Magic or mycology?

Hollie Stephens traces our evolving understanding of yeast, from spontaneous fermentation to cultivated super-strains.

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In the modern craft beer world, drinkers are becoming increasingly savvy about beer’s components. Hop varieties are often found listed on the side of a can of beer. A label might state that the beer is made with 100% British malt. Plus, many British drinkers are at least dimly aware of how and why some types of water have characteristics that affect beer in particular ways.

But yeast is different. It’s not quite as effortless and sexy to talk about, probably because it is visually unappealing. It’s the under-celebrated workhorse that helps you to get a buzz on. And it’s small, but mighty. Packaged brewer’s yeast contains several billion cells per gram, which means tens of billions of cells per packet, ready to get to work on making beer. Each cell is between three and forty micrometres (in millimetres, that’s between 0.003mm and 0.04mm.) Yeast is, frankly, absolutely amazing, and most of us don’t appreciate it enough. 

The unsung hero of beer 

Of course, we as a species have a long and proud history of misunderstanding our tiny sugar-guzzling helpers. When people in ancient civilizations such as Mesopotamia made alcoholic beverages, they knew nothing of the existence of yeast, and less still about how they might control it. Via trial and error, they might have worked out that re-using certain vessels (which still contained active yeasts) helped them to successfully make alcohol, but they didn’t understand exactly what was going on. “For most of history, fermentation was a divine mystery,” writes Chris White and Jamil Zainasheff, in their co-authored Brewing Element Series book titled Yeast. And even as people became much more pointed and particular about their brewing practices, yeast still wasn’t properly understood. Mention of its existence is notably absent from the 1516 Beer Purity Law in Germany, which states that beer must only include malt, hops, and water. 


In 1680, a Dutch scientist named Anton van Leeuwenhoek first observed yeast cells under a microscope and saw that they consisted of minute particles. Later, in the 1800s, Louis Pasteur became the first to establish yeast as a living microorganism and connect it with fermentation, drawing the conclusion that living yeast transforms glucose into ethanol. “It is not exaggerating to suggest Pasteur made the greatest advances of anyone in the history of beer,” writes White and Zainasheff. Not only did he prove the utility of yeast in the production of alcohol, but he also understood that bacteria could be responsible for causing off-flavours.

Into the brewhouse 

Most brewer’s yeasts are of the genus of yeasts known as Saccharomyces (Latin for ‘sugar fungus’), and these are composed of carbohydrates, proteins, lipids, minerals, and DNA/RNA. The yeast strain used in a beer will determine the ideal fermentation temperature, and typically, lager yeasts will ferment at lower temperatures than ale yeasts. The first scientifically isolated lager yeast strain was Saccharomyces carlsbergensis, named for the Carlsberg Brewery that introduced it (now referred to as Saccharomyces pastorianus – long story), whereas Saccharomyces cerevisiae is used in the production of ales (and also in distilling and bread making). The continuous nature of beer production – which is undertaken year-round, in contrast to wine production for example – has contributed to the domestication of brewing yeasts. In some cases, yeast strains have become so dependent on brewing to survive that they have lost their ability to reproduce sexually.

Professional brewers and homebrewers alike can choose from hundreds of yeast strains, based on the attributes that they want in the finished beer. They need to think about attenuation (the extent to which the yeast will eat the sugar in the wort to produce alcohol), and flocculation (how efficiently yeast cells will clump together and fall out of the beer). So, for example, low-flocculating yeast strains would be selected for beers that are typically hazy, and a highly attenuative yeast strain will likely be preferred by someone brewing a high gravity beer. “At high alcohol levels, yeast undergoes enormous stress, and conditions must be created to ensure this does not degrade beer quality,” writes Randy Mosher in Radical Brewing. 


Ester production varies, depending on the fermentation conditions and the yeast strain used

As well as impacting the efficacy of fermentation, yeast can leave its mark in the finished beer in other ways. Its esters, such as isoamyl acetate (responsible for the banana-like notes in Weissbier), can impart fruity aromas in beer. Esters are volatile compounds formed from organic acid and alcohol. “Without esters, a beer would seem quite bland,” White and Zainasheff write. Ester production varies, depending on the fermentation conditions and the yeast strain used. They are found in particularly high quantities in some Belgian-style beers. “Yeast readily adapts and evolves to specific brewery conditions, so two breweries producing the same style of beer with the (ostensibly) same yeast strain will actually have, over time, cultured different yeast strains that produce unique beers,” writes John Palmer in How To Brew. Because of this, breweries might guard their yeast strains – which are responsible for producing their beer’s specific flavours and aromas – very closely.

Wild and free

Brettanomyces was historically considered to be a ‘wild yeast.’ These yeast cells can thrive in a variety of conditions, and they can dramatically change the character of beer. This genus of yeast grows slowly, and is responsible for funky, leathery, ‘barnyard’ style aromas and flavours. Is Brettanomyces yeast a contaminant, or a great tool for brewers? That is all in the eye of the beholder. It is considered a contaminant if it pops up in an amber ale, whereas for brewers focused on making Lambic style ales, it’s a gift from nature. In recent years, sour and barrel-aged beers have gained popularity, as brewers have shunned commercial yeasts and set about making beer the natural way. 

Brewers might opt to introduce Brettanomyces yeast and lactic acid bacteria such as Lactobacillus and Pediococcus, via a process of mixed fermentation. The original way to achieve this is ‘spontaneous fermentation’, which involves leaving the wort exposed to airborne microbes using a vessel such as a coolship - a copper or stainless-steel vessel with a large surface area – to keep wort at an ambient temperature at which wild yeasts can work their magic. These wild yeasts can be difficult to work with, yielding variable and unpredictable results, so this fermentation method is certainly not for the risk averse. Sour ales might be aged in wooden barrels or foeders (a Dutch word, meaning, essentially, an extra-large oak vessel). Here, once again, there is a great likelihood of inconsistencies, and brewers will typically blend the contents of all the barrels together before packaging and/or serving.

Modern day progress

Innovation in yeast is good news for beer, and even those looking to cut down on drinking might benefit. Dr Matthew Winans, a Research and Development Scientist at Imperial Yeast, says that some labs are currently investigating yeast alternatives to Saccharomyces cerevisiae for malt beverage fermentations that result in low alcohol beers. “The idea here is the microbes provide flavour/volatile metabolites important in fermentation without the high levels of ethanol typically achieved, resulting in a complex beverage with depth,” he says. 

Another aspect that yeast experts are actively investigating is the occurrence of ‘hop creep’: increased attenuation after dry hopping. “The hop’s amylase breaks down dextrins/starches in the wort into fermentable sugars,” says Dr Winans. “This allows the alcohol level and terminal gravity to creep off spec.” Researchers are working to understand more about this interaction between hops and yeast during fermentation, but it’s a complex business, even for those who specialize in working with yeast. “Although the research has been progressing steadily, there are questions that still are not fully understood,” says Dr Winans, though he points out that it is a fantastic time to be a researcher. “Although strain development has been a topic of research for many many years, these efforts are starting to have real world applications which is very exciting.”

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