Yeast vs. Bacteria
This month I’m going to talk about the microorganisms that are welcome in our beer and others that are uninvited visitors. In the case of both yeast and bacteria, there are scenarios where their presence can be useful to make a great beer and other times where they can cause harm.

Yeast We Want
The yeast that we want present in a beer is aptly named brewer’s yeast. Having the proper amount and type of yeast will turn the sugars from malt into alcohol and CO2. The scientific name for brewer’s yeast is Saccharomyces cerevisiae. This is the same yeast that is involved in making wine and baking bread. We start with a small amount of yeast and give it the proper conditions to grow and multiply. When we have enough of it, it gets pitched into the sugary wort. By removing the oxygen from the fermentation vessel, we force the yeast into the fermentation pathway instead of the growth pathway it was just in. Within a couple weeks, the yeast has eaten all the sugars and made the desired amount of alcohol.

Yeast We Don’t Want
Instead of brewer’s yeast, this class of organisms is called wild yeast. Yeast is all around us in our day to day lives. It lives on trees, flowers, fruit, and vegetables. If we’re already brewing a beer with brewer’s yeast, then wild yeast is an unwanted guest. These yeast strains can be found around the brewery environment and without proper cleaning process, can be introduced into a beer. This will result in a completely different sensory profile than was intended by the brewer. Wild yeast can give off a large variety of sensory notes from medical and clove to barnyard and horse blanket. However, there are beer styles where this yeast is desired and expected. These include lambics, sours, and barrel aged beers. It might sound crazy, but with the right balance, barnyard flavors can be a nice addition to a beer.

Bacteria We Want
While yeast is our microorganism of choice when fermenting beer, bacteria is our friend when we want a sour beer. Lactobacillus and pediococcus are two bacterial strains that are often used in the brewing industry. While using the same process to create wort for yeast, the brewer will make this and pitch the bacteria. However, this solution is usually held at a high temperature compared to fermentation temperatures that are used for yeast. Sour bacteria thrive and grow at these higher temperatures. The brewer can track how sour the wort is becoming and once it reaches the ideal level, the solution will be boiled thus killing the bacteria and halting the sour process. From there, yeast is pitched like normal and they will create alcohol for our sour beer.

Bacteria We Don’t Want
Luckily for us, beer is a pretty inhospitable environment for bacterial growth. The presence of hops and alcohol are usually enough to make sure nothing is growing or kill off anything that sneaks its way into the fermenting beer. However, the handful of organisms that are able to survive can wreak havoc on a beer. These bugs can cause unpleasant off flavors such as rotten eggs, rubber, rancid fruit, and vinegar. They can also cause changes in mouthfeel by making the beer syrupy or ropey. Ew. Even if you do happen to encounter an infected beer out in the wild, the good news is that the presence of these types of bacteria won’t make you sick.