The Science of Beer: Yeast – Part 3

| November 1st, 2011 | No comments

Trying to explain yeast and what it does is much more difficult than trying to explain the other components of beer. Because of this, I have broken up my discussion of yeast into several posts. This is the third and last post in which I describe what affects fermentation.

As previously discussed, both species of beer yeast consume sugars and leave alcohol and carbon dioxide as waste products. I also mentioned previously that the process of fermentation was seen as a gift from god by early civilizations due to their lack of knowledge in unicellular microorganisms. Even today, with this understanding, it is also understood that brewers make wort, yeast make beer. Understanding what affects the yeast to do its job is what allows us today to successfully ferment at an expected level.

It is important to remember that yeast is a living organism, just like humans. While we have human civilizations living all across the world, there are certain conditions that must be met to allow us to survive and live in extreme conditions. If our body temperature gets too cold or too hot, we die. While humans can live and grow eating poor quality, empty calories, we can grow healthier and stronger with proper nutrition. It is also important that we are protected from bacteria and toxins which harm us, even one’s that our body produces.

Each strain of yeast has a temperature range in which it prefers to exist in. In general, lagers are known for their cold fermentation temperatures, in the 40’s-60’s (fahrenheit). Ales however, are known for their warmer temperature ranges, from the 60’s-70’s. The specific range that a given strain is meant to ferment at, ensures that the yeast remain healthy and actively consuming sugars. Should the temperature get too low, the yeast goes dormant and stops fermenting. Should the yeast get too hot, it will still ferment, however it will produce off-flavors that are not ideal to the flavor of the beer. To compare it in human terms, if you get too cold, you get hypothermia and go dormant (in a sense). Or compare doing housework inside your air-conditioned home as opposed to doing yard work in the middle of summer. You won’t die working outside, but you will smell a lot worse!

While yeast consumes sugar as it’s main diet, there are important other minerals that keep the yeast healthy enough to keep consuming sugar, much like us humans need adequate amounts of the appropriate vitamins and minerals in our diet. In the case of yeast, calcium, magnesium and zinc are important. Magnesium, for example, allows for better cellular metabolism, allowing the yeast to convert energy to continue consuming as well as reproduce. Some of these nutrients come from the malt and some come from the water. The water is an area that many people do no consider enough in beer making. Purified water is not ideal in brewing as it has been cleaned of all it’s minerals. Chlorinated water affects the yeasts ability to live and properly grow. Besides affecting the yeast, water chemistry also alters the mashes conversion of starch into sugar and the affect hops have in beer.

Lastly, while we may enjoy the alcohol and carbon dioxide in beer, remember that those are waste products to yeast. Imagine for a moment if you were trapped in a jail cell with no toilet. Yeah, exactly. Different yeast strains have difference tolerance levels for alcohol. If a particular strain doesn’t have a very high tolerance, than it won’t be ideal to use when making a barley wine. Instead of a high alcohol beer, the yeast will die off as the alcohol content rises leaving you a beer that has less alcohol and tastes rather sweet. Even commercial breweries like Dogfish Head have difficulty when making their 20% 120 Minute IPA. On the TV show Brewmasters, Dogfish Head was forced to dump an entire batch due to the yeast not finishing the fermentation.

Whether you are brewing a high alcohol beer, a lower session-style beer, it is important to consider all of these facts when it comes to brewing. Use a strain that’s best suited for the style being made. Ensure that you have the means to keep the yeast at the correct temperature specified for a given strain. Most people make a yeast starter, which is essentially a small batch of beer made for the same purpose that sports teams have pre-season games. It gets the yeast started fermenting and reproducing so it can be strong and ready for the main event. The higher the alcohol the beer will be, the more yeast will need to used. Consider trying to build a house by yourself as opposed to entire team of people. Sure you might be able to finish the house alone, but it’s going to take much longer and likely be of a lesser quality than one built by 10 people.

There is still much more to be said about yeast and it’s affects on beer making. As I mentioned a few times, while there are two main species of yeast, there are over a hundred strains within those two species, each with different levels of attenuation, flocculation and alcohol tolerance. All yeast give off different levels of the off-flavors that are a part of normal fermentation. In some strains of yeast it may be more of a particular flavor, that is best suited for one style of beer, but not another. In addition to these two species of yeast, there are other wild yeast strains that are used for the affects that they have on beer. For example the brettanomyces strains can consume even more polysaccharides than ale or lager yeast can, and it also produces some tart flavors. Some brewers will add this after completing a fermentation with an ale yeast, or some may completely ferment a beer using only this yeast. There are endless possibilities. To be a great brewer, you must truly understand how to use yeast.