Does yeast mate during alcohol fermentation?

March 31, 2015 2:21 PM

I've learned a bunch about the mating life of S. cerevisiae and about alcohol fermentation, but I haven't found a definitive answer to one question: Does yeast mate during alcohol fermentation? More than usual? Less than usual? Not at all? If it does, does that mean that a-factor and α-factor are present in beer and wine?
posted by clawsoon to Science & Nature (10 answers total)

Best answer: From this reference:

Most wild S. cerevisiae wine yeasts isolated from must fermentation are diploid homothallic strains (24, 30). They sporulate in the absence of nutrients, giving haploid spores that are very resistant to adverse environments (long periods of nutrient depletion, extreme temperatures, radiation, dryness, etc.). Sporulation can take place every year at the end of the vintage, and the spores can survive the periods between vintages. At the beginning of the next vintage, the spores germinate, giving haploid yeasts on the new ripened grape or in its must, where there are abundant nutrients available. These haploid strains, which can have two different mating types (a and α), mate with each other, and restore the diploid status that predominates in spontaneous must fermentations (3), in which multiplication occurs by asexual budding. The sexual part of the yeast biological cycle (sporulation and mating) is not required for wine making, although it is needed for the yeast to survive the adverse conditions during the cyclic starvation periods between vintages. If a yeast is unable to undergo meiosis, sporulation, and mating, i.e., if it is sexually sterile, it will disappear from nature because it cannot survive in the winery-vineyard ecosystem. An exception is the film yeast living in the surface of wines such as sherry that undergo biological aging. These populations can survive for years in the winery without sporulation (20), although they do need to sporulate to survive outside the winery.
posted by kisch mokusch at 2:49 PM on March 31, 2015 [1 favorite]

Response by poster: Thank, kisch mokusch. To clarify what that reference says - and please correct me if I'm wrong! - it sounds like some mating factor may be present in the final product as a result of mating that occurs "on the newly ripened grape or in its must", but there'll be little or no mating factor production through most of the growth and alcohol production phase.

Am I correct to assume that there'll also be little mating factor production, only haploid spore production, during the die-off phase when the alcohol content rises to lethal levels? I.e. mating factor production happens only during the window when nutrient availability transitions from "hardly any" to "lots and lots"?
posted by clawsoon at 3:18 PM on March 31, 2015

Best answer: In contrast to the situation kisch mokusch described above with wine, my guess is that yeast probably do not mate during commercial beer fermentation, because IIRC the fermentation process is usually much more controlled and involves inoculating with specific, clonal strains of yeast (my understanding is that people also try to do this with wine sometimes, but it all gets out-competed by the wild yeasts on e.g. the grape skins). My sense is that these commercial beer-brewing strains are also likely to be diploid (because of mating type switching, yeast tend not to be haploid for very long unless you have specifically bred the ability to switch mating types out of them), and diploids, of course, produce/respond to neither pheromone. They could sporulate at the end of the run once nutrients become limiting, but without a sugar source those spores would just be hanging out in G₀ not doing much, so you probably wouldn't get mating unless you re-added nutrients (which doesn't happen in beer making, I'm pretty sure).

Beyond this, some common beer yeasts are also sterile interspecific hybrids, e.g., strains of the lager yeast S. pastorianus.
posted by en forme de poire at 4:45 PM on March 31, 2015

I will preface this by saying that this is definitely not my field of expertise. I'm a biomedical researcher, which is why finding the reference was pretty straight forward, but I don't work with yeast. I studied it a bit in Genetics, but that was was a while back.

I take it to mean that mating is something done in less-favourable conditions, where the added diversity might improve survival. If yeast are unable to mate in harsher conditions, they generally do not survive. This is presumably because mating can produce "fitter" offspring, better suited to the changing environment. However, mating also introduces genetically less-fit offspring (more diversity = a wider spread of phenotypes = both fitter and less-fit organisms). However, these are weeded out during the highly favourable conditions (such as during fermentation) through competition. In other words, when nutrients are abundant, they all opt primarily for asexual budding as the preferred means of expansion (presumably because it is faster than mating). The fit clones will expand well, while the less-fit ones will not.

