tun to the lauter tun are among the most likely vectors, along with long hot stands for wort before it is sent to fermentation. All this concern about HSA has led to some fairly extreme measures, such as mash mixing in closed vessels that have been purged of air. However, some traditional breweries, from England to the Czech Republic, do not see hot-side aeration as a quality problem at all. Some aldehydes can be pleasant in certain British ales, and several prominent Czech brewers, using open grants to collect wort from the lauter tun, see the darkened beer color and flavor developments as positive attributes in their beers.
See also grant.