The main stages in the brewing process are the preparation of a fermentable extract (sweet wort) from malted barley and other cereal grains, boiling the wort with hops to introduce flavor, fermentation of the hopped wort by yeast, separation of yeast cells from the beer and, finally, maturation and conditioning. The finished beer is distributed in barrels or filled after filtration and pasteurization into bottles, cans or casks.
Preparation of Raw Materials
The main carbohydrate of barley is starch, which cannot be assimilated by yeast and so an important part of technology of brewing is the controlled germination of the barley to produce amylase enzymes. The process is known as malting.
The barley grains are washed and soaked for 2 to 3 days to stimulate development of the embryo; the water is then drained off and the seeds germinated. The embryo is allowed to grow until the plumule attains a length equal to approximately three-fourths of the kernel length and is then dried to halt growth.
During the brief period of germination, the embryo produces digestive amylases, which convert the starch into dextrin, maltose, and a little glucose. The maltose is the principal substrate for yeast fermentation.
Kilning dries the malted barley, during which coloured compounds may develop by the caramelization of the sugars, giving the correct colour to beer.
The next major step in beer production is known as mashing. The dried malt is ground and mixed with hot water. Most of the enzymatic conversion of starch to maltose by amylases takes place during mashing, which requires about 2 hours. Some of the starch, the dextrin fraction, is not broken down and is not metabolizable by yeast so it remains intact throughout the fermentation phase and gives body to the beer.
The aqueous extract prepared during the mashing process, called wort, is filtered off from insoluble materials and husks and is then boiled with hops, the female inflorescence of the hop plant containing essential oils and resins, which impart characteristic flavors to the beer. The hopped wort is used as the fermentation medium for beer production.
The fermentation phase is normally a single stage batch process, which can be carried out in simple wooden vats or in large stainless steel tanks. The hopped wort is cooled to a temperature between 8 and 160C (the temperature is partially determined by the type of beer being produced) and placed in fermentation vats. It is then inoculated with a single strain of Saccharomyces cerevisiae, which has been selected by the brewer to produce a beverage of the correct strength and flavor.
During a fermentation period of about 5 to 14 days, the sugar is converted to alcohol and carbon dioxide. The bubbles of carbon dioxide rise to the top of the vat and carry with them the yeast cells and dark flocculent materials from the liquid. The resultant foamy scum must be removed periodically.
After inoculation, the fermentation proceeds with a rapid growth phase, which is associated with a rapid period of ethanol formation and sugar consumption. After this period, the rate of alcohol formation slows down.
Maturation and Conditioning
At the completion of the fermentation period, alcohol as well as some other flavor components (esters, organic acids, and sulphur compounds) are present. However, the flavors in the beer are not fully developed. The immature beer is transferred to closed vessels for a conditioning period and is maintained at 3 to 6oC. During this time, changes in the flavor occur as the yeast cells assimilate some of the diacetyl produced during the primary fermentation, some of the sulphur compounds are lost as carbon dioxide bubbles through the beer and some of the yeast cells undergo autolysis, adding their cell contents to the beer.
Although there is some carbon dioxide derived from the activities of the yeast, typically additional carbon dioxide is injected into the beer under high pressure. The beer is both filtered and treated with proteolytic enzymes to improve clarity. Finally, the beer is bottled and pasteurized.
Biochemistry of Beer Brewing