Yeast breathes life into beer: An ode to oxygen

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Oxygen is crucial to life and arguably oxygen is the most important element for life on earth. Oxygen is released during photosynthesis by plants and many microbes. Oxygen is consumed by humans, by animals and even by yeast in beer during respiration. Brewer’s yeast is a facultative aerobe. This is biologist’s term for a special “super power” of yeast. Yeast can respire aerobically (using oxygen) and also anaerobically (without using oxygen), but only anaerobic respiration produces alcohol.

Yeast needs oxygen in order to multiply significantly. To brew beer, but also to make bread or vinegar, we need many, many yeast cells, and oxygen is needed to produce all these necessary yeasts. While using oxygen, yeasts convert sugars to carbon dioxide and water. Once they consume all the available oxygen, yeasts go into their anaerobic respiration. During this period, yeasts convert sugars to carbon dioxide and alcohol. A key component in beer and bread, alcohol is also an intermediary in vinegar. In bread the alcohol evaporates during the baking process leaving those spongy holes we recognize. When making vinegar, the alcohols are converted to acetic acid and if the yeast continues, the vinegar is converted to water. In beer, alcohol is a key ingredient and hopefully remains.

If there is too much oxygen in beer, the beer can take on a stale or cardboard-like taste. Oxygen can make a great beer go bad quickly. Brewers take great care to ensure that there is the correct amount of oxygen for yeast to reproduce and thrive. Because we live in an environment that is 21 percent oxy-
gen, once there is ample yeast to digest all the sugars, brewers must take great pains to keep the oxygen levels as low as possible in the beer-making process. After beer is packaged, any excess residual oxygen will affect the flavor and quality of the beer.

Oxygen is both good and bad for the little critters called yeast that create great beer. Yeast can thrive with oxygen and do good works without it. Brewers are very concerned about oxygen from two different perspectives. Life as we know it requires oxygen, but beer as we know it needs to be nearly free of oxygen.