The Magic of Yeast
If yeast didn’t exist, Douglas Adams would have had to invent it.
Among a host of imaginary species, the author of The Hitchhiker’s Guide to the Galaxy created the babelfish, a creature that feeds on the soundwaves of one language and excretes the soundwaves of a different language. The interplanetary traveler slips a tiny babelfish into his ear—Adams wrote science fiction, after all—and can instantly understand any spoken language in the universe. The babelfish is “so mindbogglingly useful” that it triggers an intergalactic theological debate.
Almost as useful—and as unbelievable—as yeast. Like the fantastical babelfish, yeast goes about its biological business, which is making more yeast, and just happens to produce a byproduct that transforms human perception. Yeasts ingest sugars and excrete a substance that is literally mindboggling—alcohol.
The effect of alcohol on the human mind is profound enough that every ancient society that stumbled on alcohol associated it with the divine. Little wonder that the foamy evidence of yeast in action was known to brewers in the Dark Ages as “Godisgood.” They couldn’t see where it came from, they couldn’t explain it, but they knew it turned mundane ingredients into something inspirational.
Trial and Error
In its beginnings, brewing had to have been a pretty haphazard process. All fermentation would have been of the sort we now call “spontaneous fermentation,” carried out by so-called “wild yeast”—modern terms to distinguish it from the highly controlled fermentation practiced today with known yeast sources.
And yet, brewers in antiquity clearly learned by trial and error how to make conditions most hospitable for the magic of fermentation to occur. Some physical locations must have been better sites for brewing than others, in part because the ambient yeast populations vary from one micro-environment to another. And brewers would have learned the best phase in the brewing process during which to expose their beer to the elements, much the way that modern lambic brewers in Belgium still throw open the brewhouse windows to let in the local wild yeast.
Brewers also learned to save some of one brew to inoculate the next. Brewers’ yeast is unique among yeast species in its tendency to clump together, or flocculate, at the end of fermentation. This quality allowed brewers to collect yeast from the top or the bottom of the fermentation vessel to start the next batch.
In Norse brewing tradition, a family kept a brewing stick that was used to stir the wort (the sweet liquid that will ferment into beer) at a critical stage. The family knew the sticks were valuable enough to pass from one generation to the next, even if they didn’t necessarily know that they were maintaining a colony of yeast on the stick.
By replicating the methods that produced good beer and rejecting the ones that didn’t, by skimming yeast that floated to the surface of the fermentation vessel or collecting what settled to the bottom, brewers inadvertently favored yeast with certain characteristics over others. With careful selection of yeast, brewers could increasingly replicate beer qualities over and over.
And brewers weren’t just looking for the efficient creation of alcohol. Whether the brewers knew it or not, the different types of yeast that thrived in their beer left their mark on the flavor and aroma, as surely as the type of hops, the roasting of the barley, the nature of the water, or the idiosyncratic techniques of the brewer.
Legislation and Science
Of the four ingredients of beer, yeast was the last to be identified. The original Reinheitsgebot, the Bavarian Purity Law of 1516, specified barley, hops and water as the only legal ingredients of beer. Yeast, the agent that turned the other three ingredients into beer, was not mentioned.
In the next century, Dutch amateur naturalist Anton van Leeuwenhoek found life everywhere he turned his microscope—tiny “animalcules,” including yeast cells, which he described for the first time in 1680. Nearly two hundred year later, in 1866, Louis Pasteur confirmed the role of yeast in the fermentation of beer.
These men observed living organisms whose life cycle made scientific sense of the brewing process. The infant science of microbiology joined the centuries-old skills of brewers.
Yeast is a single-celled organism: specifically, a type of fungi. Yeasts are found everywhere, on surfaces and floating in the air. They thrive and multiply quickly on sources of sugar—which describes beer wort perfectly—and release equal parts of alcohol and carbon dioxide, which gives beer its fizz.
In a third scientific breakthrough, in 1883, Emil Hansen at the Carlsberg Laboratories in Copenhagen isolated a pure yeast type with individual characteristics—which we know now as a distinct yeast strain. His techniques allowed the identification of different strains that operated best at different temperatures, utilized raw material at different rates, and produced different mixes of byproducts. With access to pure strains, brewers had much greater control over the beer they brewed.
