— The most important ingredient in brewing was the last one discovered...

The most important ingredient in brewing was the last one discovered, because yeast is a single-celled organism that is invisible to the naked eye. Still, brewers have long known that some unseen agent turned a sweet liquid into beer. Long ago, the action of yeast was such a blessing, yet so mysterious, that English brewers called it “Godisgood.”

Modern brewers usually brew with purified strains of yeast that give exactly the result they want. Some yeast strains are fairly neutral, creating alcohol and little more. Others add a whole range of complex side flavors that make beer more interesting.

How does yeast work? When it is added to a sugar-rich solution, it immediately begins to consume the sugars and create more yeast. But from the brewer’s point of view, the important thing is not the growth of more yeast, but the waste products of yeast metabolism: alcohol and carbon dioxide, that gives beer its fizz.

As the food supply runs down and the alcohol levels rise, the environment becomes literally toxic to the yeast, which becomes dormant. The brewer may draw off some of the yeast for the next cyc


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