Beer School: Culture club

Happy yeast brews better beer, as Louise Crane discovers


In the grand scheme of things, house yeast strains are a fairly new phenomenon, the modern semi-detached to yesteryear’s hut with a view. Until Emil Christian Hansen first isolated a single yeast strain in 1883 at the Danish Carlsberg Laboratory, brewers had made do with the foamy gunk known as kveik (or gjester, or gong, or multiple other names across the world). This was a mixture of strains of yeast and some other microorganisms along for the ride, and while its variations were local, they weren’t exactly restricted to a single brewery the way they are today. A yeast will develop changes unique to where it is propagated but, until the modern understanding of what yeast is, these changes were not fanatically kept consistent between batches. Today, some breweries pride themselves on their single, unique yeast strain, keeping it in a special facility under lock and key.

“The source of yeast that we are using here, I brought it over in my suitcase from San Diego,” says Thomas Tyrell, PhD, master brewer at Stone Brewing Berlin, the two-year-old European outpost of Stone Brewing in San Diego. “Then we thought we need to have somebody taking care of it.” Enter the Versuchs- und Lehranstalt für Brauerei in Berlin (VLB), which translates as the Research and Teaching Institute for Brewing, a world-famous centre for education, research, consulting, information and services for the drinks industry that’s been around since 1883. The Biological Laboratory at the VLB has over 120 yeast strains available for home and small-scale brewing, and banks an undisclosed number of proprietary strains for commercial brewers.

Geneticist and writer Adam Rutherford explains the science behind strains: “Saccharomyces cerevisiae is the Latin name for the yeast we use in baking and brewing. Indeed, brewer’s yeast and baker’s yeast are the same species, but they have slightly variant metabolic functions. The different strains can be used in brewing because they work slightly differently to each other. This is down to the expression of the cumulative effects of thousands of centuries of minute genetic changes due to environmental differences and adaptation - the same process that has given rise to different breeds of dog, or ginger and blonde hair. The different metabolic functions of yeast strains give rise to subtly specific tastes for each brew. Strains will be selected over time. So if one batch produced a desired flavour, brewers would use that again and cultivate it.”

Every time Stone Brewing needs to refresh its yeast at the brewery, it orders up a sample of its ‘banked’ house yeast from the VLB, where it’s kept at cryogenic temperatures (-80°C, or -112°F). This halts all of the yeast’s life processes, but doesn’t damage it so it can be revived. Quickly enough, a little flask of the culture is sent from the VLB to Stone. From then on, it’s the brewery’s job to keep the yeast happy, healthy and prosperous. 

To begin to grow the small sample into a big enough quantity for Stone’s 100 hectolitre batch, the yeast needs some food. “Our thinking is that we want to train the yeast directly for the medium where it’s going to end up, so we’re using brewer’s wort.” A single cell of yeast is mixed with 50ml of wort, and this is gradually scaled up. “When you start up you bring a single cell into 50ml, and when this is grown through you have 80 million cells in every millilitre,” says Thomas. “Our batch is 100 hectolitres so we have to bring it up to that volume, so the yeast is multiplying itself by a couple of thousand times. At pitching point we need about 15 million cells per millilitre. I can write you that number down - it has a lot of zeros!”

Propagation is all about timing, temperature and getting air to the yeast. “Corresponding to the amount of yeast cells present, you need to bring in more oxygen or maybe at the beginning a little bit less,” says Thomas. “And then the yeast takes up this oxygen to produce unsaturated fatty acids, sterols and all these substances that are cell material necessary for growth. That’s why oxygen is so essential in the growing stage. It hardly produces any alcohol during that time. It really puts everything, all the effort, into growth.”

