Yeast of Eden

Breandán Kearney explores how important yeast is to Belgian beer and why Belgian yeasts are so different


At the end of 2019, an international team of scientists discovered that some of the most renowned classic Belgian beers, including its Trappist Ales, are fermented with a rare and unusual form of hybrid yeasts. The yeasts studied seemed to combine the DNA of the traditional ale yeast, Saccharomyces cerevisiae, with that of more stress-resistant feral yeasts such as Saccharomyces kudriavzevii, a strain initially isolated from decayed leaf but now often found on the bark of oak trees. “These yeasts are hybrids between two completely different species” says Dr. Jan Steensels of the KU Leuven Centre for Microbiology who coordinated the lab work of the study. “Think of lions and tigers making a super-baby.”

Belgium is not alone in having its beers closely associated with characterful yeast strains. English Ales are known for their subtle fruit profile. Germany is famous for its clean lager yeasts, as well as for its Weißbier strains with pronounced clove and banana character. 

However, Belgium stands on its own when it comes to the sheer variety of fermentation flavours across its beers, whether it’s the banana and pear drop aromas of the Tripel, the raisin and plum notes of the Belgian Dark Strong Ale, the citrus profile of the Belgian Golden Strong Ale, the spicy tang of Belgian Witbiers, the orange and pepper bite of the Saison, the vinous nature of Flemish Red Brown, or the complex array of lemon sherbert and horse blanket flavours found in Geuze.

It’s not just the existence of such a diverse range of indigenous yeasts. It’s how the Belgians use them. Brewers from other countries have attempted to recreate many of Belgium’s classic beers and while a few have succeeded with aplomb, often creating something new and interesting, many are unable to mimic the sophisticated fermentation character of these beers. How has Belgian yeast evolved to be so diverse? And why is Belgian yeast so different to yeast in other countries?

Local character

There are several species of yeast, and this is going to get a little technical, but hang in there. Saccharomyces cerevisiae is an ale yeast. The yeast generally used to produce lagers is saccharomyces pastorianus, a hybrid of Saccharomyces cerevisiae and Saccharomyces eubayanus. Spontaneous or wild fermentations can occur with a range of yeast species, the most well-known of which is Brettanomyces, literally “British Fungus”. Belgian beers feature all of these types of yeasts in abundance.

There isn’t one single reason that Belgium’s yeasts have evolved so differently to those in other countries, but a combination of factors. The first might be the country’s geographical location. To its west is France and its myriad winemakers, from whom Belgium absorbed knowledge about the microbiological benefits of oak aging in large foeders. To its east is Germany, where technical proficiency in lager fermentation and yeast management in laboratories infiltrated the Belgian appetite for quality. To the north, just across the channel, is England, where yeasts with subtle fruit notes helped English style ales become popular in Belgium, particularly after World War I.

Another factor may have been the localised nature of Belgian brewing in farmhouses, small towns, and monasteries, and Belgian brewing’s resistance to the mass production techniques that took hold much more quickly in neighbouring countries. In the early to mid-19th century, the average Belgian brewery was very small, mostly because there was a tax incentive to remain so, each producing around 2000 HL per year. Rather than packaging beer, they were selling beer from wooden barrels directly to consumers in their own pubs. In 1845, there were 3,089 breweries in Belgium (today, there are around 380). As a result, production and consumption stayed local, preserving distinctive varieties of yeast that were evolving in the environment of a particular brewery and thus maintaining their unique character.

Flavour and aroma: taste the difference 

A major differentiator of Belgian yeasts is the nature of the aroma and flavour compounds they kick off during fermentation. These compounds are generally known as esters and phenols. What’s important to remember is less the technical terms and more the flavour implications.

Descriptors for ester compounds are often fruit-related, whether that’s banana (isoamyl acetate) or apples (ethyl hexanoate) or pineapples and mango (ethyl butyrate) or even perfume and roses (phenylethyl acetate). Esters are part of the reason Delirium Tremens is all pears and oranges, and why Chimay Bleue is all figs and prunes.

At high levels, phenol compounds can be undesirable, contributing “medicinal”, “smoky”, or sometimes even “band-aid” characteristics, but at low levels they are considered the norm in certain Belgian styles. The most common phenol in Belgian beer is 4-Vinyl guaiacol or 4VG, which gives clove-like, spicy, or herbal aromas and flavours. Yeasts produce 4VG through processing ferulic acid, found in elevated levels in wheat malt, and increased in concentration when brewers introduce a rest step in their mash at around 45°C, or when they raise fermentation temperatures. Belgian witbiers such as Hoegaarden Wit, Sint Bernardus Wit, and Wittekerke tend to have a higher phenolic profile than other styles because of 4VG, but a whole swathe of Belgian blondes can be peppery and clovey for this reason. 

Belgian brewers will select yeast strains based on their ester and phenol profile, but will also dial in their desired levels through a toolbox of tricks, including their pitch rate, the level at which they oxygenate the cool wort, their fermentation temperature schedule, the original gravity of the wort, and the sugar and nutrient make-up of the wort. Other elements such as fermenter geometry play an important role: Yvan De Baets of Brasserie De La Senne has chosen squat, wide fermenters to create a lower-stress environment for yeast to work, impacting ester production. Similarly, open fermentation creates a different environment for ester production at breweries such as De Dolle Brouwers in Esen, Bourgogne de Flandres in Bruges, Liefmans in Oudenaarde, and Rulles in Habay.

PHOTO: Emma Vendette

Attenuation: how deep is your love?

