Lesson 6 - Beer Flavour and Stability
In this lesson we will discuss beer flavour and beer flavour stability. This will include the effects raw material has on beer flavour, followed by a discussion on common taints, their flavours and consumer reaction. We will examine the flavour changes that occur during beer storage: focussing on oxidation reactions, the species of oxygen involved, and how we can utilise antioxidants to protect against aged beer flavour change. Finally, we will summarise the selection of raw materials and process optimisations available to improve beer flavour stability.
Oxidation Reactions and Flavour Instability
As beer ages, flavour-active compounds change. Flavour deterioration can be caused by an increase in an undesirable character from de novo synthesis of new compounds or by the release of pre-formed material that was chemically bound. Let us look at some of the key changes.
Lipid oxidation
Unsaturated fatty acids are a subset of lipids derived from barley and other extract bearing material. They are solubilised during the mashing process and can then be oxidised in two ways: enzymatically or non-enzymatically.
Enzymatic oxidation of lipids
LOX or lipoxygenase is the barley embryo-derived enzyme catalysing the oxidation of polyunsaturated fatty acids during the mashing process.
It is heat-labile and rapidly inactivated at a temperature of 65 ºC. It is likely that it is only active during mashing in. In addition, its activity is strongly pH-dependent and is halved when moving from pH 5.5 to pH 5.0.
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The unsaturated fatty acids, typically linoleic acid, are transformed by the presence of both LOX and oxygen, to hydroperoxides; these are sometimes referred to as oxidised fatty acids.
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These hydroperoxides are, in turn, converted by the more heat-labile hydroperoxide isomerases and other enzymes, to a complex mixture of compounds including saturated and unsaturated aldehydes.
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Further non-enzymatic reactions result in the formation of E-2-nonenal, formerly known as trans-2-nonenal (T2N).
E2N has an extremely low aroma threshold of 50-250 ng/l and the compound is a flavour standard for cardboard, one of the prominent flavours in ageing beer. It binds to malt proteins and is carried through the brewing process in this bound form before being released during storage of finished beer.
Non-enzymatic oxidation of lipids
Linoleic acid can undergo oxidation non-enzymatically after an initial reaction, probably involving oxygen radicals such as the hydroxyl and perhydroxyl radical (•O2H). This triggers a cascade of reactions resulting in E2N formation amongst other compounds.
There are two opinions as to whether this happens in finished beer or during the brewing process. In the brewing process it thought that that;
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30% of oxidation happens in mashing
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70% happens during wort boiling