There are many factors that determine beer flavor, but the most finicky and temperamental of them all is yeast. These microorganisms belong under Kingdom Fungi, and as the name suggests, they are indeed fun guys. Before we get this party started though, a little beer science.

Fungi spores

Preparing the Ingredients
There are three main ingredients that go into making beer: water, barley, and hops. Yeast is considered to be an activating agent. To prepare the barley, the grain must first be malted or partially sprouted. Malting causes the grain to produce a starch-breaking enzyme that turns starch into simple sugars, which can then be consumed by the yeast. Hops are added for extra flavor as well as being a mild preservative.

The malted grain is first boiled or heated in water (a process called mashing) to activate various protein-digesting enzymes called proteases. Some brewers will heat the mash at different temperatures; this process (called multi-step mashing) results in a heavier beer, creamier mouth feel, and stiff foam. Hops can be added during this stage to give the beer a bitter flavor. The longer the hops seep in the mash, the more bitter the beer will be.

After the solids have been strained out of the mash, the remaining brown sweet liquid is called the wort. When cooled to room temperature, yeast is added into the wort and the entire batch of liquid is left alone for the magic of fermentation to proceed.

Small but mighty ale yeast:
Saccharomyces cerevisiae

Fermentation
Fermentation is a chemical reaction inside the cell that extracts energy from sugar in the absence of oxygen. This energy is used by the cell to make proteins, to grow, and to reproduce. In beer making, we feed sugar to yeast, not to harvest the energy, but to drink and enjoy the waste products: carbon dioxide and ethanol. Yes, beer is liquid yeast waste.

Finicky Yeast
Beer is mostly brewed like a chemistry experiment. Minerals and solutes in water can be adjusted, analyzed, and filtered. Brewers can pick the strain of hops that contain the perfect combination of alpha and beta acids. Barley can be mixed with other grains such as wheat and rye.

There is, however, an element of artistry in brewing that has nothing to do with the brewer and everything to do with the yeast. During fermentation, yeast not only produce carbon dioxide and ethanol as waste products, they also produce hundreds of other molecules called secondary metabolites. These compounds give each batch of beer a unique flavor. A brewer can start off with the same type of water, the same grain bill, the same strain of hops, but adding a saison yeast versus an ale yeast will produce a completely different beer.

Factors that can anger or please the yeast include: time, temperature, light, extra sugar, low sugar, too much alcohol, other microorganisms (competing yeast or bacteria), oxygen, minerals, and even the material of the pot used for the mash. Some factors are controllable, that’s why breweries are so clean and sterile, but other factors can throw a batch of beer completely out of whack.

So next time you’re enjoying a beer, remember the fun guys who labored away: eating sugar, farting carbon dioxide, and peeing ethanol. Without them, that beer you are drinking would just be flat, bitter barley soup.

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