Beer Science: Yeast Species and Strains

A recent beer tasting at The Moor Pat (see review of Logsdon Seizoen Bretta here) had me thinking about the types of yeast used to construct beer.  The purpose of this two-part write-up is to elaborate on the similarities and differences between the yeast used to construct a "Brett" beer and the more common yeasts used in brewing.

This will be a two-part blog.  Today, I will focus on the differences between some commonly used words in yeast vernacular.  Next week's Beer Science entry will focus on the specifics of using Brett and why using this yeast will make your beer taste so (in)famous!


Species vs. Strain
Let's begin by defining the following words as they relate to beer-making: yeast, fungus, species, and strain.  These words can be confusing and are often used incorrectly.  Yeast is the generally used name for many different kinds of organisms used to brew beer, ferment wine and cook bread.  There are many different "kinds" or yeast, and these different yeast can belong to different species and strains.  But all yeast in the world belong together to a larger classification group known as Fungi (plural).  Each yeast is therefore a fungus (singular).

A species of yeast most commonly used in beer brewing is Saccharomyces cerevisiae.  When writing the name of an organism, convention dictates that the first name begins with a capital letter, the second name is with a lower-case letter, and the entire name is presented in italics.  Saccharomyces cerevisiae is used in beer, wine, and bread making.  The name Saccharomyces cerevisiae comes from the root "saccharo-" which means sugar (think saccharin, saccharide), and the suffix "-myces" means fungus.  Brettanomyces bruxellensis is another species of yeast used in a small proportion of beer (and often this species of yeast is not wanted in wine at all!).  Very commonly, yeast and Saccharomyces cerevisiae are used interchangeably.  The phrase "I brewed my beer with yeast", although correct, does not identify what particular species was used.  99% of the time yeast = Saccharomyces cerevisiae.  However other species such as Saccharomyces pastorianus (formerly Saccharomyces carlsbergensis) are used to brew different types of beers.

Saccharomyces cerevisiae, otherwise known as yeast.

Even though the same species, Saccharomyces cerevisiae, is used to make many items such as beer, wine, and bread, there are different strains used for each purpose.  What makes one strain different from another strain?  That question is a little more complicated to answer.  Different strains of Saccharomyces cerevisiae have been grown and tailored over the years to produce different flavors.  Some strains of Saccharomyces cerevisiae metabolize sugars differently and therefore produce different chemical byproducts.  Bread yeast have been designed to make bread and beer yeast have been designed to make beer.  There are literally thousands of yeast variations, or strains, that can produce different byproducts.  Different strains metabolize sugar and produce different chemicals that impart unique flavors.  And sometimes an entirely different species, such as Brettanomyces bruxellensis, is used for brewing.  In these beers, the chemical byproducts and thus the flavors are completely different from more traditional Saccharomyces species!

Side note: Interestingly, yeast (Saccharomyces cerevisiae) possess a great deal of similarity in their cell structure to human cells, and information from research conducted on yeast can often be useful in understanding human cell science.  At Yale University, I worked on a project that identified and categorized all the proteins in yeast.  Why?  It turns out that about 40% of yeast proteins have a similar protein that can be found in humans.  So if you understand what these proteins are doing in yeast, you might be able to understand how the human protein works as well.

To review: yeast is a fungus.  There are many different species of yeast.  Here I have mentioned only three: Saccharomyces cerevisiae, Saccharomyces pastorianus, and Brettanomyces bruxellensis.  Different yeast metabolize sugars differently, and different chemistry will result in different flavors in beer.  Different strains of  Saccharomyces cerevisiae and Saccharomyces pastorianus can be used in brewing, and adventurous brewers may choose another species of yeast such as Brettanomyces.

Next week I will focus on using the species Brettanomyces to make beer, and explain the chemistry of how this yeast can create such funky flavor profiles.

Some yeast resources:
Wikipedia yeast entry
WYeast Strain Guide
Homebrewers Yeast Strain Chart