Brewers are all too aware that beer is probably the beverage that reacts most sensitively to oxygen. What is known as the oxidation off-flavour occurs if there is too much oxygen pickup. Trend beers made using the rediscovered cold or dry hopping technique quickly alter their taste with oxygen pickup. Standard light beers are also especially sensitive to oxygen, however. Dark beers, on the other hand, often display considerable flavour tolerance to an oxygen content of above and beyond 0.1mg per litre.
Oxygen pickup in beer production starts with the crushing of the malt and continues even after the beer has left the brewery, as oxygen penetrates seals on crown corks, for instance, up to the point where the beer is drunk. Beer ingredients react so fast with oxygen that dissolved oxygen measurements have to be taken immediately after the respective processes of tank filling, filtration or bottling, for example. An oxidised beer with its typical changes in taste often contains barely measurable amounts of oxygen yet the negative impact on quality is substantial.
The use of additives such as bisulphite or ascorbic acid as oxygen scavengers is common in countries other than Germany, especially when filling beer into PET bottles. This is because these containers, plus the PE or PP caps most commonly used to seal them, allow relevant amounts of oxygen to migrate very quickly. Using ascorbic acid as an antioxidant in beer is a two-edged sword, however. Oxidation processes — not just in beer but also in other beverages such as soft drinks and juice — are complex procedures with many intermediate stages and thus much more than simply the transfer of oxygen to a specific receptor molecule. Are the polyphenols in the malt or hops beneficial antioxidants or damaging haze formers? This is a different story altogether that scientists continue to research!
In order to rise to this challenge, breweries have developed a number of approaches to quality assurance: from the use of malts low in polyphenols to polyphenol-rich whole cone hopping or from strategies for preservation or precipitation of the polyphenols to those aimed at polyphenol reduction using PVPP.
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