Microbes and food 1. Menu - you are what you eat
Hops photo
Picture 1.10a Hops are tall plants that are grown up strings. Dried hop flowers are added to beer for flavour.
1.10 Beer
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What is it?
Beer is an alcoholic drink. It is made by a process called brewing from barley grains, hops, sugar, yeast and water. The barley is turned into malt. The unique bitter taste of beer comes from the resins and oils in the hop flowers. In some countries other cereals such as wheat or millet are used to make beer. Beer is sold in bottles, cans, barrels or casks. It is a very ancient drink and has been made in Britain since well before Roman times.

Ales and lagers are different types of beer. They are produced by using different strains of the yeast Saccharomyces cerevisiae and by the way in which the fermentation process is controlled.

Alcoholic drinks include wine, cider and beer. They are made by fermentation during a process called brewing. Most fruit and vegetables can be used to make an alcoholic drink because they contain carbohydrates such as starch and glucose sugar. When the carbohydrate is fermented by yeast, one of the products is alcohol.

Glucose -> alcohol + carbon dioxide + water

Generally grapes are used to make wine, apples for making cider and cereals such as barley or wheat for brewing beer.

How is it produced?
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Barley grains photo
Picture 1.10b Barley is malted and fermented to produce the alcohol in beer. The malt's colour depends on the heating time.
Malting. The barley is soaked in water and the grains are spread out to germinate. When roots and shoots appear the barley is dried in kilns to stop further growth. During germination enzymes change starch in the grains to sugar. Heating in the kiln gives the malt its colour and flavour. The longer it is kilned, the darker the beer.

Mashing. The malt is ground up to become grist. It is mixed or ‘mashed’ with water at a carefully controlled temperature (65 °C). The water or ‘liquor’ is quite important as the minerals in it, especially the calcium content, affect the quality of the beer. The sugars in the malt dissolve in the water and this liquid is called wort.

Fermenting vessel photo
Picture 1.10c The hops and malt are boiled in a copper before going into the fermenting vessel. The product can be sold in kegs, bottles or cans.
Boiling. Hops are added to the wort which is boiled for up to two hours. Extra sugar is sometimes added at this stage depending on the type of beer required. Boiling extracts the bitter flavour from the hops, stops further enzyme activity, sterilises the wort and, according to the boiling time, controls the strength of the beer. The wort is then strained through filters and cooled to about 20 °C before being pumped into the fermenting vessels.

Fermentation. The yeast is added to the wort where it grows and converts the sugar to alcohol and carbon dioxide. The temperature is controlled at 8 – 12 °C to produce lager and 12 – 21 °C to make ale. An ale fermentation takes 4 – 6 days, whereas lager production is much slower and takes up to two weeks. During fermentation the yeast population multiplies about six times. The excess is skimmed off. It can be re-used, usually after an acid wash to control any contamination by bacteria, but eventually the yeast performance drops and it is sold on to be made into yeast extract or animal feed.

Downstream processing. Some beers are run into casks where more sugar is added and a secondary fermentation takes place. Keg beer is filtered then pasteurised to kill the yeast. Bottled or canned products are usually filtered and centrifuged before packaging. Some beers are carbonated to improve their sparkle.

Beer photo
Picture 1.10d A glass of beer.

How does it spoil?
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Some microbes in the brewery such as lactic acid bacteria and acetic acid bacteria can produce off-flavours and cloudiness in the liquid at various stages in the process, including the final product. Wild yeasts can also cause problems. These are all controlled by careful attention to hygiene and the various heat treatments that the beer undergoes during manufacture.

Beer is a very safe food; until quite recent times it was safer to drink beer than the local water. It is preserved by the alcohol, pH and low nutrient content.

Alcohol can be dangerous to health.