Beer Families, Beer Flavors
There are hundreds of different species of yeast floating around out there. We humans take advantage of just a few species, and their hundreds of varieties, to make our bread rise, turn grape juice to wine, and brew our beer. There are about seventy commercial strains of brewing yeast, almost all of which are strains of two species in the genus Sacchromyces.
The variation in ideal fermentation temperature splits brewing yeast into two general groups, and the world of beer into two broad families. Those yeast strains that operate better at cooler temperatures give rise to the family of beers we call lagers; yeast strains that operate better at warmer temperatures give rise to the ales.
Fermentation generates a range of byproducts in addition to alcohol and carbon dioxide. Yeast fans will tell you these compounds, which number in the hundreds, are the true heart of beer flavor and aroma. They include esters, ketones, phenols and fusel alcohols, and yeast strains vary widely in the mix of compounds they produce.
Esters are the source of the fruity notes in beer that are dominant in ales and nearly absent in lagers. When beer reviewers refer to peach flavors, apricot, raisins or even bananas, they are describing the contributions of esters.
Phenols lend spicy notes to a beer, and sometimes medicinal flavors.
Ketones include compounds such as diacetyl, that can give a butterscotch flavor to beer—sometimes desirable, other times unwelcome.
And the contribution of fusel alcohols are almost entirely unwelcome: they are thought to be a leading cause of a bad hang over.
Strains also vary in their attenuation: the percentage of sugars the yeast consumes and converts to alcohol and carbon dioxide. Yeast with a high attenuation will use up most of the available sugars, resulting in a dry beer. Yeast with a low attenuation will leave some sugar behind, which not only results in a sweeter beer, but one with more “mouthfeel,” that is, a more viscous texture in your mouth.
Shopping for Flavor
How does a brewer shop for the flavors, aromas, attenuation and other yeast-derived qualities he or she wants in the finished beer? He could locate a commercial beer with the right flavor profile and grow the yeast from the living cells floating in the brew: beers that are cask- or bottle-conditioned (sometimes labeled as “on lees” or “sur lies”) still contain live yeast. Or he could let a commercial or government yeast laboratory do the hard work of isolating desirable yeast cells, growing colonies from single healthy cells, and maintaining the pure strains in good condition until needed.
These labs serve as banks to propagate and store popular commercial strains of yeast, as well as proprietary strains developed by individual breweries. A large brewery can afford its own quality control division to monitor the yeast for contamination by bacteria or wild yeast, or mutations that can start to influence beer flavor. However, smaller breweries and homebrewers will turn to commercial labs to provide the quality control and support.
David Wendell of Wyeast Laboratories in Mt. Hood, OR, is accustomed to steering brewers, and especially homebrewers, towards yeast strains that will give the desired outcome. “Often they know a commercial beer that is similar to the taste they want,” he says. “Or they are looking for a particular profile, for example, if a beer has a fruity, dry character, a signature ester profile. Or they have more technical requirements, maybe a highly flocculant yeast”—meaning a yeast that clumps together after fermentation.
Homebrewers may have misconceptions about yeast. Wendell often fields the question “How much alcohol strength can I get with this yeast?”
“The answer,” he says “is that you can make strong beer with almost any strain if you pitch properly [add enough yeast, but not too much], aerate properly, use the ideal temperature, and the yeast is unstressed [hasn’t been through a large number of cell divisions].”
It’s worth remembering, however, that alcohol may be a desirable byproduct as far as the human beer drinker is concerned, but from the yeast’s point of view, alcohol is a toxic waste, and yeast strains vary in their ability to tolerate rising levels of alcohol. High alcohol concentrations will shut down, then ultimately kill off the yeast. Brewers who are aiming for very high alcohol beers will often turn to champagne yeasts, which are more alcohol tolerant.
The yeast requirement of homebrewers and commercial brewers will be very different. Joane Carilli, with White Labs, a yeast laboratory and bank in San Diego, CA, says “Home brewers are making five gallons at a time in your house. Homebrewing is not commercial brewing.”
Homebrewers can experiment with a different yeast strain for every beer, but a commercial brewery will generally be more conservative. “One ale strain can make a good range of beers,” says Wendell. “Production breweries that are making beer day in and day out will keep one yeast they know well and use it for all their ales. They’ll need a pilsner strain as well.”
Carilli knows that, despite the high-tech capabilities of modern yeast propagation, there’s still as much art as science in brewing. “As much as brewers want to control it, yeast is a living organism, and the good brewers know all they can do is manage it. The best brewers never forget that.”