Yeast Mechanics

Photo courtesy of Denny Conn

It seems there’s no way to write about beer and yeast without saying: “Brewers make wort, yeast makes beer.” So, there, we’ve done that bit so now we can take a closer look at how yeast turns our wort into beer and how to get the most out of your yeast. By the way, don’t take that to mean we’re not serious about yeast’s impact on our beer. Don’t forget, Drew brews arguably the most yeast-influenced style in existence — saison.

First a disclaimer . . . while we’re both science-y guys, neither one of us is a microbiologist. So we’re going to discuss yeast the way we think of and understand it. We’ll be talking about concepts, not hard-core biology. That’s the way we like it and that’s the way we think most of you like it, too. After all, it’s what our ancestors did! (But feel free to tell us how we’re wrong!)

Brief history of yeast – Magic Sticks to Pasteur

Yeast is a member of the fungi (fun-guy) kingdom, which means that strictly speaking it’s neither plant nor animal. Technically it’s a eukaryotic microorganism, which is a fancy way of saying it’s a more structured cell with clearly delineated parts — unlike say bacteria, which tend to have less structured gooey internals. So guess what? You, me, the tree, and yeast — we’re all classified the same . . . as eukaryotic-based organisms!

While both bread yeast and beer yeast are Saccharomyces cerevisiae, there are differences based on historical selection criteria. Bread yeast is suited to producing CO2 rapidly, while the main purpose of beer yeast is alcohol production (and these days — flavor production). [Side experiment – go and make a loaf of bread with bread yeast. Put the dough, covered, in the fridge and allow it to proof, aka ferment, overnight. Pull it out, let it finish rising and treat as normal. You’ll be rewarded with an amazingly flavorful loaf
of bread.]

Keep in mind this doesn’t always mean much. Across history, brewers and bakers were usually tied at the hip — some cases culturally (see Egyptian salaries in buckets of beer and loaves of bread), phraselogically speaking in others (“beer is liquid bread”). Bakers often used leftover barm from their brewer cousins before we got into a time and period of differentiation. And in the bad old days of homebrewing — before we had the plethora of yeast choices before us — homebrew recipes usually instructed would-be zymologists to pitch their wort with a fresh package of bread yeast. Yes, you can make beer this way, but it’s going to be a better idea to go for a carefully selected beer yeast.

But this is all fairly modern information. The story goes that in the “old days,” before even Denny was born, there was a “magic stick” that was passed down through the family brewers/brewsters. No one knew exactly what it did, but they knew that if they wanted to make beer they had to stir the wort with that stick. Supposedly that was because the stick had become loaded with yeast from the previous batches that went into the new batch? True? Apocryphal? The answer has been lost in the mists of time, but it
at least makes a darn good story, am I right?

Drew: Ok, actually — not really all that unknown today. We know that medieval and later brewers understood that the kräusen of a fermenting beer was capable of starting the fermentation process and yielded cleaner beer more quickly. We also can still see remnants of the “magic stick” tradition in the wooden hoops used to store Kveik strains in the Norwegian farmhouse brewing traditions that Lars Garshol has been documenting ( Fundamental thing to remember — our predecessors may not have had all the fine details, but they weren’t dumb. They built the foundations of where we’ve gotten to! (But let’s face it — history is “boring” while stories are “fun.”)

Fast-forward to 1680, Antonie van Leeuwenhoek used a microscope to observe globules of beer yeast, but he didn’t understand what they were — cell theory wouldn’t come for another 150-ish years, so he thought these curious little blobs were a byproduct of grain flour.

Jump again to 1857, Louis Pasteur used subsequent discovery to figure out how fermentation worked, how microorganisms move, and what role yeast played in fermentation. He also proved that yeast could function aerobically (with O2) or anaerobically (without O2). He basically figured out how to keep your beer, wine, milk, etc. from spoiling — pretty important stuff.

How yeast makes beer

Ever heard somebody say “Yeast eats sugar, farts CO2, and pees alcohol?” Not exactly the most mouth watering way to think of our beer, but there’s at least a grain of truth there. The semi-technical explanation is that fermentation converts the glucose (and other simple sugars such as fructose and sucrose) found in wort into ethyl alcohol and CO2, producing energy as well. From the point of view of the yeast, the alcohol, ester, phenols, and CO2 are byproducts.

