Yeast Inspection

Written by on April 6, 2011 in Homebrewing - 2 Comments

We begin our tour through beer ingredients with yeast, in my opinion the single most important component in beer but often the most overlooked.  I think that when consuming beer, most drinkers think very little about the role that yeast played in creating the pint in their hand.  This is largely because we don’t actually consume much yeast, with the notable exception of the ‘hefe’ style beers, which owe their distinctive cloudiness to the presence of suspended yeast.  In the majority of commercial beers, the yeast doesn’t even make it into the final package, as it is often filtered out of the finished beer before packaging in kegs, bottles or cans.  For those beers that are bottle conditioned, the yeast settles to the bottom and forms a layer of sediment that is left behind when the beer is carefully poured.  Despite the general lack of appreciation for yeast, if it weren’t for them we wouldn’t have beer.  Or bread. Or wine.  Or spirits.

So, what is yeast, and how does it make beer?  Yeast is a unicellular fungus that produces carbon dioxide and ethanol (alcohol) as waste products of fermentation.  An oversimplified analogy would be that yeast pees ethanol and farts carbon dioxide.  How appealing!  In alcoholic beverage production, it is the ethanol that we are mainly interested in, the carbon dioxide produced is largely lost to surroundings, except for bottle conditioned beer or wines.  In bread making, it is the carbon dioxide bubbles that provided the rise, and the ethanol is evaporated out in the baking.  As metabolic byproducts, yeast produce many other compounds that give many beers their characteristic flavors.  A beer in which the yeast produced only alcohol and carbon dioxide would be a bland beer indeed.  The topic of yeast metabolism is a huge one, and does play an important part in a deeper understanding of the role yeast plays, but for now  I am going to leave it at this more basic level.  If the interest is there, we can always delve farther into yeast biology.  If you can’t wait and feel the need to know more, I’d recommend Yeast: The Practical Guide to Beer Fermentation by Chris White and Jamil Zainasheff.

From a historical perspective, humans have likely been fermenting food products into alcohol for approximately 7-8 thousand years.  The link between fermentation and yeast has only been known for the last 150 years, in large part due to the work of Louis Pasteur.  Early fermentation was likely spontaneous, started by wild yeast already present on fruit/grains, or drifting in on specks of dust.  Over time many brewers began inadvertently reinoculating each batch with yeast from the previous, whether it be by reuse of fermentation vessels and equipment, or by adding back a portion of the previous beer.  This would result in carry-over of yeast from batch to batch.

In part, these types of practices selected for certain characteristics in the yeast.  Many people have heard beer often described as being either an ale or a lager.  The majority of the differences in the yeast strains used in ale and lager brewing, and the characters and flavors they impart to the beer, are due to centuries of selection by brewers. Lager yeast are often called bottom fermenting, while ale yeast are called top fermenting. However, this is mostly a function of the temperatures at which they are used to make beer, as well as the vessel in which it is brewed.  Lagers (from the German for ‘to store’ or ‘storage’) are usually fermented at much lower temperatures (45 -55 degrees Fahrenheit), with a long lagering or conditioning period following fermentation.  Ales are fermented at a higher temperature (65-70 degrees Fahrenheit), and are typically finished much faster.  Ale yeast will tend to produce more secondary compounds that may be described as ‘fruity’ in the finished beer, as opposed to lagers, which are generally considered more ‘clean’.  The biological difference between ale and lager yeast is that lager yeast are able to digest certain trisaccharides (tri-three, saccharides-sugar; molecules that are three sugars long) that ale yeast cannot.

Although both ale and lager yeast are species of the genus Saccharomyces, and are responsible for the majority of beer produced, certain strains of the genus Brettanomyces are used in the production of many sour beers. While Brettanomyces is often considered a containment or infection (especially in the wine industry), it is responsible for the characteristic flavors of many of the beers done in the Belgian-style, including lambic and gueuze.  Beers done with ‘Brett’ will generally finish out drier, or with less residual sugar than other beers as Brettanomyces strains will digest additional sugar sources.

While usually very little yeast (if any) may be present in a pint of beer, it is these microscopic workhorses that we owe thanks for the beverage know and love. So, raise a glass in gratitude to yeast!  Cheers!

Up next: MALT!

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Ryan Bell

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2 Comments on "Yeast Inspection"

  1. konuas April 13, 2011 at 2:21 PM · Reply

    What are your thoughts on blending yeast strains? Have you tried to use a pitch of two different strains in one batch, and then repitched on or cultured the resultant cake? Does one strain seem to overtake the others, or can they intermingle in the later generations to create a third strain entirely?

  2. Ryan April 13, 2011 at 10:56 PM · Reply

    I have pitched with two strains before, but honestly wasn’t familiar enough with either to say whether one or the other dominated, and didn’t reuse the yeast. In general, conditions will favor a certain part of the population, and those will reproduce the most, leading to higher levels of that type of yeast (this happens even within a single strain of yeast). If you were to do batch after batch using the same yeast, I’d imagine that if you keep brewing conditions (temp, recipe, etc) the same, that one strain would become more dominant.

    Brewer’s yeast is so altered by domestication that it is no longer able to reproduce by mating between individual cells, and is only able to reproduce by budding off new cells from the existing ones, so no, they wouldn’t be able to intermingle.

    There are examples of using yeast blends, particularly when looking at Belgian inspired beers, which may start fermentation with a fairly standard yeast, then pitch Brett into it later to be able to control the level of funky character that will develop from the Brett. If you were to try brewing the a blend of yeast, you would want to keep it among the beer yeasts, and not include wine yeasts, as some wine yeast strains can produce byproducts detrimental to the health of beer yeast.

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