When it’s time to produce alcohol, brewers limit oxygen to the yeast and it switches over to fermentation, and grows little. Fermentation temperature is very strain dependent. Stone’s yeast ferments quite happily over 20°C, but Norwegian farmhouse yeast is known for fermenting at 30°C. “We just did a trial and that was pretty eye-opening,” Thomas enthuses. “That was exciting to see how fast it can go. It speeds up everything. Interesting beer! I expected it to be more fruity or phenolic or estery, but it wasn’t, it was pretty clean.” Each yeast strain has a temperature optimum. 

Yeast health is also crucial to keeping out other microorganisms. Thomas draws a picture to explain: “If you as a human have a plate in front of you, and you have salad, potatoes, steak. Let’s say the steak is the most easy to access, in a psychological way. If the yeast is not so healthy, it will eat the steak but leave the potatoes and the salad, but if the yeast is healthy and hungry, it eats everything. And that’s what the yeast should do, it should ferment all sugars of the wort so fast that at the same time the yeast hasn’t even noticed it has produced alcohol.” In this way, yeast leaves no sugars for other bugs to thrive upon, and it creates the best flavour and aroma profile.

Once fermentation is complete, most brewers harvest yeast from the tank so they can use it to propagate the next batch. Why then, does it need to be stored in a laboratory at all and re-ordered from there? The answer is that yeast shows a predictable decline in vitality when used over and over again. “We learn that the activity goes down a bit after a couple of generations. In theory it could be fifty weeks or a complete year, but we use yeast for ten generations before re-ordering. You could use yeast again and again and again with the right treatment, but this is not always possible. You would have to take out the yeast and bring it into something like a sanatorium, give it a little sugar, keep it cool, and revitalise it.”

According to, Chico is probably the most widely used craft yeast strain on the West Coast of the USA, if not in the entire country. The name refers to the hometown of Sierra Nevada Brewing Company, from which the strain is said to have been sourced. (In turn, Sierra Nevada is said to have obtained it from Ballantine, but no one knows where they got it from, or when). It’s available to buy from various labs servicing the brewing community, including White Labs as ‘California Ale’ (WLP001) and Wyeast ‘American Ale’ (1056). It is renowned for being clean as a whistle, all the better for highlighting malt and hop expression, though ferment it above or below its preferred temperature range and it will throw shapes in the direction of some pretty funky esters.

Stone’s yeast strain originally came from a now-defunct Canadian microbrewery, via a brewery in the Northwest U.S. that co-founder Steve Wagner came across while living there. Steve always thought it would be a great yeast for the kinds of beers he envisioned making, so when it was time to set up a house yeast for Stone, he asked a friend to bring some from Oregon to San Diego. Rick Blankemeier, the San Diego brewery’s QA Analyst and all round beer nerd states on their blog that it’s one of the least temperamental yeast strains he’s worked with. “It doesn’t like wild pH changes, but it works well with all different levels of alcohol with ease. Heck, we’ve got it so tuned in and tuned up now, it can scream through anything. It can ferment through a batch of Stone Pale Ale in just three days!”

Though a house yeast is so-named because it is the chosen yeast of a brewery, that doesn’t stop Thomas Tyrell from experimenting with other strains. “Most of our beers are done with our house yeast, but we are also looking at the moment at a lager yeast beer - just on pilot systems.” The choice of yeast depends on the beer style. “Some beer styles are more ingredient based (hop or malt driven). In this case we focus on these materials. But if you think of Belgian beer styles or Bavarian Hefeweizen, or others (like our current test with a Norwegian Kveik yeast) then we build the recipe around the yeast. In general I think, that brewers still have to learn a lot about the capabilities of yeast in respect to flavour. There is much more out there than we actually know.”

“Coming next month onto the market we have a Hefeweizen. This is known for its phenolic and banana like aroma, and we’ve blended it with blueberry puree. It’s a fun brew. It’s pretty interesting because weizen means wheat beer but the typical flavour that everybody knows this beer for comes from the hefeweizen yeast. So if you would brew this beer with just 100% barley malt, you’d probably get still the same flavour. Isn’t that funny?”

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