Yvan De Baets Explains Saison’s Greatest Myth: the Yeast

Yvan De Baets, cofounder and head brewer at the Brasserie de la Senne in Brussels, has been researching (and brewing) saison for more than two decades. He has argued that “yeast is the biggest myth about saison.” We asked him to elaborate.

Joe Stange , Yvan De Baets Jul 20, 2021 - 9 min read

Yvan De Baets Explains Saison’s Greatest Myth: the Yeast Primary Image

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When you say saison yeast is a “myth,” what do you mean?
I mean that for many people, a saison can be made only with “saison yeast.” But what is it? A “saison yeast” seems, for some, to be a yeast named “Saison Something” or “Something Saison,” sold by a commercial yeast company. It’s a sort of magic powder: You add it to a wort, and you get a genuine saison.

There are different things to say about that.

First, there are, to my knowledge, currently three main strains on the market, adopted by the yeast companies from bottles of Dupont, Blaugies, and Thiriez. The Blaugies yeast is actually an offshoot of Dupont’s—Pierre-Alex Carlier and Marie-Noelle Pourtois selected it at one moment of its development in a wort, isolated it, and cultivated it. Even if the two strains give slightly different results, we can say they’re the same yeast. The Thiriez one is different. It’s funny that it’s now called “French Saison” because it is used by Thiriez, but its origin is Belgian: It comes from the yeast bank of the brewing school where I studied in Brussels.

So, that’s only two yeast strains. To me, they are true saison yeasts—no problem there. They are very attenuative (of the variety diastaticus) and phenolic off-flavor positive (POF+), meaning they impart strong phenolic aromas and flavors. It is quite probable that a lot of the yeasts used for making saisons in the past had that last characteristic. A lot of them were “wild” yeasts at some point, developed from a mix of different yeasts (and quite probably bacteria) that occurred naturally in those breweries. A lot of wild yeasts are POF+. Over time, with the evolution of knowledge about yeast and microbiology, brewers who wanted to keep the character of their beers would have selected them—by isolating one or two strains for more consistency and to avoid “accidents.” That’s the case with Dupont, which selected its yeast(s) sometime around the 1950s. They weren’t alone—that was common practice just before and especially after World War II.


My point is that these two (or three) yeasts do not represent all the strains that have been used for making saison—nor all those that could still be used. They are clear survivors of the old saison yeasts. However, are there other available yeasts today that might be descended from some that were used for brewing saison? We just don’t know, but it’s highly likely. Meanwhile, could some other classic or newly discovered yeasts—such as a local wild yeast—be used for brewing a genuine saison, respecting its spirit? Of course!

Also, those two yeasts—certainly in their commercial versions—are isolated strains taken from an ancient consortium of yeasts, with the other strains now probably lost. So those two (or three) yeasts are good examples of saison yeasts, but they don’t represent the whole variety of yeasts that can be used for making saisons today—not even close. If the general public (in this case, the Belgians) had kept their interest in these beers 40 or 50 years ago, before they almost completely disappeared, more yeasts could have been “saved,” and their variety would have been far greater.

If we want to brew saison with different yeasts today, where do we start?
The most important thing for a saison is the finished beer—that’s the purpose. And it has to be well-attenuated, because you need refreshing properties in saison—and by the way, if you need refreshing properties, you need balance and drinkability. This is also because the brewers probably didn’t have another choice besides getting a well-attenuated beer with the techniques they were using (see below). Plus, you cannot keep a beer that isn’t well-attenuated (because it will continue to ferment, potentially in a bad way), unless it is super strong in alcohol—which is not the case with saison.

Indeed, most, if not all, of the saisons underwent an important secondary fermentation in the wooden barrels in which they matured. In all the brewing treatises that talk about bières de garde (the broader family of saisons), an important emphasis is put on that point. And those barrels contained—most of the time, if not always—Brettanomyces. I cannot imagine the brewers brewing in rustic places (farm or village brewery, whatever) with rustic methods somehow avoiding having Brett in their barrels. Anyway, this was not a problem: People not only were used to those winey, tart flavors, but they demanded them in their beers. It was essentially a mandatory flavor in Belgian ales until the 1900s. Old-style saisons had a flavor profile similar to traditional gueuze. My sources on that? Oral testimonies, the tasting of old saisons of long-gone breweries, and an article written by a university brewing professor in 1946. The presence of Brett was, for me, a sure thing.


So, what is a saison yeast? Is it the primary yeast or the secondary? Is it Brett? I know for sure—because I have written records proving it—that some breweries would change their primary yeast many times during the year for brewing saison, at least before World War II. And it didn’t seem to bother them. The reason, I think, is that the flavor profile from the primary yeast was not necessarily what you would find in the finished beer—because that was given mainly by Brettanomyces. We know that Brett character can easily dominate that of other yeasts. So you could, for example, start your fermentation with a yeast that is not especially well-attenuative—no worries because the Brett would finish the work anyway and dry your beer out.

A saison should be:

  • pale in color (up to dark amber)
  • light in alcohol
  • well-attenuated and/or dry
  • “rustic,” i.e., not too clean—not easy to define, but important to distinguish it from other styles
  • quite bitter or tart/sour
  • spicy, or not
  • without fruit
  • refreshing and balanced

With that profile in mind, when it comes to fermentation, you have many different options you can play with:

  • primary fermentation and maturation with one well-attenuating POF+ yeast (for example, Saccharomyces var. diastaticus)
  • primary and maturation with one well-attenuating POF- yeast
  • primary with one well-attenuating POF+ yeast, then secondary with Brett
  • primary with one well-attenuating POF- yeast, then secondary with Brett
  • primary with one less-attenuating POF+ yeast, then secondary with a well-attenuating POF+, POF-, or Brett yeast
  • primary with one less-attenuating POF- yeast, then secondary with a well-attenuating POF+, POF-, or Brett yeast
  • any of the above, plus bacteria or blended with older sour beer—which multiplies the possibilities

About that rustic character …
The phenolic flavor helps give some “rusticity” to the beer. That makes it a bit more difficult to reach the target with a POF- yeast—but one can play with hops and rustic grains, for example, to get there. This leads to a cleaner version, referring more to what I call the “post-WWI” saisons. (And this is what we do for our Saison du Meyboom.)

And I’m quite sure that even if a lot of those strains would have produced phenolic character, not all of them would have. Yeasts are too diverse for that. I still remember old saisons that I had the chance to taste in the 1980s that didn’t impart phenolic notes (at least not the medicinal ones of the POF+ yeasts).

From this view, finally, you can see that there are many more possibilities than you might think for brewing a genuine saison.