Digging Through The Yeast Strain Toolkit

Yeast is everywhere. It’s on your clothes, scattered across the garden, and more than likely also present on the paper you’re reading right now. Hundreds of strains have been used in brewing over centuries, but the actual number of yeast strains in the environment is unfathomable. For as long as yeast has existed, nature and domestication have altered it. Now, we also have the capability to guide and drive these changes with even greater intentionality. In the modern age of craft brewing, how are we finding, making and using new strains? What makes a strain ​“new”?

People have been domesticating yeast for centuries, resulting in the diversity of industrial brewing strains we have today. As brewing strains have become differentiated from non-brewing strains, they have acquired important traits, like the ability to utilize maltose and flocculate. And most are negative for phenolic off flavors (POF-).

Though the majority originate from English, German, and Belgian brewing traditions, the industry has recently welcomed other world heritage strains, too. These strains developed many of the same traits as other brewing strains did — but also include some important and useful differences. Kveik from Norway, for example, shows what domestication in different environments can do. Norwegian farmhouse brewers traditionally fermented at temperatures as high as 108°F. These conditions allowed strains that had high-heat tolerance to be successful, while they also happened to feature no phenolics. When introduced to the brewing world outside of Norway, kveik’s capabilities seemed like an impossible dream come true. Kveik is not new to brewing. Norwegian brewing history is long established. But it may be new for many modern brewers outside of its place of origin. And there are likely many more ​“new to us” strains that were domesticated through other traditional fermented beverages, or foods, that may have interesting applications in beer.

In a more directed way, scientists engage in a sort of lab domestication called ​“adaptive lab evolution” (ALE), which takes existing strains and makes them more suitable for industrial brewing by applying different and new selective pressures to evolve traits. They have been able to generate strains with higher ethanol tolerance, higher ester synthesis, and lower off-flavors like acetaldehyde and diacetyl in this way.

Genetically engineering yeast to make new brewing strains has only recently been introduced to craft beer. Brewing scientists in the 1980s were the first to apply engineering to a strain, looking to make it a more optimal machine for the efficient production of low-calorie light beer. Glucoamylase enzyme was already in-use as a brewing ingredient, but they wanted to enable the strain to make it itself. At around the same time, scientists were also uncovering more about yeast genetics in general, and discovering how some brewing strains, like diastatic saison strains, can already make the glucoamylase enzyme (STA1). Saison strains acquired this trait through domestication. Both
domestication and engineering can approach the introduction of very similar traits and applications. 

Now, in 2024, we know that diastatic brewing strains (strains that hyperattenuate) have the potential to wreak havoc in the brewery. When used intentionally, they have to be carefully managed. A more favorable tool for brewers today probably wouldn’t be to make any strains newly diastatic, but, rather, to change a diastatic strain into a non-diastatic version (for saison profile, it could be used with exogenous glucoamylase enzyme). This is an example of something that would be achievable, in theory, with a breeding approach. We could mate a diastatic strain and a non-diastatic strain and then screen through the offspring to see if we find one that has mostly the same characteristics as the diastatic parent, but inherited the other parent’s non-diastatic nature. With genetic engineering, however, the change could be more targeted. It could be made only to the STA1 gene, leaving the rest of the strain’s features intact. 

Hybridization is a powerful tool for generating new strains nonetheless. One very famous example of hybridization is the lager strain. Its existence resulted from a hybridization event between two Saccharomyces species, Saccharyomyces eubayanus, which gave it its cold tolerance, and Saccharomyces cerevisiae, which gave it maltose utilization.

In other naturally occurring hybrids, some Belgian brewing strains are the offspring of two, three and even four different Saccharomyces species. Omega Yeast has a couple of hybrid strains that were intentionally bred to combine certain favorable traits— like Saisonstein, whose parents are a very robust saison strain called French Saison, and Belgian Saison I, which is known for its tendency to stall out under some conditions. Saisonstein has some of the flavor and aroma of the parent that stalls out, but the fermentation behavior of the reliably fermenting parent. Saisonstein didn’t have a specific target of gene combinations. It was something that was found from mated offspring. Part of what makes hybridization so robust as an approach is that it can be used to generate hundreds or thousands of new strains with a random assortment of genes that can be studied for novel combinations of traits. The challenge there is screening through the haystack of hybrids to find something desirable.