The variety of attenuative properties among Belgian yeasts is another reason Belgian beers are so diverse. “Apparent Attenuation” is the level to which a yeast can ferment, essentially the actual percentage of sugars consumed by the yeast. 

Many Belgian yeasts are highly attenuative, eating almost everything that comes in their way. Saisons, for example, are well-known for their dryness and spritz. In his book Farmhouse Ales, Phil Markowski reports on a theory from Les Perkins of Wyeast Labs that the Saison Dupont strain may have originally been a red wine yeast that over time adapted itself to a brewery environment. Markowski summarises the similarities they found in the Saison Dupont strain with a typical red wine yeast, including “the capacity to be super-attenuative (which indicates an exceptional ability to produce hydrolyzing enzymes to break down complex sugars, albeit very slowly)”.

Other yeasts in Belgium are chosen for their medium attenuation. A certain level of sweetness is desired in many Abbey style ales such as Dubbels and Tripels, and selecting the correct yeast can help the brewer land where he wants in terms of alcohol level and residual sugar. 

There are also yeasts in Belgium chosen for their very low attenuation properties, those for example used in producing Flemish Red-Brown beer. These producers first want a young ale with lots of residual sugar so that the lactic acid bacteria and other microorganisms living in their oak foeders have enough food to eat during the acidification process.

Attenuation is also an important consideration for Belgian brewers when selecting a conditioning yeast to re-ferment their beers in bottles or kegs. Most often, they will select yeasts for their ability to ferment simple sugars added by the brewer to achieve a desired carbonation level, while at the same time being neutral in flavour contribution. 

Tolerance: how high can you go? 

Another characteristic of many Belgian yeasts is their tolerance to alcohol. When other yeasts such as certain Lager or English Ale strains are pushed to ferment in these higher alcohol conditions, they may struggle to ferment at all, resulting in stuck fermentations and under-attenuated beer. Worse, such an environment can stress these yeasts, forcing them to produce compounds which may be considered undesirable.

In Belgium, however, many ales finish in the region of 6 or 7% ABV, but often reach as much as 9 or 10% ABV. The Belgian yeasts used to create such beers have no problem fermenting at these higher alcohol levels. Belgian Dark Strong Ales such as Rochefort 10 from Brasserie de Rochefort and Tripel de Garre from Brouwerij Van Steenberge are 11.3% ABV and 11.5% ABV respectively. Bush from Dubuisson, a Belgian Amber Strong Ale from Brasserie Dubuisson finishes at 12% ABV.

One reason such tolerance may have evolved in Belgium is the Vandervelde Act, a piece of legislation passed in 1918 by the temperance movement which prohibited the selling of spirits in pubs. The act was in effect until 1983 and prompted brewers over time to raise the alcohol level of their beers to fill a gap in the market, selecting and developing yeasts which would be able to ferment at these higher alcohol levels. 

In the years following the Vandervelde Act, many of Belgium’s iconic beers were born: In 1921, the Westmalle monastery starting producing Westmalle Dubbel (7% ABV); in 1923, the Moortgat brewery released Victory Ale, a beer which would later come to be known as Duvel (8.5% ABV); and then in 1933, Westmalle released another beer they called Superbier, later renamed Westmalle Tripel, a golden ale of 9.5% ABV.

Irrepressible culture

Wild yeasts are deployed by Belgian brewers in all Lambics (and therefore Geuzes), contributing flavours and aromas ranging from “floral” and “earthy” to “mousy” and “horsey”. 

The most common wild yeast found in these beers is Brettanomyces, originally discovered in English stock ales in 1904. Brettanomyces has a handful of recognised subspecies, the most common in Belgium being Brettanomyces bruxellensis and Brettanomyces lambicus. These yeasts will continue to consume sugars for many years, including complex sugars that are difficult for other yeasts to ferment. 

Wild yeasts are also permitted to develop in Flemish Red Brown beers, although at a much lower level than in Lambic. Young beer is aged in oak foeders where Brettanomyces and a range of bacteria ferment and acidify. Think Rodenbach, Petrus Oud Bruin, Liefmans Oud Bruin, Vander Ghinste Oud Bruin, and Duchesse de Bourgogne. 

Wild yeasts such as Brettanomyces are also celebrated by more recent generations of brewers in Belgium who create beers of mixed fermentation which don’t necessarily fall into any one style descriptor. The older guard features names such as De Dolle, De Ranke, Struise, Alvinne, De La Senne, and Minne. The newer kids on the block include ‘t Verzet, Brussels Beer Project, and Borinage.

Some Belgian brewers use Brettanomyces as a conditioning yeast after primary fermentation is finished. Such use creates a further fermentation which adds complexity and evolution, as in the Trappist ale Orval, which is packaged at 6.2% ABV with Brettanomyces and can reach 7.2% ABV when the wild yeast has done its work.

Looking after the “ligers”

Scientists in laboratories will continue to explore the unique nature of Belgian yeasts. Brewers in Belgium will continue to stretch them to the limits of their capability. Consumers will continue to revel in their wide spectrum of flavours.

But to focus on yeast alone as the main proponent of Belgian beer is to ignore the intimate relationship that Belgian brewers enjoy with interesting malts, home-grown hops, water chemistry, and spices and fruits, not to mention the processes in which they engage to create their beers in a different way from brewers in other countries. 

Yeast may be one of the most pronounced tools in their box of tricks, but it forms part of an holistic approach for Belgian brewers. Their yeasts might be special medieval hybrid strains described by scientists as “lions and tigers making a super-baby,” but even “ligers” need something to eat, a space to live, and someone to care for them as they grow.

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