Lesson 8 - Beer Properties End of Unit SAQ

*** Critically discuss the influence of metals and oxygen in beer production on the following aspects of beer:

               a. Beer foam

               b. Beer haze

               c. Beer flavour

               d. Beer gushing

Response:

In the brewing process, both the presence of oxygen and metals is critical for yeast health and performance. Without oxygen, yeast cannot sustainably create the fatty acids and sterols needed for cell division. Without trace metals, their metabolic activities would be hampered, and alcohol would not be formed. Even though they are essential, their presence and reactions are sometimes good but mostly undesirable. Like many things in brewing, we need to balance these.

For oxygen to be reactive, it needs the presence of metal ions, notably iron, copper or manganese, to create oxygen radicals; the reactions involved are the Fenton and Haber-Weiss. Iron or copper catalyses the formation of the very reactive hydroxyl radical from hydrogen peroxide.

• Beer foam

While oxygen does not play a direct role in beer foam, metal ions, and particularly divalent (²⁺) and trivalent (³⁺) cations, function to strengthen foam bubbles due to their ability to cross-link the hydrophobic isomerised α-acids; these bind with the foam stabilising proteins in the bubble boundary layers.

Manganese, aluminium, iron, magnesium, calcium, and zinc all bind hop acids. Zinc, added at 2 mg/L post-filtration, has been proposed as a foam stabiliser.

• Beer haze

Both oxygen and metals play a key role in haze formation. Oxygen can affect biological as well as non-biological haze. High levels of oxygen post-early fermentation encourage the growth of aerobic and microaerophilic bacteria, which can produce turbidity. High in-package dissolved oxygen (DO) will allow growth in unpasteurised beer.

The most important effect oxygen has, together with metal ions, is to catalyse the formation of a permanent haze. This happens by initiating the formation of proanthocyanidins via the oxidation of flavanols, which condense further to form tannins. It is the tightly bound tannin protein complexes which form and grow as permanent haze particles.

Haze complexes contain high levels of the metal ions, which, in high enough concentrations, can form hazes by themselves. Calcium, typically bound to oxalate, can form haze in-package if insufficient calcium is presented to precipitate the oxalate in the brewhouse or beer processing areas.

Oxygen also plays an indirect role in beer haze. High DO BBT/FBT (indiscriminately purged with CO₂ to reduce DO) often form collapsed foam bits that are clearly visible in the packaged product.

• Beer flavour

Oxygen plays a pivotal role in beer flavour together with metals, particularly iron, copper and manganese. These metals catalyse oxygen radical formation. This can oxidise fatty acids on their own, or together with the enzyme lipoxygenase (LOX), to form staling carbonyl groups such as E-2-nonenal (trans-2-nonenal) and other aldehydes. In turn, these are normally bound as adducts by sulphur dioxide (SO₂) produced by yeast. They are released as the ageing process progresses, giving the characteristic cardboard or papery stale flavour.

There are other oxidation reactions affecting flavour, notably, hop iso-α-acid oxidation which results in the decrease of bitterness over time. There is also the oxidation of melanoidins, which are formed from the reaction of sugars and amino acids in malting, boiling and ageing. This forms the toffee and sherry flavours that are desirable in certain high alcohol beer styles, e.g. barley wine and imperial stouts.

Introducing oxygen to correct a stuck fermentation can also cause oxidation of ethanol to acetaldehyde, producing a green apple flavour.

On their own, metals (notably iron) can alter the flavour. Packaging in cans is a particular risk where metallic flavours can develop if the lining has defects. Iron is often introduced when carbon steel comes in contact with beer, as well as from high beer soluble iron (BSI) grades of diatomaceous earth (DE or kieselguhr).

• Beer gushing

Secondary gushing relates to the formation of nucleation sites that allow for the rapid and uncontrolled release of CO₂ bubbles from the beer.

Calcium oxalate is a well-known cause of secondary gushing. About 15 mg/kg of calcium and oxalate will cause crystal formation and it is important that this occurs before filtration, and that calcium is excluded after filtration. Control is practised in the brewhouse where calcium is supplemented in the water, mash conversion vessel and/or kettle to obtain optimum levels (considered to be 50-150 mg/L in brewing water).

Various other metals have been implicated in gushing including iron, nickel, tin, and molybdenum. In the presence of hop acids, Fe³⁺ and Ni²⁺ cause gushing. The exact mechanisms are not fully understood but would, in part, involve stabilisation of the foam bubbles by cross-linking the iso-α-acids.