eTopic 16.1 From Barley and Hops to Beer
Making beer is a surprisingly complicated process. Beer production encompasses five main stages (Fig. 1): malting, the germination of barley grains; mashing, a stepwise heating process to promote starch hydrolysis; wort boiling with hops; fermentation; and postfermentation treatments.
Malting of barley. The barley grains are malted, or soaked in water, to encourage germination (Fig. 2). During germination, the endosperm (stored food) of the grain secretes gibberellin, a hormone that stimulates the growth of rootlets and the emerging stem. Gibberellin induces the aleurone (lining of the endosperm) to produce hydrolases, enzymes that break down starch to maltose and proteins to amino acids for use by the growing plant. The hydrolases actually become activated during the second stage, called mashing, when the grains are crushed and stirred in huge vats of water.
Mashing. While the grain is being mashed, the temperature is raised in steps, each of which optimizes the activity of a different hydrolase (Fig. 3). At 52°C, the protein hydrolases are activated. Then at 68°C, the starch hydrolases convert long-chain sugars to the disaccharide maltose, which can support yeast fermentation. The final temperature (77°C) inactivates all enzymes; then the mash is cooled, pressed, and filtered. The liquid filtrate of the mash is called wort.
Wort boiling. The wort is supplemented with hops, a traditional herb used for centuries in Europe to contribute a distinctive flavor of beer. After boiling with hops, the wort is again filtered.
Fermentation. The wort is inoculated with a special strain of Saccharomyces cerevisiae known as brewer’s yeast. The yeast conducts ethanolic fermentation on the maltose from hydrolyzed starch. At the same time, minor by-products, such as long-chain alcohols, impart good flavors. The time of fermentation is a key factor in the quality of the beer.
Postfermentation treatment. This step includes filtering of the wort to remove the bulk of the yeast. The product, now recognizably beer, still contains undesirable levels of acetaldehyde and diacetyl generated by partial oxidation. These oxidized by-products can be reduced by the few remaining yeast cells during a period of secondary fermentation. During secondary fermentation, oxygen is completely excluded and the temperature is decreased to 15°C or lower. The best German beers are aged at 2°C for several months.
Figure 1 Process of beer production. Barley is malted, milled, and mashed to break down long-chain sugars and proteins into short chains and monomers that yeast can digest, forming wort. The wort is fermented by yeast to make beer.
Figure 2 Malting the grain. Malting involves germination of the grain to induce expression of hydrolase enzymes.
Figure 3 Mashing the wort. During mashing, stepwise heating activates the hydrolases to break down carbohydrates and proteins.