Fundamentally beer is the product of the alcoholic fermentation by yeast of extracts of malted barley. Whilst malt and yeast contribute substantially to the character of beers, the quality of beer is at least as much a function of the water and, especially, of the hops used in its production.
Barleystarch supplies most of the sugars from which the alcohol is derived in the majority of the world’s beers. This starch is enclosed in the cell wall and proteins within the barley, and these wrappings are stripped away in the malting process (essentially a limited germination of the barley grains), leaving the starch preserved.
In the brewery, the malted grain must first be milled to produce relatively fine particles, which are for the most part starch. The particles are then intimately mixed with hot water in a process called mashing. The water must possess the right mix of salts. For example fine ales are produced from waters with high levels of calcium. Famous pilsners are from waters with low levels of calcium. Typically mashes have a thickness of three parts water to one part malt and contain a stand at around 65 deg C, at which temperature the granules of starch are converted from an indigestible granular state into a ‘melted’ form which is much more susceptible to enzymatic digestion.
The enzymes which break down the starch are called the amylases. They are developed during the malting process, but only start to act once the gelatinization of the starch has occurred in the mash tun. Some brewers will have added starch from other sources, such as maize or rice, to supplement that from malt. These other sources are called adjuncts.
After perhaps an hour of mashing, the liquid portion of the mash, known as wart, is recovered, either by straining through the residual spent grains or by filtering through plates. The wort is run to the kettle (sometimes known as the copper, even though they are nowadays fabricated from stainless steel) where it is boiled, usually for I hour. Boiling serves various functions, including sterilisation of wort, precipitation of proteins (which would otherwise come out of solution in the finished beer and cause cloudiness), and the driving away of unpleasant grainy characters originating in the barley. Many brewers also add some adjunct sugars at this stage and at least a proportion of their hops.
The hops have two principal components: resins and essential oils. The resins (so-called a-acids) are changed (‘isomerised’) during boiling to yield iso-a-acids, which provide the bitterness to beer. This process is rather inefficient. Nowadays, hops oils are often extracted with liquefied carbon dioxide and the extract is either added to the kettle or extensively isomerised outside the brewery for addition to the finished beer (thereby avoiding losses due to the tendency of the bitter substance to stick on to yeast).
The oils are responsible for the ‘hoppy nose’ on beer. They are very volatile and if the hops are all added at the start of the boil then much of the aroma will be blown up the chimney. In traditional lager brewing a proportion of the hops are held back and only added towards the end of boiling, which allows the oils to remain in the wort. For obvious reasons, this process is called late hopping. In traditional ale production, a handful of hops is added to the cask at the end of the process, enabling a complex mixture of oils to give a distinctive character to such products. This is called dry hopping. Liquid carbon dioxide can be used to extract oils as well as resins and these extracts can also be added late in the process to make modifications to beer flavour.
After the precipitate produced during boiling has been removed, the hopped wort is cooled and pitched with yeast. There are many strains of brewing yeast (Saccharomyces cerevisiae),and brewers jealously guard and look after their own strains because of their importance in determining brand identity.
Fundamentally brewing yeast can be divided into ale and lager strains, the former type collecting at the surface of the fermenting wort and the latter settling to the bottom of a fermentation (although this differentiation is becoming blurred with modern fermenters). Both types need a little oxygen to trigger off their metabolism, but otherwise the alcoholic fermentation is anaerobic. Ale fermentations are usually complete within a few days at temperatures as high as 20°C, whereas lager fermentations at as low as 6°C can take several weeks. Fermentation is complete when the desired alcohol content has been reached and when an unpleasant butterscotch flavour which develops during all fermentations has been mopped up by yeast. The yeast is harvested for use in the next fermentation.
In traditional ale brewing the beer is now mixed with hops, some priming sugars and with isinglass finings from the swim bladders of certain fish, which settle out the solids in the cask.
In traditional lager brewing the ‘green beer’ is matured by several weeks of cold storage, prior to filtering.
Nowadays,the majority of beers, both ales and lagers, receive a relatively short conditioning period after fermentation and before filtration.This conditioning is ideally performed at -1°C for a minimum of three days, under whichconditions more proteins drop out of solution, making the beer less likely to go cloudy in the package or glass.
The filtered beer is adjusted to the required carbonation before packaging into cans, kegs or glass or plastic bottles.