The production of beer is a multistep process designed to break down the starches of grains such as barley to produce sim ple sugars, which can then serve as a substrate for alcoholic fermentation by yeast. Yeast alone cannot convert grain to alcohol because they lack the enzymes that degrade starch, the primary carbohydrate of grain. Sprouted or germinated barley, however, known as malted barley or malt, naturally contains these and other important enzymes.

Dried, roasted malt is ground, mixed with adjuncts (starches, sugars, or whole grains such as rice, corn, or sorghum), and then soaked in warm water in a process called mashing (figure 32.5). During this process enzymes of the malt act on the starches, converting them to fermentable sugars. The final characteristics of the beer such as color, flavor, and foam are derived entirely from compounds in the roasted malt. The adjuncts simply serve as readily available, less expensive sources of carbohydrates for alcohol production.

After mashing, the residual solids or spent grains are removed to yield the sugary liquid called wort. Hops, the flowers of the vinelike hop plant, are added to the wort to impart a desirable bitter flavor to the beer and contribute antibacterial substances. The mixture is boiled for several hours to extract the flavor components of hops, concentrate the wort, inactivate enzymes, kill most microorganisms, and precipitate proteins, facilitating their removal. The wort is then cen-trifuged to remove the solids, including hops and precipitated proteins, and cooled before being transferred to the fermentation tank.

Special strains of brewer's yeasts (as opposed to baker's yeasts) are commonly used in beer-making. Bottom yeasts such as a Saccharomyces cerevisiae strain referred to as S. carlsbergensis, which tend to form clumps that sink to the bottom of the fermentation vat, are used to make lager beers. These yeasts ferment best at temperatures between 6°C and 12°C and usually take 8 to 14 days to complete fermentation. In contrast, top-fermenting yeasts, such as certain strains of Saccharomyces cerevisiae, are used to make ales. Top-fermenting yeasts are distributed throughout the wort but are carried to the top of the vat by the rising CO2. They also ferment at higher temperatures (14°C to 23°C) and over a shorter period (5 to 7 days). Most American beers are lager beers produced by the bottom yeasts. Porter and stout beers are made using top-fermenting yeasts.

Malt barley is cracked open before entering the mash tun.

The ingredients of mash-malt, water, and sometimes adjuvants-are mixed.

The mash is heated, allowing enzymes in the mash to convert starches into fermentable sugars. The liquid wort is then separated from the spent grains.

The wort is pumped into the brew kettle, where it is boiled while hops are slowly added. The wort is separated from the hops and cooled.

A special strain of Saccharomyces, or brewer's yeast, is added to the wort. Fermentation begins. Excess yeast cells are removed after fermentation.

Spent grains

Spent hops

Spent grains

Spent hops

Surplus yeast


Mixing tank

Mash tun

Brew kettle

Fermenting tank

32.3 Microorganisms in Food and Beverage Production 809

allowed to escape during fermentation. After aging, beer is clarified by filtration, microorganisms are removed or killed using membrane filtration or pasteurization, and the product is packaged.

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