Sunday, April 29, 2012

Yeast Basics

Saccharomyces cerevisiae (brewers and bakers yeast) has long played a part in human society. It has allowed humanity to create alcoholic beverages and baked goods. The Latin name Saccharomyces means "sugar fungus", crediting this yeast with its ability to chew through large amounts of glucose and maltose. The second part of the name, Cerevisiae (think cerveza in Spanish) is Latin for beer. Another name for this same yeast is "budding yeast" commenting on the way in which it reproduces.

In order to understand how yeast is working in your beer you need to understand their life cycle. Yeast are a single celled fungus that reproduces asexually (most of the time). When yeast are reproducing asexually they are not sharing any genetic material with other yeast. This makes all yeast in a given culture identical to each other (essentially clones). Yeast reproduce through a process known as budding (hence the name budding yeast). In order to do this a yeast cell, known as the mother yeast, grows a bud that enlarges until DNA can be moved into it and it matures enough to split off from the mother cell. The newly created cell is known as the daughter cell.

This is a light microscope photo of budding yeast. The nucleus of each cell is highlighted in blue. Cells boxed in green are not currently budding. Cells boxed in red have a daughter cell beginning to bud off of them.
Yeast are able to grow in both the presence and absence of oxygen. However, oxygen is needed for yeast to create sterols which are used in the creation of the yeast cell wall (basically). Without enough oxygen present yeast will soon stop reproducing but will continue to eat available sugars. For this reasons yeast are cultured in the presence of oxygen.

Although maltose is the predominant sugar in wort it is not the only sugar available to yeast or the only sugar that yeast are able to metabolize. Budding yeast are also able to consume glucose, trehalose, fructose, and galactose.As yeast are breaking down sugars for energy they produce alcohol, carbon dioxide, and a myriad of other compounds such as esters and phenols (flavor and aroma compounds).

It is the job of any good brewer to create and maintain a happy yeast culture so that the yeast can go about their business in the wort. In order to accomplish this task the brewer needs to provide yeast with not only sugars and oxygen, but also a nitrogen source, vitamins, phosphorus, and some trace metals. Fortunately almost all of these compounds are present in wort if it is made from barley. All that the brewer needs to do is add oxygen and zinc (which is not present in high enough amounts in barley). A brewer could go their whole carrier without adding zinc and everything would be fine. However, if the same yeast is used again and again they will eventually begin to suffer from a lack of zinc. For this reason I add a small amount of yeast nutrients to my wort (I have also had success with adding a 1/4 cup of old yeast slurry to my wort at the beginning of the boil. These old dead yeast have the trace metals and vitamins that I am interested in and by boiling them I break them open allowing those compounds to be added to the wort).

What the basic brewer needs to known about adding yeast to a beer is...
  1. How much yeast to add
  2. When to add the yeast
  3. How to control fermentation
  4. At what point fermentation has ended
Anything beyond this can be saved for another day.

1) How much yeast to add: An ale needs fewer yeast cells than a lager, about 0.75 million cells per milliter of wort per degree Plato. A lager is going to need more like 1.25 million cells. Although this may sound very complicated most of the yeast cultures that you buy are going to have an appropriate amount of cells for a 5 gallon batch. If you are brewing something that is really high in gravity or larger than 5 gallons you are going to need to grow up additional yeast.

2) When to add the yeast: This is simple, add the yeast at a temperature lower than 80 (ale) or lower than 65 (lager). Although yeast can survive at 100 F very easily you do not want to stress them out before they begin their big job. You can also choose to add your yeast when the wort has reached the temperature at which you want to ferment it (say 72 F for an ale).

3) How to control fermentation: There is a lot of activity going on during fermentation, and the yeast are producing a large amount of heat. It is important to make sure that they are fermenting at an appropriate temperature, which  often means cooling them down. Otherwise you may get some unintended by-products of fermentation (aroma and flavor compounds that you did not want).

4) At what point fermentation has ended: This may seem very easy, when it stops bubbling right? Wrong. When yeast have finished eating the sugars they go back and clean up some of the other compounds left in the wort. If you bottle the beer before the yeast clean it up off flavors will be present. Make sure the yeast have had time to do their job (~10 days for ale ~21 days for a lager).

This is about all 1 post can handle. I will address how to culture yeast and some other details on yeast in following posts.

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