The Budding Histories of Yeast, Part One: Beer Before Panties and Microscopes

The yeasts used in modern brewing have a rather diverse and divergent history for a set of organisms that were only "discovered" less than 200 years ago. I've developed a particular interest in brewing yeasts of late, and as such have decided to devote an as-yet-embryonic segment of this blog called "Drinking Science" to the history, development, and characteristics of the yeasts used in beer brewing. For the next few weeks, I'll begin by discussing the history of brewing yeast and go on to describe the mercurial taxonomic and qualitative differences between ale and lager yeast, then will diverge onto a number of topics of personal interest including flocculation, effects of environmental stresses on flavor expression, the 'wild' yeasts, and modern industrial and scientific advances in yeast.

A good yeast strain's importance to the consistency and quality of a brew has been known nearly as long as people have been brewing. Before Louis Pasteur (a man whose profligacy of achievements continues to amaze me) printed the "Etudes sur la Biere", in addition to his many other works on fermentation (see Pasteur Brewing for a good overview), the presence of the fermenting agent in beer, wine, and their derivatives was unexplored. Charles Cagniard de la Tour had reported yeast as being the "ferment", or fermenting agent, in alcohol in 1838, but before that, ferment was basically synonymous with magic. Brewers knew to add some of the sediment from a previous batch to a new batch (Dowhanick, 1999), to use the same vessels, or stirring utensils as used for a good previous batch, for their magical properties of course. It was only the advent of microbiology and discovery of the presence of yeast in the 19th Century allowed this fourth ingredient of beer to finally be observed and classified.

It is likely that alcohol was "invented" by accident, when jars of mushed grapes or soaked grains were left in the air to ferment. These early beverages would have been pretty nasty with airborne and food borne contaminants, but like we say here in college "it'll get you drunk." And if you're trying to get some Mesolithic poon, buckets of soaked and fermented grains were your Pink Panty Pulldowns, or perhaps your Stinky Loincloth Raisers, given the state of women's undergarments at the time. The water purifying, antibacterial, nutritional and storage benefits of alcohol would have been just as importance as its ability to facilitate an altered state. All of these immediately observable benefits would have inspired the production of more fermented beverages with the selective forces of taste, fermentation temperature, sugar composition, native competition, and others leading to the isolation of a few strains of yeast suited particularly to the fermentation of beer. These were then cropped and propagated by the brewers, eventually creating the distinctive yeast strains known in Pasteur's time before they were categorized, dicked around with, and categorized again with microscopes, genetic sequencing, and other procedures of the nature.

Brewer's yeast, or Saccharomyces cerevisiae, was the first eukaryote to have its genome completely sequenced, in 1996. Even this sequencing event did not resolve the massive taxonomic orgy that characterizes the Saccharomyces sensu stricto species complex to this day. In the early 20th century, ale and lager yeasts were considered two different species. Ale yeast consisted of the familiar S. cerevisiae and lager yeast belonged to a separate species named Saccharomyces carlsbergensis, after the brewery in Copenhagen, "where it was first isolated in pure culture by Dr. Emil Christian Hansen in 1883". This was the first pure culture brewing yeast of any type to be isolated. According to which source one consults, this yeast species can be synonymous with S. pastorianus (Nguyen and Galliardin, 2005), S. uvarum (Dowhanick, 1999), S. bayanus (Cavalieri, 2003) or can be grouped in with the species S. cerevisiae (yeastgenome.org). The taxonomy becomes even more complicated when one considers that S. uvarum is generally considered synonymous with S. bayanus, which is accepted to be the commonly active yeast in wine must fermentation (EPA.gov) and a probable ancestor of S. pastorianus (carlsbergensis) through a hybridization event with S. cerevisiae (Lodolo et al., 2007). Also, thanks to the wonders of genetic sequencing, the existance of two different types of genome in lager strains has been identified (Rainieri et al., 2006), which implies that lager yeasts may not belong to a definable species, but are rather a collection of pure yeast species and recently hybridized strains (Rainieri et al. 2006), as well as members of S. cerevisiae which have been recently altered to be able to express the phenotypic characteristics associated with lager yeasts (yeastgenome.org). The subject of yeast taxonomy, needless to say, is kind of like the "I'm My Own Grandpa" song. For now, let's return to the great and simple time before genomes were "discovered" and describe the phenotypic (physically expressed and observable) traits of the two commonly used types of yeast (that is to say, Ale and Lager, regardless of species), how they came to be differentiated, and what their noticeable contributions have been to the beer styles developed before the 20th century.

