I’ve had an interest in wild yeasts ever since I learned that they can in fact be used to make beer – good beer, even! I mean, the yeast we’ve domesticated was at one time wild, so it shouldn’t be too difficult to tame some wild yeast.
By sheer coincidence, I happened to meet both a professional microbrewer and a professional cider maker in the same week who each mentioned an interest in wild yeast derived from fruit. The concept is appealing: by isolating local yeast, we can aim to create a more truly local product with unique characteristics! Having lots of experience in microbiology, my labmate and myself jumped at the opportunity and a short while later we were swabbing apples in an orchard alongside the aforementioned beverage industry professionals.
Our goal was simple in concept but potentially more complicated in execution: to isolate at least one strain of Saccharomyces yeast that will not only ferment beer and cider, but produce appealing and possibly interesting sensory characteristics.
We also wanted to explore the wilder side of yeasts by isolating and characterizing other fermentative yeasts (eg. Hanseniaspora, Brettanomyces) with potential for blending. In my strain collection, I have a strain of Hanseniaspora uvarum/Kloeckera apiculata I acquired from Sui Generis, which was originally isolated from some self-fermenting apples. What’s particularly interesting is that this yeast strain can only ferment to ~2% alcohol (these yeasts are not as ethanol-tolerant as their Saccharomyces cousins), yet imparts great flavours typical of spontaneously-fermented ciders. It turns out that these apiculate yeasts (so-called because of their bowling pin or lemon-like shape) are well known to improve the complexity of wines, ciders, and other alcoholic beverages as well! I’m sure these interesting yeasts could work their magic in beer as well and this will be the subject of an experiment on this blog later on. For more info, see this blog post.
Next, of course, was actually designing an experiment/plan. A wide range of yeast species have been isolated from apple skins as well as pressed ciders, which means that there may be a wide diversity in what we isolate. Of course, there will be plenty of bacteria and mold spores present on the apple skins as well. Because of this, we had to use a method that would allow us to isolate organisms from the apple skins while also selecting for useful fermentative organisms at the same time. I had some sterile, hopped wort available in the lab from another experiment, and figured that would be a pretty good media to use. By swabbing apples, then immersing them in sterile wort placed into sterile plastic tubes, we could transfer some organisms into the wort and hopefully select for fermentative ones by closing the caps and limiting the amount of oxygen available.
We met on a dewy September morning and swabbed a wide variety of eating apples: Cortland, Macintosh, Russet, Honey Crisp, Empire, Spartan, and an experimental variety. We also strategically placed two agar plates used for isolation of yeasts (YPD media) around the orchard, in hopes that we might pick up some kind of interesting airborne yeast. Of course, these plates were far more likely to just grow massive quantities of mold and indeed they did.
Our tubes of wort fared a little better – out of 15 tubes, 4 showed signs of fermentation after three days! However, most of the other tubes never fermented and instead grew populations of bacteria (visible under the microscope) or patches of mold (clearly visible to any passerby). I plated out some of the culture in two of the tubes (Honey Crisp and Russet) onto YPD agar plates after three days, and left all four tubes to ferment out for a week before culturing again.
It became clear that there were a lot of things living in these tubes – I saw lots of different yeast colonies, bacteria and molds. I picked up some yeast colonies and streaked them onto another plate to try to get rid of the bacteria/molds.
From this effort, I ended up with 4 strains banked, and potentially lots more to explore on the plates if need be. Spartan I appears to be Saccharomyces, with spherical cells that bud. Spartan II is some kind of apiculate yeast, forming lemon-shaped cells that bud on either end. Honey Crisp III has a really interesting morphology under the microscope, forming long chains of cells, elongated hyphal cells, and normal-looking round cells as well. I streaked this onto copper sulfate YPD and cycloheximide YPD and it grew on both, indicating that it is likely some kind of Brettanomyces (See here). Honey Crisp IV also appears to be some kind of Saccharomyces strain. All of these strains grew to good cell density in sterile wort (OG 1.052), indicating that they are candidates for further fermentation experiments. If you’re wondering where Honey Crisp I and II went, they were huge fission yeasts which didn’t survive so well in wort – which is fine, since I haven’t found much positive information about these yeasts in beer/cider anyway.
Interestingly, I saw a rather diverse yeast population in the plates made from the young cultures versus the ones allowed to ferment out. I streaked out the cultures from three of the tubes after 7 days of growth (Macintosh Ground I, Honey Crisp I, Russet), and what I saw was interesting: very uniform colonies of yeast, which were revealed by microscopy to be uniformly round, budding cells I’ve tentatively identified as Saccharomyces. It’s likely that there was a very small population of these yeasts on the original swab, which were able to reproduce quickly and outcompete the other organisms in their tubes, producing carbon dioxide and alcohol, and purging the solution of oxygen. I’m not ready to say they are all the same strain however, and why would I? They came from different apples after all. I banked one yeast strain from each of these plates, bringing the total up to seven.
Now that there are seven yeasts isolated which can survive in wort, it’s time to get some real data and test them for fermentation performance! Here’s what we’re going to do:
- Grow yeasts in liquid media for pitching, making sure to smell them as we go.
- Scale up with promising isolates into small-scale wort and cider ferments, monitoring FG, fermentation performance and taste/aroma.
- Scale up to commercial pilot batches!
Stay tuned for part II. Test ferments are under way…