Examining what a new strain might look like from both an R&D and a brewer’s perspective, the concept of a new strain could be distilled down to two broad categories:

1. A sharpened tool: A strain whose traits represent an improvement on the behavior or characteristics of a strain that is already in-use — something that builds on its capabilities to add a new layer of utility in line with how it is already being used.
2. A whole new tool: A strain whose traits introduce a new functionality or feature — something that no other brewing strains can do.

Thinking of the strains you use in the brewery, do you wish one had less clove aroma, or another had more tropical aroma, or a third contributed a little less haze? These traits can be engineered into the strains you know and love. Yeast scientists can bring about these changes in a variety of ways. With the past several decades of yeast genetics research, and access to new technologies — like full genome sequencing, and gene editing tools like CRISPR/Cas9 — it is possible to use genetic engineering as a much faster and more specific method to make these strains.

In the 1990s, there was a virus spreading around the world that decimated papaya crops. Researchers found ways to modify the papaya crop to be resistant to this virus, and effectively saved papayas from extinction. By building in this resistance, modification ​“sharpened” the performance of the papaya tree and its ability to successfully produce healthy fruit given its environment. The quality and character of the papaya fruit itself remained exactly the same. It was simply now able to survive an environmental challenge. We wouldn’t consider this modification to have created a ​“new” papaya, really. It simply improved the existing papaya.

Through genetic modification with yeast, we have a similar ability. We can improve upon a given strain’s traits in order to optimize its abilities in brewing and fermentation processes in line with how that strain is already being used. For example, if a strain is known for being sluggish or for producing unwanted levels of a certain flavor, we can modify that strain to ferment more efficiently or reduce (or even eliminate) its capacity for producing certain compounds. Diacetyl Knock Out (DKO) is a great example of a modification that ​“sharpens” a strain’s abilities rather than changes it into a whole new tool. DKO-enabled strains can themselves produce ALDC. Integrating the ALDC enzyme directly into cell production greatly reduces diacetyl in finished beer brewed with these strains. This is just one example of an improvement. Practically speaking, there are endless possibilities for sharpening the tools we have at our disposal. It comes down to what brewers find the most useful and what changes would have a meaningful impact.

Another great opportunity for strain improvement is with haze. Both the presence and absence of haze are a hot topic for brewers. A haze-reduced version of an already familiar strain commonly used to make non-turbid beer would give brewers the chance to try a new spin on a favorite strain, or even improve clarity in a tried-and-true non-hazy recipe. And a hazy strain made to be even more reliably hazy means the strain would be more effective for producing hazy beer. These changes reinforce how the strain is already being used.

Let’s look again at fruit to understand the concept of a new tool compared to a sharpened one.

Imagine you’ve just brought home a pineapple from the store, and you’re preparing to chop it up for a little snack. You make your first slice and find that the fruit isn’t the vibrant yellow that you expected, but pink! In the early 2000s, researchers used genetic modification to cultivate a pineapple with increased levels of lycopene. This turns the fruit pink. The pink pineapple, described as sweeter and less acidic than its traditional counterpart, doesn’t necessarily replace typical pineapples. The pink flesh is not an improvement, per se. It gives consumers the variety of choice in pineapples. The pink pineapple could be used in place of regular pineapple for a different profile in your pineapple upside down cake, or could inspire a whole new dessert that showcases its novelty or new characteristics.

Just so, with yeast we can also modify strains in such a way that it creates an entirely new world of functionality and flavor combination. This is a bit more of a visionary step, requiring us to look beyond the traditional, toward a never-before-imagined future.

The Thiolized series of strains is an example of this. These strains represent a new component in the recipe-building process. For some brewers, the addition of thiol-generating capability add layers of flavor to their existing recipes, while others embrace them as the primary driver of aromatics in their beers. These are whole new tools.

One of the more prominent uses of Thiolized strains has been in fragrant dry-hopped beers. Thiols add a new layer of tropical fruit-like flavor amid a bouquet of floral and fruity hop aromatics, but these beers always come along with the risk of dry hop-induced diacetyl formation. By combining a new tool, a Thiolized strain, with the tool-sharpening capabilities of DKO technology, the fermentation will result in new and novel tropical aromas, and it will also prevent diacetyl from forming throughout the entire fermentation and maturation process.

If you look at yeast like you do your other ingredients, what are the tools you have at your disposal? You may find some processes painstaking, or maybe one of your raw materials is difficult to source, or is expensive— consider whether fermentation may be a path toward easing those issues, whether it’s expressing an enzyme to aid diacetyl cleanup, or the removal of the gene that produces phenolic flavors in beer, or even taking the best qualities of two strains and blending them into one.