Why do yeast make alcohol?

Ever wonder why yeast make alcohol? Probably not, I realize, but you should. Yeast throw off ethanol in the process of metabolizing sugar, so alcohol is a byproduct of survival; fair enough. But alcoholic fermentation is, in fact, a surprisingly inefficient way to get energy. The standard oxygen-requiring way of breaking down sugar used by most cells, our own included, wrings somewhere between 30 and 38 ATP (38 is the ideal number; it’s probably never quite that high in practice) out of a single glucose molecule. (ATP is the cellular currency in which energy is transferred and spent.) Nevertheless, alcoholic fermentation has the distinct advantage of not needing oxygen and so it makes perfectly good, intuitive sense for Saccharomyces cerevisiae to use it when oxygen isn’t available.

Here’s the quirk: S. cerevisiae uses inefficient alcoholic fermentation even when it does have access to oxygen, even though it has the machinery for the much, much more energetically worthwhile aerobic metabolic process. Yeast will only switch to aerobic metabolism when the amount of sugar available for them to eat is very low. Why? A good question, and one microbiologists haven’t had much success answering.

Our best hypothesis according to a brand-new review on the subject comes in two parts:

  1. Alcoholic fermentation lets yeast act fast to use up the “public goods” while squirreling away private resources for later. Every microorganism you’ll encounter in grape juice can consume sugar. Very few can also consume (and get energy out of) ethanol, but yeast can. So, by converting sugar to ethanol, S. cerevisiae can starve out other microbes and leave itself with a food source for later.
  2. As an additional and maybe even bigger benefit, ethanol is toxic to most yeast and bacteria at concentrations that Saccharomyces can tolerate with relative ease

Possibly the most bizarre thing? We don’t know much about what determines the circumstances under which S. cerevisiae, our long-time compatriot and coworker, produces alcohol versus making energy in some other way. We’ve looked at when and where different yeast genes are expressed and when and where it makes different byproducts but, like so much else in the wonderful and frustrating world of modern-day genetics, putting together the whole story is still a work-in-progress.

S. cerevisiae: Friendly to us, but…

Saccharomyces cerevisiae is one of the friendliest microbes around. The good, old, familiar yeast used for (nearly all) bread, ale (lager yeast are different), (nearly all) wine, saké’s second (alcoholic) fermentation, and an improbable array of industrial applications: making CO2 to bubble into aquariums and keep subaquatic plants from suffocating and manufacturing insulin, for example. Heck, my favorite state even elected it as the official state microbe this past May, making Oregon the first – and only – state to have a patron yeast.**

The yeast so friendly to us, though, is a good deal more sinister to its peers.

Branco and colleagues, at the Laboratorório Nacional de Energia e Geologia in Portugal, have figured out that some S. cerevisiae strains release peptides that kill other types of yeast and bacteria found in the early stages of wine fermentations, including the love-it-or-hate-it Brettanomyces bruxellensis (responsible for barnyardy Burgundy and a whole lot of spoiled wine that tastes like old gym socks).

Microbiologists have also known for decades that some strains of S. cerevisiae are “killer yeast,” releasing peptides (very small proteins) that cause the death of other “killer sensitive” strains. Killer yeasts can be a cause of stuck fermentations: if your carefully selected yeast is killer sensitive, an accidental contaminant population of killer yeast could wipe it out, leaving only the less well-adapted accidental yeast unable to finish fermentation, and leaving residual sugar in a wine that was supposed to be dry. Brewers who want to play with wine yeast in ale recipes need to be careful, too, because the majority of wine yeasts release killer peptides and all ale yeasts are killer sensitive; mixing the two requires choosing a non-killer wine yeast. More recently, we’ve learned that some malolactic fermentation bacteria (like Oenococcus oeni) are also sensitive; a reason, perhaps, for some malolactic fermentations not kicking off as expected.

So we knew that already. But the new peptides that Branco and colleagues have identified are important for two reasons.

First, we’ve been operating under the general assumption that killer yeast will only kill other yeast (and maybe some of those malolactic bacteria). If that’s not true, we may have a new and improved explanation for why some microbes interact the way they do.

Second, if some S. cerevisiae make a peptide that will kill off undesirable or spoilage yeast, we could use it to our advantage. Produced in bulk, they could be added as a “natural” preservative against Brett and other nasties. Other yeast toxins have been used in similar ways.

S. cerevisiae always takes over wine fermentations, even when musts aren’t inoculated with commercial active dry yeast, and even though grapes in the vineyard play host to a large and diverse population of other yeasts and bacteria. We’ve generally assumed that S. cerevisiae takes over because it’s best able to withstand the rather noxious fermenting grape environment – high sugar, high acid, increasing amounts of alcohol (S. cerevisiae‘s high alcohol tolerance is especially significant) – and because it grows fast.

What this research calls to my mind are recent explorations into adding microorganisms other than S. cerevisiae to fermentations, not to carry the burden of sugar-to-yeast conversion, but to add flavor and complexity. New killer peptides may offer a welcome explanation for why some microbes work well with S. cerevisiae and others don’t. For at least some of those that don’t make it, maybe we need to change the cause of death from “suicide under high stress” to “murder.”


**Wisconsin made an attempt in 2010 to elect Lactococcis lactis, the workhorse bacteria for culturing cheddar and many other cheeses (not to mention lactic-fermented pickles, but those are more Seattle than Wisconsin), but the movement stalled in the Senate after being passed by the House. And Hawaii’s House made a movement to elect Flavobacterium akiainvivens (which doesn’t appear to be of any particular good to anyone, but was found on a rotting bush on O’ahu) in February but, again, it failed to clear the Senate. The official state microbe is evidently catching on, to which I give an earnest “Hooray!” if it means more popular awareness for how much bacteria and yeast contribute to our livelihoods. The marvelous Elio Schaechter thoughtfully collected crowd-sourced suggestions for some other states on the dedicated microbe blog Small Things Considered here