So while there may be some mating during fermentation, the offspring evidently opt primarily for asexual budding as the preferred means of expansion. As nutrients are depleted, the yeast will produce spores. Again the "fitter", more numerically abundant yeast will likely produce more spores than the less-fit yeast.

Which means that your take is pretty much the same as mine. A bone fide geneticist may come in and offer a better opinion, though. I know that there are a couple of them on Metafilter, or at least there used to be.
posted by kisch mokusch at 2:40 AM on April 1, 2015

Upon second re-reading there must be some mating during fermentation, even if the later stages are dominated by asexual budding. So my take is now exactly the same as yours.
posted by kisch mokusch at 6:57 AM on April 1, 2015

Best answer: I just finished listening to the 14 lectures of Charles Bamforth's "The Brewmaster's Art" series, so, like, I is informed about this!

Dr. Bamforth said that all that yeast growth during fermentation is from budding, which is asexual.

However, there is less-frantic growth at other times, and I think that that may be sexual. He didn't get into the birds-and-bees aspect of that, because he is concerned with fermentation. *shrug*

Now I feel like I let you down. Sorry. Bamforth seems a lovely guy; want me to email him for you? It would be fin!
posted by wenestvedt at 7:27 AM on April 1, 2015

I just emailed Bamforth; If he doesn't reply, I will find the professor in my .edu employer's culinary department who handles our little research brewery and ask them, too!
posted by wenestvedt at 7:36 AM on April 1, 2015

Response by poster: I just got an answer back from Dr. Linda Bisson, and it looks like y'all are correct:

I do not think anyone has shown mating during fermentation but the belief is mating occurs on surfaces so yeast biofilms in wineries or on the surface of the grape which is consistent with the diversity found on those surfaces. Bob Mortimer found homozygous strains (an indication of very recent sporulation and mating events) only on the surfaces of grapes but he did not look all that hard on winery surfaces. the mating factors would not be present in beer or wine and beer strains are often polyploidy and not sporulating productively so mating would not be observed although people think some of the polyploidy may have arisen from failed sporulation attempts or mating between diploids that is illegitimate - but those would not involve mating factors.

I also suspect from the genetic variability in commercially grown yeast that if the yeast is settling at the bottom of a tank or held in high concentration sporulation and mating might occur but it would be a rare event.

Sounds like it goes even further than en forme de poire suggested for beer, with the polyploid strains not even able to mate.
posted by clawsoon at 9:46 AM on April 1, 2015 [1 favorite]

Best answer: However, there is less-frantic growth at other times, and I think that that may be sexual.

So I'm not a Yeast Geneticist™ but I did work on yeast metabolism in grad school and I can answer this part, at least. Sporulation (going from diploids to haploid spores) is strongly controlled by nutrient availability and, in particular, is essentially totally repressed by the availability of fermentable sugars. You would only get sporulation after extended cultivation in a poor nutritional environment. In the lab we'd use acetate as a carbon source and no nitrogen source whatsoever (and also lower the temperature from 30 to 25°C) and hold them there for at least a couple of days, possibly up to a week.

Also, I totally forgot to mention this, but when diploids sporulate, they generate a set of four spores (tetrad) inside a tough wall; if you don't digest and/or dissect this away, when glucose is re-added (i.e. germination) the spores will tend to mate with each other right away, since there are typically both mating types represented within a given tetrad.
posted by en forme de poire at 12:26 AM on April 2, 2015 [2 favorites]

Best answer: I received this email last night WRT to yeast sexing it up in your wort:

Hi Will

Thank you for your kind words.

No significant mating during fermentation.

(Too much alcohol, perhaps?!)

Seriously: budding only.

Best
Charlie



Charlie Bamforth
President, Institute of Brewing and Distilling

Distinguished Professor
Department of Food Science and Technology
2158 Robert Mondavi Institute North Building
University of California
Davis
CA 95616

posted by wenestvedt at 5:39 PM on April 3, 2015