Brewing would have begun with the advent of agriculture. In settled, agricultural communities, soaked grains and fruit could sit and ferment, and the foam at the top could be scraped off and either directly propagated in a new stock of ingredients or saved in a hard cake to be pitched later (Dowhanick, 1999). The yeasts that evolved from this process would have been the top-fermenting Ale, Wine, and Cider yeasts, now accepted as belonging to the S. cerevisiae and S. bayanus, respectively. The oldest known association of S. cerevisiae with human activity is its existence in an over 5000 year old clay pottery wine jug, the Scorpion I jar (Cavalieri, 2003). The fact that S. cerevisiae was present in such quantities that it could be isolated and analyzed over 5000 years later implies that the species had already been significantly cultured. The Saccharomyces yeasts are uncommon in nature and are not airborne, being detectable only once grape skins have been broken, and as such it's likely that the first fermented beverages were a product of a cocktail of airborne, food-borne, and surface-borne yeasts which were gradually selected down to the Saccharomyces genus. This selection would have happened in a combination of ways. First, the fermented beverages with the best flavor would have likely had a higher percentage of Saccharomyces yeasts, and the propagation of these beverages would have been more likely than those which were less palatable. Second, Saccharomyces yeasts tend to proliferate in fermented beverages once they have reached 5% alcohol, as they are still tolerant to this condition that kills many competing yeasts and molds (Cavalieri, 2003). Therefore, any beverages fermented to a higher alcohol by volume percentage than about 5% would have a high percentage of Saccharomyces species present and able to be cropped, as opposed to other fungi which would have died out before they could be harvested.

Michael Jackson's "Beer Hunter" outlines the probable development of the bottom-fermenting strains of Saccharomyces as being a result of the necessity for seasonal brewing. Monasteries and brewer-farmers before the advent of steam-power and refrigeration would carve or find large caves in the icy hills and store (the etymology of the word lager is "to store") their beers there during the hot German summers. The yeast would flocculate to the bottom of the vessels, and those which expressed the desired flavors, alcohol content, and robustness at the cold temperatures of the caves would be selected and repropagated. These yeasts generally expressed the cleaner taste associated now with lagers, which is a hallmark of colder fermentation temperatures at which the yeast don't express the fruity esters associated with hotter-fermented British and Belgian ales. Around this time, British maltsters were inventing new ways to gently and evenly malt barley, and the advent of steam power allowed breweries to move to a larger scale than the farmhouse. This allowed for the development and spread of many of the beer styles known today, all of which began as unique products of the seasonal, grain, water, and yeast characteristics of their particular region.

Current research indicates that the many strains of yeast known to produce Lagers belong to genetic hybrids of historically pure S. uvarum and S. bayanus strains with S. cerevisiae. Most of these hybrids belong to the species complex that has been historically classified as S. pastorianus (carlsberensis), the yeast "species" known to produce Lager beers. Interestingly, data supports the observation that more modern type Lagers strains contain almost complete S. cerevisiae genomes, while the older Lager strains contain much smaller portions of this genome (Rainieri et al., 2006). In light of the known history of Lager brewing, this paints a very interesting picture of the development of these lager yeasts as well as their potential in the future.

When they were storing their beer in caves for the summer, the old European monks and farmers were storing beer that had in large part been fermented by S. cerevisiae, but likely also had S. uvarum (often considered very genetically close or synonymous with S. bayanus) present in small quantities. The fermentation temperatures likely promoted a hybridization event between the S. cerevisiae and the S. uvarum (or S. bayanus), allowing for a more robust yeast strain that could survive the colder conditions. As time went on, the characteristics of the strain more closely associated with "beer", that is, those more present on the S. cerevisiae genome, were increasingly selected for, in addition to the yeast having more opportunities to re-hybridize with pure S. cerevisiae while still retaining the hybrid robust nature allowing it to survive the colder temperatures as well as metabolize melibiose, a hallmark trait of Lager beer yeast strains. Despite the fact that the exact genetic history of beer yeast, especially Lager yeast, may never be known, and despite the fact that a strain may be a species today and another species tomorrow, the demonstrated ability of the Saccharomyces sensu stricto species complex to hybridize for survival is exciting. We may have predictable yeast that we can buy and pitch and we will know exactly what they are going to do, but the future of brewing yeasts and their range of expression in beer is definitely wide open. Natural hybridization of ale and lager yeasts with each other and with yeasts like S. bayanus has happened before and could happen again to express new flavors. On the scientific front, parts of genomes can be swapped around to increase temperature or alcohol tolerance, or increase the expression of some flavor or flocculation characteristic.

The rediscovery of old beer styles is exciting and has given us things like saisons, bieres de garde, altbier, and California Common beer that may have been lost forever. Now, knowing what we do about the nature of brewing yeast, we can take the next step and begin reaching out and developing new styles that are inspired by forces beyond the season, the local water, or what types of hops are expressed by the local soil. Brewing is one of the greatest testaments to the ability of mankind to marry nature and ingenuity into pleasure, and we have really only just begun.


Sources Cited
U. Bond (2009), "The Genomes of Lager Yeasts". Advances in Applied Microbiology. 79, 159-182

D. Cavalieri, P. McGovern, D. Hartl, R. Mortimer, M. Polsinelli (2003), "Evidence for S. cerevisiae Fermentation in Ancient Wine". Journal of Molecular Evolution. 57, S226-S232.

T. Dowhanick. "Yeast - Strains and Handling Techniques". The Practical Brewer. Master Brewers Association of the Americas. 1999. P263-298.

E. Lodolo, J. Kock, B. Axcell, M. Brooks (2008), "The yeast Saccharomyces cerevisiae - the main character in beer brewing". FEMS Yeast Research. 8, 1018-1036.

S. Rainieri, Y. Kodama, Y. Kaneko, K. Mikata, Y. Nakao, and T. Ashikari (2006), "Pure and Mixed Genetic Lines of Saccharomyces bayanus and Saccharomyces pastorianus and Their Contribution to the Lager Brewing Strain Genome". Applied Environmental Microbiology. 72, 3968-3974