C6H12O6 ====> 2(CH3CH2OH) + 2(CO2) + 2 ATP (energy)
Sugar ====> Alcohol + Carbon dioxide gas + Energy
(Glucose) (Ethyl alcohol) (ATP)


The basic mechanism is there, it works . . . it really works. Fermentation, in all of its forms, is a critical component to how life keeps rolling on. So with, or without our input, fermentation happens. The key to what we do is shepherding the fermentation along a happy path with flavors we enjoy and low final gravities.

To best achieve this, we need the happiest yeast out there. Much like us, yeast do best if they’re raised right. So let’s dig into what we can do to assist.

Getting Your Yeast Ready to Work

If you’re using dried yeast, there’s really not much to do to get it ready. Dried yeast doesn’t need a starter to be ready to work. In fact, a starter with dried yeast actually might be detrimental by depleting the dried yeast nutrient reserves. You want the yeast to consume those in your wort, not in a starter. Some manufacturers recommend rehydrating dried yeast before using it, others don’t. We’ve found that rehydration is seldom necessary. While some studies have shown that you may be slightly reducing viability by not rehydrating, there are so many cells in a pack of dried yeast that a slight reduction in viability probably doesn’t really matter. If you’re making a lager, or a larger and/or high-gravity batch, pitch 2 or even 3 packs of dried yeast.

Liquid yeast is a whole other matter and generally requires more care. Different brands and packages of yeast contain differing amounts of cells. Some arrive in your hands with enough yeast that you probably won’t need a starter, but the brewer should always read and follow the manufacturer’s recommendations.

For liquid yeast with lower package cell counts, a starter is a good idea. We make starters for any batch of 5.5 gallons (21 L) that has a starting gravity in excess of 1.040. Most yeast companies say that’s overkill — you can use our yeast without starters! But remember, they’re working hard to make the brewing process seem easier. We’re working hard to make it simple . . . and successful. Our experience is that it works better for us if we take the extra step.

It took years to get homebrewers to understand the importance of yeast starters. Then we took it too far — a few years ago, it became popular to use a yeast calculator to see how many cells you needed, then use a stir plate to get that cell count. We both did that for quite a while. But then we ran across a revival of an old method that is easier and works at least as well as the calculator/stir plate rigamarole.

It’s called the “Shaken, Not Stirred” method, also referred to as SNS or even the 007 Method. It was espoused by a guy who went by S. cerevisiae on the AHA discussion forum and is based on the idea that yeast are like nuclear weapons when it comes to growth. The old saying “close only counts in horseshoes and handgrenades” is a similar concept. Cause when it comes to yeast pitching rates, he argues that growth is so explosive, close is good enough. (You can find the full blog post here: In short, the theory is that cell count doesn’t matter as much as yeast health.

You begin by boiling up a 1-qt. (1-L) starter wort in the 1.035–1.040 gravity range. Once it’s cooled down, put it into a 1-gallon (4-L) container (Denny uses repurposed apple juice jugs) and shake it until the container is full of foam. Don’t use foam control in your starter wort or you’ll never be able to do this! Let it ferment overnight and pitch the whole thing (no decanting) into your wort the next day. That’s all there is to it.

Denny has used this method exclusively for the last couple years, for both ales and lagers, with excellent results. The beer starts quickly, ferments happily, and tastes great. His tests have been on 5.5-gal. (21-L) batches of ales of up to 1.075 OG and lagers up to 1.065 OG. For larger batches or higher gravities, he brews a lower gravity or smaller size batch first and harvests the slurry from that for pitching into the larger or higher gravity batches.

If you’re a pitching rate fanatic, this whole idea may seem a bit foreign . . . OK, downright weird to you.

Remember, you want your starter container to be 4 times the size of your starter wort so you can fill it with foam by shaking. The yeast will utilize the oxygen absorbed in that foam to synthesize sterols. The yeast cells utilize those sterols to keep cell walls flexible and encourage yeast cell budding. Remember, we’re not as concerned here about the number of cells as the health and vitality of those cells. It’s gonna go nuclear! Again — it’s not the numbers, it’s the health. Think a soccer game played with a large number of pizza eating slobs vs a small squad of fit athletes.

