Saturday, 11 February 2012

Yeast Part 1: Ale, Lager and Brett

My friend Max requested that I write a few entries on yeast, with explanations of the types of yeasts there are and why I choose to use them in different contexts. This is the first such entry giving an overview of the two main classifications of yeast (ale and lager), with a caveat into the recently revived Brettanomyces strains. Check back for more over the coming weeks. Yeast is the aspect of brewing that most fascinates me so I hope you can learn to enjoy the topic of yeast as well.

Yeast is the magical ingredient in beer. It Is this organism which takes what is essentially sugar water and turns it into what I believe to be the most special and variable beverage on the planet. The spectrum of flavors that arise from yeast is absolutely amazing. From bone dry beers with almost no yeast flavor profile, to the immense, fruity, flavor bombs from Belgium. What is more interesting is that even though yeast is the most important ingredient in beer, its existence was unknown for the vast majority of brewing history. Yeast was first microscopically observed by the Dutch naturalist Anton van Leeuwenhoek in 1680, but its importance to the brewing process was not fully understood until the mid 1870’s when Louis Pasteur published his seminal book Etudes sur la Biere. He was the first person to recognize that beer was made by the anaerobic metabolism of sugars in wort to produce carbon dioxide and ethyl alcohol.

Today, brewers have access to an unprecedented number of different yeast strains, all with distinctive characteristics. Just as dogs have been bred into different breeds, so have yeasts been bred into different strains. Some of this was by design, while at other times it was the natural processes of evolution and mutation that gave rise to various yeast strains. Either way, brewers today are reaping the benefits of this versatile and wonderful organism.

Ale, Lager and Brett Yeasts

While there are roughly 1,500 different types of yeasts, there are generally two that are of primary concern to brewers: Saccharomyces cerevisiae (ale yeast) and Saccharomyces  pastorianus (lager yeast). There is another genus of yeast, Brettanomyces, which has caught the interest of some craft brewers of late, which I will touch on briefly as well.

The main distinguishing factor generally used to classify yeast as ale or lager yeast is whether the yeast ferments at the top or the bottom of the fermentation vessel. Ale yeasts are said to ferment at the top of the vessel, while lager is said to ferment at the bottom. In addition, ale yeasts generally ferment more quickly (2-7 days) at warmer temperatures (60F-78F), than their slower (5-10 day) and colder (45F-55F) lager yeast counterparts. I use the term “generally”, because there is a degree of overlap between lager and ale yeasts.  Certainly with the way craft brewers are experimenting with yeasts, the line is often ever more blurred. Regardless, the conditions under which yeasts ferment have profound effects on the flavor profiles produced in beers.

As I mentioned earlier, ale yeast generally ferments at the top of the fermentation vessel. This is because the warmer temperatures promote more aggressive fermentations creating carbon dioxide forcing the yeast to the surface of the wort. Such fermentations produce a wide array of flavor compounds that are largely subdued under lager fermentation conditions. When you taste an ale, you will often note “fruity” flavors and aromas that you would probably not taste in lagers. The family of compounds that produce these flavors are called esters, and the flavors can range anywhere from banana to apple, or even roses and honey. The type of yeast strain is extremely important in determining which combination of esters will end up in a finished beer. Some strains, most famously Belgian strains, will produce large amounts of esters which essentially define their style.

Temperature is another factor that largely effects the composition of flavor and aroma compounds in beer. Given the same yeast strain, higher temperatures tend to promote higher levels of ester compounds with correspondingly fruitier beers. Brewers must be cautious, however, because if fermentation temperatures are too high, off-flavor compounds such as diacetyl (butterscotch flavor) or fusel alcohols (heavier alcohol molecules that lend hot or solvent-like flavors) may be produced in undesirable levels.

Differing fermentation conditions from ales are the main reason why lager yeasts are bottom fermenting and also largely determine the flavor profile imparted by the yeast. As opposed to ale fermentations, which are fast and very active, lager fermentations are slow due to lower temperatures. Less fermentation activity means less carbon dioxide to push the yeast cells to the top of the fermentation vessel. Lager yeasts, therefore, stay nearer to the bottom. Lower temperatures also retard the formation of the ester compounds mentioned above, leading to beers that are “crisper and cleaner” than ales, but with less flavor contributed from the yeast. Many lager styles can produce wonderful flavors such as the double bocks of Germany, but the flavors are largely the result of the grains utilized in the mash.

Lastly, there is the wild yeast Brettanomyces (abbreviated Brett). In most beer styles it is considered a spoilage organism, but in some historical styles, most notably Lambics and Flanders red ales, it is an integral part of their spontaneous tertiary fermentations. Further, as is the nature of craft brewing, many adventurous brewers have embraced Brett with a vengeance, using it in conjunction with conventional yeast strains to add complexity to their beers. Some even going so far as to use Brett exclusively in their brews. Flavors most often associated with Brett range from “earthy” to “floral” or the complimentary “horse blanket”. In addition, brewing with Brett can be a bit of a challenge, with the final product difficult to predict. Headstrong brewers have taken this challenge, sacrificing control for the reward of surprising results.   

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