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Yeast 101

Yeasts are microorganisms classified as Fungi with 1,500 species currently chronicled. Yeasts are single cell organisms.  Some species of yeast may become multicellular through the formation of strings of cells known as pseudohyphae (false hyphae). Hyphae are a long, branching filamentous structure of a fungus as seen in most molds.


Yeast size can vary greatly depending on the species, typically measuring 3–4 micrometers in diameter, although some yeast can reach over 40 micrometers.   Brewer’s yeast average between 5-10 micrometers in diameter.


Saccharomyces cerevisiae and Saccharomyces pastorianus orBrewer’s Yeast (ale and lager yeast respectively) reproduce asexually by mitosis using an asymmetric division process called budding.  Budding is a form of asexual reproduction that occurs when a new organism develops from a bud on a yeast cell. The bud remains attached as it grows and separates from the parent cell when it is mature.  The newly created organism is a clone and is genetically identical to the parent organism.

Saccharomyces cerevisiae and Saccharomyces pastorianus are instrumental in brewing.  It is thought they were originally isolated from the skin of grapes (seen as thin white film on the skins of dark-colored fruits). 

With adequate nutrient, yeast cells can double in 100 minutes and the replicative lifespan is about 26 cell divisions.

Brewer’s yeast can grow on glucose, maltose, and fail to grow on lactose. Growth on other sugars is variable; Galactose and fructose are shown to be two of the best fermenting sugars.  The more complex the sugar is, the more difficult it is for the yeast to break down.  The longer more complex the sugar molecules are, the more difficult it is for the yeast to extract the required nutrients to reproduce.

Brewer’s yeast metabolizes sugars extracted from grains producing alcohol and carbon dioxide. In addition to fermenting the beer, yeast influences the character and flavor.   Brettanomyces, which is a wild form of yeast, ferments lambics.  Torulaspora delbrueckii, another form of wild yeast, ferments Bavarian weissbier.

Before the role of yeast in fermentation was understood, fermentation involved wild or airborne yeasts, and a few styles such as lambics still use this method today.  Emil Christian Hansen, a Danish biochemist, developed pure yeast cultures that were introduced into the Carlsberg brewery in 1883.  Pure yeast strains are now the main fermenting source used worldwide.

So there you have a semi-scientific look at what yeast is, what it does and how it reproduces.  These are the basics of what you need to know for the BJCP Entrance Exam.

Of course, as always if you have any concerns or questions, email me atbill@homebrewingnews.com and I will respond personally.