If you’re a pitching rate fanatic, this whole idea may seem a bit foreign . . . OK, downright weird to you. It’s taken Drew a good long while to adjust. We’ve had it drilled into us that cell count really matters and that a low cell count will make for a more estery beer. But will it really? Nearly 20 years ago, Denny asked Dr. Clayton Cone of Lallemand about how pitch rate affects beer flavor. The conventional wisdom has always been that lower pitch rates will stress yeast and increase ester production. Dr. Cone’s answer blew that away. Here’s a summary of what he said . . .

“Ester production is related to yeast growth, but not in the way you might think. The key to yeast growth and ester production is acetyl Co-A. It is necessary for both yeast growth and ester production. When it is busy with yeast growth, during the early part of fermentation, it is not available for ester production. Ester production is directly related to biomass production. Everything that increases biomass production (intensive aeration, sufficient amount of unsaturated fatty acids, stirring) decreases ester production. The more biomass is produced the more Co-enzyme A is used and therefore not available for ester production. Anything that inhibits or slows down yeast growth usually causes an increase in ester production: Low nutrient, low O2, etc.”

There was a lot more detail to Dr. Cone’s answer (you can read the rest at
. Go ahead and have a laugh, Denny is misidentified as Todd from Idaho!), but the takeaway is this: If you pitch less yeast and encourage it to grow with appropriate oxygenation and nutrients, you will make a less estery beer, not a more estery beer. In other words, science once more thumbs its nose at conventional brewing wisdom.

Don’t forget, underpitching is relative. You want to pitch enough yeast, but not too much. Since homebrewers are often told that it’s impossible for them to overpitch, this is an important point. So let’s look at that next.

Saving and reusing yeast

In his excellent article cited earlier, Dr. Cone also addresses saving and re-pitching yeast slurry. Many homebrewers, again going on the oft repeated advice that you can’t overpitch, use the entire slurry from a batch for their next batch. Dr. Cone recommends using no more than 25% of the slurry, and that fits with our experience. Overpitching a slurry can be detrimental for several reasons. But don’t fret on hitting these guidelines precisely. If you accidentally pitch 30% of the cake, your beer won’t fail.

Homebrewers have also been fed the idea that you need to wash the yeast. First, unless you acid wash it, what you’re actually doing is rinsing it. Second, there’s only minimal advantages to doing that (those minor exceptions include removal of colors, certain hop compounds, and trub.) It won’t necessarily improve your beer, while on the other hand it’s just one more thing you can potentially screw up and contaminate your slurry.

To store slurry for re-pitching, leave a little beer behind when you rack your beer for packaging. Use that bit of beer to swirl up the slurry, then pour it into 3–4 sanitized containers and keep them in the fridge. Many people use capped beer bottles or Mason jars to do this. If you use glass, keep in mind that the yeast will continue to ferment slowly even in a cold fridge. Denny ended up with glass shards in the wall if his fridge as proof of this! These days, he prefers to use 1⁄2-gallon (2-L) plastic tubs that his local homebrew shop uses for liquid malt extract. Not only will they not shatter, the snap on plastic lids will simply bulge if pressure builds up in them. (Seriously, don’t tightly seal up a fermenting mass. It’s a recipe for grenades!)

What yeast attenuation rating is for

A lot of brewers look at the attenuation ratings for a particular yeast and assume that’s the attenuation they’ll get with it. That’s not necessarily the case. The attenuation rating is meant to just be a comparison of the possible attenuation of one strain to another using the same wort. In reality, wort composition has far more effect on attenuation than the rating on the yeast. Depending on the wort makeup and fermentation conditions, you could get anywhere from 60–90% attenuation from the same yeast. If you want to control attenuation, recipe design is probably the easiest and most controllable way to do it.

So, there you have it . . . both sides of the yeast story. We hope you’ll try some of our unconventional ideas and see how they work for you. For us, they’ve resulted in great beer with less effort and more fun. And isn’t that why we all homebrew?

Issue: July-August 2019