Hordeum vulgare (barley)

The science of brewing

Learn more about fermentation

The oldest known alcoholic drink, identified from traces left on pottery excavated from China, dates from 7000 BCE. Today, drinking alcohol is popular in many parts of the world, including the UK. Here, we explore the process by which alcohol is produced fermentation.

Fermentation is an anaerobic process that produces ethanol (alcohol) and carbon dioxide from glucose. The process is catalysed by enzymes that are contained within yeast, a vital ingredient in brewing and baking.

Glucose → ethanol + carbon dioxide

Stages of brewing

  1. Malting. To prepare barley for use in brewing, the grains are soaked in water (steeped) and then spread on the floor and left for three to five days to allow germination. During this process, the starch stored inside is converted to sugars (catalysed by the enzyme amylase). The grains are dried (kilned) to produce malt.
  2. Milling and mashing. The malt is cracked (milled) and then dissolved into water and mixed at a warm temperature (mashed). Sugary water, known as ‘wort’, is strained off.
  3. Brewing. Hops are added for flavour, and the wort is boiled for several hours to sterilise and concentrate it.
  4. Cooling. The hops are filtered out, and the wort is cooled to a temperature suitable for yeast.
  5. Fermentation. Yeast – which digests sugars – is added, and fermentation takes place over ten days. In top fermentation (for ales) yeast cells rise to the top at the end of fermentation and are skimmed off. In bottom fermentation (for lager or pils) the yeast sinks to the bottom. Some Belgian beers rely on airborne yeasts for fermentation.
  6. Maturation and finishing. The beer is stored to allow it to mature and for secondary fermentation to take place. The beer is then filtered and bottled, canned or put into kegs.

Lead image:

Hordeum vulgare (barley).

Efraim Lev and Zohar Amar/Wellcome Images


Further reading

About this resource

This resource was first published in ‘Careers From Biology’ in June 2012 and reviewed and updated in November 2017.

Cell biology, Careers
Careers From Biology
Education levels:
16–19, Undergraduate, Continuing professional development