Does Yeast Multiply During Fermentation?
The short answer is yes. Yeast does multiply during fermentation.
The long answer, as usual, is a bit more complex and complicated.
Let’s take a look at the wonder that is yeast.
Yeast
Yeast is a member of the fungus family, the single oldest living organism on earth.
Before there was any other life form on earth, there was fungus.
Mushrooms, yeast, and molds are all fungi.
Fungi are highly resilient, reproduce quickly and with very few demands, and are hard to kill.
Hence the reason yeast has not only been around for what is likely millions of years, but is also one of the most prevalent life forms on earth, taking up more space even than bacteria, of which there is more on earth, because of its size.
Yeast is several times larger than bacteria, on average about 30 to 40 times larger.
Yeast can multiply, or reproduce sexually and asexually, and with or without oxygen, though it seems to prefer to reproduce sexually and with plenty of oxygen.
Brewing with or Without Oxygen
In general, brewers are advised to brew in an airtight container, and this is because of the fear of too much oxygen allowing the yeast to produce off flavors.
However, yeast needs oxygen to multiply, reproduce, and ferment.
So, the way it is typically done today is that a brewer will harvest, roast, and grind the grain, boil and steep it, and then add yeast and hops for fermentation once the wort is cooled down.
It is at this point that the entire batch will be locked into an airtight container.
The wort has been fully aerated, and there is plenty of oxygen for the yeast to feed on to ferment and reproduce.
The general thinking is that if during fermentation the vessel is left to the exposed air, either wild yeast and bacteria may enter and/or the yeast in the wort will get too much oxygen and produce off flavors.
However, studies and ample evidence, not to mention thousands of years of beer making in history, show that, given enough time, racking, and aging, those off flavors will indeed be mopped up and smoothed out.
Indeed, science has shown that, in an open-air vessel, the carbon dioxide released during fermentation will hover over the wort and act as a sort of blanket, protecting the brew as it ferments.
Nature is pretty cool, isn’t it?
In any even, with or without oxygen, the yeast will indeed multiply/reproduce.
Each yeast cell will typically bud 20 to 30 times during its life, with a mother cell producing and supporting a daughter cell on her own until the daughter reaches a volume of 50% the size of the mother, at which point she can break off and begin to produce her own daughters.
This process takes place rather quickly and can occur multiple times during a single fermentation.
Case in point: a 10 milligram starter culture can grow to 150 tons in just a week under the proper conditions.
The process of yeast multiplication during fermentation looks something like this:
Lag Phase
The initial phase of fermentation for yeast is called the lag phase, this is the first 24 hours during which the yeast is adjusting to the temperature and conditions of the wort into which it has been pitched or, in the case of wild yeast, into which it has settled.
Healthy yeast should have no problem adapting to these new conditions, particularly if the yeast is wild, or local, or if the yeast has been grown from a starter wort that is similar to the wort into which it is pitched.
It is extreme changes that shock and potentially kill of yeast, like heat, cold, or a dramatic shift in gravity or acidity.
Log Phase
Once the yeast has settled in, it will begin to reproduce rather rapidly; this stage is called the log phase.
The reproductive process, as mentioned above, is exponential, with any weaker yeast cells simply dying off and leaving yeast hulls behind for the healthy and active yeast to consume along with the sugar in the wort, which provides ample nutrients for the yeast to produce not only alcohol and carbon dioxide but also hundreds of healthy secondary metabolites.
Anaerobic Stage
Finally, many brewers will ensure yeast runs out of oxygen so that it will be forced into producing more alcohol and carbon dioxide as it has no more oxygen to feed on for energy.
As it runs out of energy, it feeds more on sugar and produces even more alcohol than it would had it been left exposed to oxygen.
So the real purpose of cutting oxygen levels is to drive up alcohol levels.
This is why beer made hundreds of years ago had much lower alcohol content, usually hovering around 3% or 4%, and some beer, like the beer made for families, had closer to 1% ABV and was drunk with each meal and considered safer than water.
Again, the anaerobic stage is not necessary for fermentation, either for alcohol production or for controlling off flavors, as nature does indeed clean up after itself quite nicely.
Furthermore, yeast will multiply under either condition, oxygenated or not, but if you are looking for greater multiplication of yeast, which will result in lower alcohol, you want to have oxygen present at all times.
To drive up alcohol, and slow the reproduction of your yeast, you will need to use an airlock or other airtight container that will allow the carbon dioxide generated to release while also preventing new oxygen from getting in.
And as for off flavors, you can trust nature to take its course, the yeast still reproducing will continue to consume any metabolites released and smooth out your brew for a crisp, clean finish.
Cheers!
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