This section describes the process of converting beer’s raw materials into the final product and how each stage in the process has an effect on the product’s many and varied sensory properties.
1.5.1 Introduction to the brewing process
The main ingredients from which most beers are brewed are malted barley, water, yeast, and hops. The processes that convert these raw materials into beer can be broadly grouped into malting, wort production, and fermentation.
Maturation, finishing, and packaging are the final steps in the process. A simplified representation of the main stages of this process is shown in Figure 1.2 for reference.
Figure 1.2 A simplified flow diagram of the main stages of the brewing process.
Malting in the maltings
Barley seeds are steeped in water, triggering their germination and the conversion of starch into fermentable sugars. The process
is halted by kilning.
Milling, mashing & mash separation in the mill and mash tun
Milling the malt makes it more easily hydrated in mashing, where wort - a sugar-rich liquid - is produced and then separated from the spent
grain.
Boiling in the kettle
Hops are added to the wort and boiled in the kettle.
Cooling & fermenting in the heat exchanger and fermenter
The liquid that leaves the kettle is cooled before entering the fermenter to ensure that the effective metabolisation of pitched yeast.
Maturing in the maturation tank
The ‘green’ beer that leaves the fermenter is matured, which has an effect on the development of the character of the beer.
Finishing & packaging into bottles, cans, kegs, etc.
Beyond the maturation tank, the beer may be filtered and the CO2 level may be altered. The
final product is packaged for transport.
1.5.2 Malting
When discussing barley in a brewing context, it is the plant’s grain that is being referred to. During malting, the kernels’ embryos are hydrated by
steeping the seeds in water. This activates hormones which trigger the production and secretion of enzymes, starting the partial breakdown of the starchy endosperm. This starch is the source of fermentable sugars in brewing.
Before excessive tissues associated with germination are produced, the process is stopped by heating the grain in a process called kilning. The aim is to reduce the moisture level in the grain, thereby halting the metabolism of the barley and stabilising the product. Lager-style beers are generally kilned to lower temperatures than ales. High kilning temperatures result in a darker colour product with more complex flavours. This process is the origin of the malty characteristics associated with beer. Dimethyl sulphide (DMS) can be produced from malt-derived precursors at this stage, although it can also be caused by bacterial infection during fermentation. DMS is considered a characteristic property of some beers at a relatively low concentration but can also be an off-flavour (Bamforth, 2009).
1.5.3 Wort production
First, the malt is milled so as to be easily hydrated. This is important for the activation of enzymes and solvation of substrates during the mashing stage, where the malt is mixed with water under controlled conditions (e.g. time, temperature, pH) in the mash tun to start the hydrolysis process. The resulting sugar solution is called wort, which is separated from the spent grains in the mash separation stage.
From here, the wort enters the kettle for hopping. Hops are perennial climbing plants. The important components of hops for the brewing process are located in the lupulin glands of the cones. The use of whole hop cones is now rare and most commonly, pelletized hops are used although resin and oil extracts can also be used.
It is in the kettle that hops are added. Hops contain resins (the most important of which are -acids) that are extracted in the wort boil and isomerized into
more soluble forms (iso- -acids). These acids are perceived to have bitter sensory characteristics. Hops also possess a complex mixture of essential oils which provide the wide range of different hoppy characters associated with beer (e.g. floral, fruity).
The timing of the addition of hops during the boil is important. Hops added at the beginning of a boil will lose virtually all their oils through evaporation, therefore contributing bitterness but hardly any flavour character. As a result, it is commonplace for a proportion of hops to be held back for addition during the final few minutes of the boil in order to contribute flavour. This is known as late hopping. An alternative and more traditional procedure for imparting hop-derived flavours is to ‘dry hop’ the product by adding hop cones to the cask.
1.5.4 Fermentation
Fermentation is primarily concerned with the conversion of carbohydrates (approximately 70% of which are fermentable sugars: maltose, glucose, fructose, sucrose, and maltotriose; Gibson (2006)) into alcohol, although it is also about producing a subtle mix of flavours. Esters, such as isoamyl acetate
which imparts an ‘artificial banana’ flavour to the beer, are typical of flavours
produced during fermentation. Before yeast can be pitched for fermentation, the liquid must first be cooled to such a temperature that yeast can metabolise effectively. This temperature is different depending on the genus/species of a yeast strain and the intended characteristics of the final product. Brewing yeasts are divided into two categories: Saccharomyces cerevisiae (associated with the brewing of ales) and Saccharomyces pastorianus (associated with the brewing of lagers). Ale yeast strains ferment at a higher temperature (typically 18-22°C) than lager yeast strains (typically 6-15°C) (Bamforth, 2009). Any unfermented sugars contribute to the sweetness of the final product.
The metabolism of yeast produces by-products. Some are desirable and characteristic of most beers. For example, carbon dioxide contributes to the perception of ‘tingliness’ and ‘bubbliness’ in the final product. Others are largely undesirable, such as diacetyl (Bamforth, 2009) which contributes a
‘buttery’ sensory characteristic to the final product. Yeast is able to ‘mop up’
the diacetyl again, converting it into compounds which do not have the same intense aromas. Another undesirable flavour produced during fermentation is acetaldehyde, a precursor of ethanol (Bamforth, 2009). To reduce levels of both diacetyl and acetaldehyde, fermentation time must be increased, which adds to the cost of brewing.
1.5.5 Maturation, finishing and packaging
The ‘green’ beer must be cooled in order to stabilise the product. It is then matured in order to, among other things, develop the flavour of the beer. After
this period of storage, the beer may be clarified using filters. Finally, the level of CO2 can be altered before packaging.
Packaging is an important consideration for stability of the product during storage before reaching the consumer. For example, the exposure of beer to certain wavelengths of light can lead to photolysis of iso- -acids and the production of 3-methyl-2-butene-1-thiol, commonly called ‘light struck’ by brewers (Stephenson and Bamforth, 2002) and described as having ‘skunky’
or ‘freshly brewed coffee’ aroma. This undesired characteristic can be avoided
by not using clear bottles and instead using green or, better still, brown bottles.
The coloured glass filters out the wavelengths of light that cause the photolysis. Alternatively, beers that use clear packaging (for brand or aesthetic purposes) can make use of tetrahydroiso- -acids as bittering agents for the reduced susceptibility to photolysis (Briggs et al., 2004).