There are few things better than an ice cold beer on a hot day. Chances are, when you crack open a beer this summer, you probably won’t be thinking much about chemistry – but it’s the particular chemicals in beer, produced in the brewing process, that give beer both its bitterness and flavour. It’s a real chemical team effort, with several important chemical families, each contributing something different but vital to the eventual taste of the beer.
Firstly, it’s worth discussing the brewing process that leads to the creation of beer. Before brewing can even start, the barley for the brewing must be prepared. This involves steeping it in water, then allowing it to germinate for several days, before then drying in a kiln. After this, it is broken down in a mill, so that enzymes can more easily reach the starch molecules during fermentation. Once broken down, it’s mashed with hot water, producing wort, a sugar-rich liquid.
The wort is then strained and transferred to the brew kettle. This is where the hops are added, along with any other ingredients. The boiling process sterilises the wort, whilst the hops add bitterness and flavour to the beer. Any solids are separated, then the mixture is cooled to a temperature where the yeast can be added safely, as it is otherwise unable to grow at high temperatures. The yeast is added in a fermentation tank, where the beer is left to ferment, and sugars are converted into alcohol. It’s then moved to a maturing tank, where it remains for varying lengths of time dependent on the style of beer, after which it is filtered and bottled.
In terms of the bitterness of the beer, the hops that are added are key, as it’s compounds originating from them that account for the bitter flavour. Hops contain organic compounds called alpha and beta acids. Most of the bitterness comes from alpha acids, of which there are many, but five main compounds: humulone, cohumulone, adhumulone, posthumulone and prehumulone. During the brewing process, they are degraded to form iso-alpha acids; these compounds are more soluble, and contribute much of the bitterness associated with beer. The main alpha acid present in the majority of hops is humulone, although hops with varying compositions may be selected in order to vary the type and level of the bitterness in the beer.
Beta acids are another class of compounds found in hops, and transferred to beer in the brewing process. There are three main types of beta acid, lupulone, colupulone and adlupulone. They impart a harsher bitterness than the alpha acids, but as they are insoluble, their contribution is much lower. They do not isomerise in the same way as the alpha acids during fermentation, but instead slowly oxidise to produce their bitter flavour. Because they take much longer to do this, their effects become more potent the longer the beer is fermented and aged.
Alpha and beta acids have a number of other properties that can be both beneficial and detrimental to beer brewers. Firstly, both classes of compound have antiseptic properties, preventing unwanted growth of bacteria and prolonging shelf life, and also enhancing the ability of the yeast to grow during the fermentation stage. Alpha acids, however, have an unwanted effect; the iso-alpha acids produced by their degradation can react with light and riboflavin from the malt to produce unpleasant tasting compounds. Beer in which this occurs is known as ‘lightstruck’, and in order to prevent this from occurring, beer is always stored in opaque containers or vessels made of dark glass.
Whilst alpha and beta acids provide the bitterness of the beer, essential oils from the hops are responsible for the bulk of the aroma and flavour. Some of these oils are very volatile, evaporating easily; for this reason, they are usually obtained by adding hops later in the brewing stage, or by utilising ‘dry hopping’, a technique which involves soaking hops in the finished beer for several days or weeks. Some hops are actually specifically grown as ‘finishing hops’ for the purpose of adding later in the brewing process to produce these essential oils.
Over 250 essential oils have been identified in hops. Of these, myrcene, humulene and caryophyllene are the main oils found in the highest concentrations; humulene, in particular, is responsible for the characteristic hoppy aroma of beer. American hop varieties tend to have higher amounts of myrcene, which adds a citrus or piney aroma, whilst caryophyllene contributes a spicy flavour.
A final class of compounds, esters, can also play an important part in the flavour of a beer. They are present in varying degrees, dependent on the type of beer – lagers will contain minimal concentrations, whilst in ales they are commonly found in concentrations that are significant. They form through the reaction of the organic acids in hops with the alcohol in beer, along with a molecule called acetyl coenzyme, also found in hops. As volatile flavour compounds, they are responsible for fruit-like flavours in beers. The final concentration of esters varies wildly depending on the conditions during brewing; pH, temperature, and even agitation of the mixture can all have an effect.
There are a number of different esters, many of which lend differing smells. Ethyl acetate is one of the most common – it actually has a aroma not dissimilar to that of nail polish at high concentrations, but at the concentrations in beer it imparts a fruity aroma. Isoamyl acetate gives a banana-like scent, ethyl butanoate gives a tropical/pineappley one, and ethyl hexanoate an apple/anise note. It perhaps isn’t surprising to note that, as well as being found in beers, esters are also often used in perfumes for their pleasant olfactory qualities.
Finally, it’s worth noting that these are just the main families of compounds; as with all foods and drinks, there are a vast number of compounds present, many of which will have some impact, however minor, on the flavour and aroma. So, the next time you order a beer, consider the hundreds of chemical compounds working together to produce that perfect taste.
References & Further Reading