This past weekend I made plates using potato sucrose agar for the first time. The composition is given on BKYeast as well as the reason for using sucrose instead of dextrose. In my case I was less worried about being able to grow larger colonies of lactobacillus but I figured I might as well save some more expensive dextrose and use cheap, plentiful table sugar. Aside from a fumble with the pressure cooker that made me run out of water 5 minutes after it pressurized and thus having to start all over again after it cooled (can’t open a pressure cooker until it cools down fully), things went OK. It seems the additional time at high temperature caused the agar to get a little flimsier, because a plate I sterilized later for only 15 minutes was more firm.
I also made a smaller second batch I left as liquid. I’m seeing what happens trying to step up a culture from a plate using this broth instead of wort. I ended up not picking from a plate but taking a small amount (~3mL) of sediment from a tej yeast 50mL culture I had growing for a few days. It looked rather unusual: large clumps formed that were swimming around. Swirling it around broke some of those and allowed them to settle, but some remained. I at first attributed it to the fact that I had used Whirlfloc when preparing the wort. It was somewhat successful because the wort was clearer but it had the troublesome effect of making larger clumps of break material than would normally be. Since some more break material formed during sterilization it was not too helpful.
Anyway, I brought it up in the potato sucrose broth for a few days. It was probably too small of a sample to begin with, so it took a while to show any signs of activity. But it did, and after three days I turned the stir plate off and saw this:
These flocs were very loose and lumpy, not unlike what was in the previous culture in wort. I did not see any of the larger clumps like I saw in the wort, but that may be due to the yeast clumping with the protein there. After I left it to settle overnight it began to compact somewhat to the point where it didn’t stir up the second I picked up the bottle (part of that is the stir bar moving around) which means it may still be useful for brewing. I will leave it for a little while longer to see how much settling takes place eventually. If it stays too loose it will be more difficult to transfer after fermentation.
So what’s going on here? Without a microscope I can only speculate (well, even then I would still be speculating) but this is a great reason for me to study more on flocculation.
First, I am fairly sure the culture is pure yeast, since at least in the previous step the pH was normal (~4.4 after fermentation) and there was no sour smell or taste. It tasted quite nice in the wort, actually (although it looked kind of scary the lack of any unusual smell finally convinced me to try it). It was not viscous or turbid; when looked against a light, the part above the clumps looked very clear.
In Brewing Yeast and Fermentation the authors cite a 1951 study dividing flocculant brewing yeast into flour classes:
- Class I – compact sediment which on resuspending in 0.5 ml is completely homogeneous
- Class II – compact sediment which on resuspending is distinctly granular
- Class III – sediment peels away in large flakes/dense round clumps which will not disperse
- Class IV – sediment consists of loose flakes/clumps of yeast
(Chris Boulton and David Quain. Brewing Yeast and Fermentation (Blackwell, 2006), 178)
The fourth class sounds a lot like what I’m seeing. I haven’t tried looking for it, but the study is by R.B. Gilliland, “The Flocculation Characteristics of Brewing Yeasts During Fermentation,” if you are so inclined. Some other species do flocculate, but I have not been able to get much on that yet as only a few studies exist. The one I am looking for doesn’t appear to be on any of the big online journal databases.
So it seems this is nothing too out of the ordinary for brewing yeast. I’m not sure that this is Saccharomyces but a lot of factors are pointing in that direction. For one thing, it ferments maltose, which Brewing Yeast and Fermentation seems to indicate is actually somewhat rare in the yeast world. Harry Kloman also cites an old Italian study that found Saccharomyces ellipsoideus to be the primary fermentation agent. A cursory Googling shows that it might be an older classification that today would be under S. cerevisiae.
Since it did eventually compact a little bit, it should be useful if not ideal for brewing. It might be an interesting study of flocculation trends over time.