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.
Reactive Oxygen Species (ROS)
Oxidation reactions in beer do not involve oxygen in its ground state but rather in the form of ROS, some of which are free radicals. The ROS form when electrons are acquired, leaving the molecules with unpaired electrons.
The reactions to form ROS from the reduction of molecular oxygen (O2) through a gain of an electron produces superoxide (•O2−) which is the precursor of most other reactive oxygen species.
O2 + e− → •O−2
Dismutation of superoxide, with the antioxidant enzyme superoxide dismutase (SOD), produces hydrogen peroxide (H2O2) and molecular oxygen.
2 H+ + •O−2 + •O−2 → H2O2 + O2
Dismutation is a redox reaction in which a compound of intermediate oxidation state converts to two different compounds, one of higher and one of lower oxidation states.
Hydrogen peroxide in turn may be partially reduced to the hydroxyl ion (HO-) and the hydroxyl radical (•OH) or fully reduced to water.
H2O2 + e− → HO− + •OH
2 H+ + 2 e− + H2O2 → 2 H2O
The transition metals present in beer, in particular copper and iron can catalyse the production of ROS, notably the hydroxyl radical from hydrogen peroxide, via the Haber-Weiss and Fenton reactions, where Fe donates an electron to hydrogen peroxide. It is for this reason that we aim to exclude these metals from the brewing process.
Amongst the ROS superoxide, hydrogen peroxide and hydroxyl radical, the hydroxyl radical is the most reactive and is involved in the initiation of auto-oxidation of lipids.
Catalase is a common enzyme found in nearly all living organisms exposed to oxygen. It catalyses the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting the cell from oxidative damage by ROS.