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 of Hop Acids
Hop α- and β-acids, and particularly iso-α-acids, undergo oxidation reactions during ageing, resulting in the loss of bitterness.
It has been shown that volatile carbonyls can be produced from a solution of bitter substances, higher alcohols and melanoidins. The trans isomers are more prone to degradation than the cis isomers. Reduced hop products do not participate in these reactions.
Other aromas, described as rancid, sweaty socks and cheesy can be formed from the oxidation of α- and β-acids. These aromas are imparted by free acids originating from the oxidation of the acyl side-chain of the acidic resin fractions. The main reactants are:
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Isobutyric
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Isovaleric
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2-Methyl butyric acid
In addition, ethyl esters consequently formed from these acids, contribute to a stale hop character.