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 Protection in Beer
We have discussed flavour changes relating to beer oxidation. Fortunately, there are means to delay or prevent these changes during beer storage. We will now consider these and how they can maintain a fresh beer flavour.
Yeast
As we have learnt, brewer’s yeast reduces acetaldehyde to ethanol during fermentation. In the same fashion, it can reduce staling carbonyls to their corresponding alcohols; in doing so, it prevents the formation or transforming of stale flavour compounds. Once the beer is filtered and pasteurised, this protection is lost.
The other well-known reduction of carbonyl compounds by yeast is the reduction of diacetyl to acetoin and 2,3,-butanediol. After yeast is removed, any remaining precursor can form diacetyl through non-enzymatic oxidative decarboxylation of the precursor α-acetolactate.
Sulphur dioxide
Sulphur dioxide, added to beer or more commonly produced naturally by yeast, prevents carbonyl cardboard flavours from expressing through binding with carbonyl compounds; this forms the flavour-neutral adducts. These adducts carry through to the packaged beer where the carbonyls are slowly released during storage as the sulphur dioxide is consumed.
Adducts are considered as a single product resulting from the amalgamation of different compounds combining all the reactant molecules’ atoms. If the adduct is broken, the combining compounds return to their pre-adduct forms.
There are health concerns around the use of sulphites in food products, and in many countries, the maximum level allowed without label declaration is 10 mg/L. As an additive, sulphur dioxide is used in the form of potassium metabisulphite (KMS), which is powder added to de-aerated water and dosed into beer post-filtration.
Ascorbic acid
Ascorbic acid, a typical reductone, is sometimes added to beer as an antioxidant or oxygen scavenger. In the presence of iron, ascorbic acid becomes a potent oxygen carrier and acts in a reverse manner to that intended.
Reductones are strong acidic reducing agents that are ene-diols, often derived from sugars by oxidation. Reducing agents become oxidised themselves, preventing other molecules from becoming oxidised.
Oxygen radical scavenging by polyphenols and melanoidins
We have previously discussed the reactivity of oxygen radicals and subsequent cascading reactions. Polyphenols and melanoidins may act as radical scavengers, stopping these reactions. Polyphenols may have other antioxidant effects by chelating metal ions as well as inhibiting LOX.
Maillard reaction products may be either pro-oxidants or antioxidants. Dark roast malts are well-known for their antioxidant properties whereas recent research has shown that certain crystal malts are pro-oxidants.
Hydrogen peroxide removal by peroxidases
As we have learnt, hydrogen peroxide is a key intermediate in the formation of oxygen radicals, both in the brewhouse and finished product. Peroxide can be removed by a reaction with polyphenols. This reaction is catalysed by the peroxidases.
Thus, the risk of radical generation in mashing depends on the ability of peroxidases to out-compete with non-enzymatic systems, which trigger the formation of hydroxyl from peroxide, via the Haber-Weiss reaction.