The stability of beer
Trying to maintain the flavour stability of beer begins before the bottles even leave the brewery. Due to the presence of hop-bitter compounds, ethanol and acidic compounds (such as propanoic and ethanoic acids), the finished beer has a high level of resistance to being spoiled by microbes; in fact, all cask beers and some keg and bottle beers, are untreated (the resistance relying on the stability of the beer). Leaving a beer untreated can have an impact on its shelf life, but the attraction of drinking fresh beer outweighs the shortened lifetime.
The presence of microbes can result in an off-flavour in a beer however, so mainstream commercial breweries treat their beers in one of two different ways in order to try and extend the shelf life. This involves using very fine filters or pasteurizing the beer by heating to 60 °C, which eliminates any microbes before packaging in a sterile environment.
After packaging, as time passes, the taste of beer diminishes and it can develop a stale quality – its bitterness is replaced with sweet, toffee-like flavours. Eventually, as these sweet flavours continue to rise, the beer can take on a 'catty or cat-pee like' taste, officially called ribes. When the 'catty' taste reaches its maximum, a cardboard or 'straw-like' taste dominates, which indicates staleness. Unfortunately, little is known about how the 'catty' taste develops in beer, but it has been linked to the presence of a smelly thiol called 3-mercapto-3-methylbutyl formate, Me2C(SH)–CH2–CH2–O–CHO, amongst other compounds. What is known is that if there is no oxygen in the packaged beer, then no catty taste develops.
(E)-2-Nonenal (or trans-2-nonenal) is the main contributor to the stale flavour in beer. However, other compounds with similar structures (containing both a carbonyl (C=O) group and a carbon-carbon double bond (C=C)) will also contribute to the cardboard or straw-like flavour.
If oxygen is present in the packaged beer it does not contribute to the production of the staling compound (E)-2-nonenal; instead, it is suggested that this compound is made earlier during the brewing process. In the fermentation process, (E)-2-nonenal binds to amino acids, which function like a protecting group, and stop the hydrogenation of (E)-2-nonenal to a tasteless alcohol. Then, during storage, (E)-2-nonenal is released, resulting in the beer gaining a stale taste due to its presence.
You might have noticed that beer bottles are normally made of glass that is brown in colour. This is important as it reduces the beers exposure to sunlight, which can result in 'lightstruck' beer; when a beer becomes light-struck, it takes on a skunky flavour. This process can only occur in beers made from hops as the mechanism takes place on hop-derived iso-alpha acids.
The process of a beer becoming lightstruck occurs through a mechanism involving free radicals – species containing an unpaired valence electron. Energy from the sunlight is transferred to the hop-derived iso-alpha acids, giving the compound enough energy to break the (HO)C–C(=O) bond leading to the six-carbon side chain (in pink). This occurs through homolysis, where the bond cleaves symmetrically.
The smaller side-chain radical can then 'kick out' carbon monoxide in a propagation reaction, commonly called a decarbonylation reaction.
The resultant free radical, with a five-carbon chain, can then react with sulfur-containing proteins (abbreviated below as HS-R), to make 3-methylbut-2-ene-1-thiol (or MBT), which has an odour similar to that of skunk spray. The precise mechanism of this step is still unclear.
Using brown glass bottles helps to ensure that very little light, with the correct wavelength, can reach the hop-compounds to produce light-struck beer. Beer is also frequently sold in cans or kegs, which eliminate the possibility of light-struck beer altogether, as no light can penetrate the container. If clear or green glass bottles are required for marketing reasons, then hydrogenated hop extract replaces the natural version, which effectively 'skunk-proofs' the beer.
The use of hydrogenated hop extract results in the removal of one or more of the side chain carbon-carbon double bonds, which can result in two different scenarios: either the light is hindered from being able to interact with the molecule, or the breaking of the bond to the side chain can still occur, but the radical that is formed lacks a C=C bond. The absence of a C=C bond means the radical is unstable, so only very small amounts are formed.
After the lengthy process of brewing beer, it is important to take measures to increase shelf life and the stability of the final brew… because no one wants to drink stale or skunky beer (even if it is safe to drink)!
See below for a set images that further illustrate the flavouring components (including the chemical structures above).
© University of York