I recently attended this year’s Craft Brewing Conference in San Diego, and one of the talks, “Yeast/Brewer Cooperation” by Dr. Michael Lewis, meshed nicely with this article. I had been planning on discussing pitching rates, how many billions of cells to pitch per liter of wort for light ales, strong ales, light lagers and strong lagers, but his presentation really helped define the big picture.
Yeast do not want to make beer. They want to grow. The yeast do not care what kind of beer you want to make. They simply take stock of the food resources, and environment, and set out to grow as much as they can before the resources run out. To grow, yeast cells need oxygen to synthesize sterols, which governs the number of times the cell can bud to produce daughter cells. The growth cycle produces three things: metabolic building blocks, catabolic waste and daughter cells. The yeast produce lots of metabolic building blocks in preparation for growth, such as esters, aldehydes, and fatty acids — usually producing more than they actually need, and these are key flavor compounds in beer. The waste products are alcohols and carbon dioxide.
Yeast do not care about making beer any more than sheep care about making sweaters. Left to themselves, sheep would simply eat everything in sight, grow lots of wool and make more sheep. The wool would probably be dirty, snarled, and poorly suited for making sweaters. A rancher manages the production of sweaters by managing the sheep. The rancher controls the number of sheep with respect to the environment and food resources to ensure that one doesn’t overwhelm the other. Yeast are the same way, they only produce three things: metabolic byproducts, waste and more yeast. The beer that brewers want is the direct result of this activity, and the three aspects are directly related by mass balance. The more yeast you make, the more flavor compounds, carbon dioxide (CO2) and alcohol you make. Manage the yeast and you manage the beer.
It is up to us as brewers to control the resources and environment so that the yeast produce the quality of sweaters, er, I mean beer, we want. Dr. Lewis noted that a professional brewer has two options when he ferments an ongoing beer product. Lewis said (paraphrasing): the brewer can produce the same beer, or he can produce a different beer. How does a professional brewer consistently produce the same beer batch after batch? By producing consistent fermentations. How does a professional brewer produce consistent fermentations? By consistently managing the yeast so that each fermentation produces the same rate of yeast growth and the same total amount of growth. If you reproduce the same fermentation with the same ingredients, you will reproduce the same beer with the same flavors. This is the big picture and it helps us put yeast pitching rates in context.
The pitching rate most commonly mentioned in the homebrewing literature is 1 million cells per milliliter of wort per degree Plato. For the mathematically challenged, I want to point out that 1 million per milliliter is the same as saying 1 billion per liter, and that there are typically 100 billion cells per liquid yeast package. So, one package could ferment 10 liters of 10 °Plato wort. Furthermore, if you consider that specific gravity is about four times the °Plato, and that there are roughly 4 liters in a gallon, then this example roughly equates to one package being capable of fermenting 2.5 gallons of 1.040 wort.
What is often not mentioned is that this recommendation is for re-pitched yeast — such as you would get from the bottom of the fermenter from a previous batch. That yeast is not at peak vitality (i.e., health) and viability (i.e., % alive), depending on age in the fermenter, previous original gravity, etc. For pitching rates when using yeast harvested from a previous fermentation, see the chart at right.
Fresh yeast from a well-prepared starter is at the peak of viability and vitality and only 50–75% as many fresh yeast are needed to do the same job as re-pitched yeast.
Going back to our example above, this means that one fresh package of liquid yeast is capable of fermenting 5 gallons (19 L) of SG 1.040 (10 °Plato) wort, and this is the basis of the common claim that today’s liquid yeast packages are ready-to-pitch, and don’t need a starter for a typica five-gallon (19-L) batch. But this singular example is only the tip of the iceberg when it comes to pitching rates.
As in all processes, there is a minimum and maximum operating window that is necessary for the process to run. The operating window for both ale and lager for a good fermentation using re-pitched yeast is roughly 0.5–2 million per mL per degree Plato. One yeast cell pitched to a 5-gallon (19-L) batch is not going to make a good beer, no matter how much you aerate it. Likewise, filling a fermenter half full of yeast slurry from previous batches and topping up with wort is not going to make a good beer either. Yeast, like other herd animals, are constrained by their environment. If you put 100 sheep in a 1 acre pasture, very little reproduction will occur in the time that it takes to eat 1 acre of grass. On the other hand, if you put 100 sheep on 10 acres, the resources will last longer and more reproduction will occur in that time.
This comparison illustrates two aspects of fermentation and yeast starters. The first is that small yeast starters do not generate much growth. There is simply not enough nutrients in a small starter to support much growth by the time those nutrients are consumed. Specifically, pitching a typical liquid yeast package to a 1-quart (1-L) starter will only result in about 1.7X growth, but pitching to a 2-quart (2-L) starter will generate about 2.3X growth and a 4-quart (4-L) starter will generate about 3X growth. Using a stir plate to maintain oxygen levels will help with the total growth, but growth will not occur in the absence of sugar.
The second aspect is that the amount of growth dictates the amount of flavor compounds generated. Lower pitching rates within the operating window will result in more growth before the resources are consumed compared to higher pitching rates. In other words, lower pitching rates tend to result in more yeast character in the beer (more esters, etc), while higher pitching rates tend to result in less yeast character, i.e., a “cleaner” fermentation. There is a limit of course, and extreme over-pitching tends to result in the production of unused metabolic building blocks, starvation, yeast death and the subsequent release of those compounds into the beer.
When Jamil Zainasheff and I wrote Brewing Classic Styles, we included specific pitching rates for all the recipes, something no other recipe book has done. (And to clarify, the term “pitching rate” typically refers to the number of yeast pitched per unit volume of wort, i.e. millions of cells per milliliter. In homebrewing, this is understood to be the initial quantity of yeast, ex. 1 package, because we are typically talking about 5-gallon/19-L batches.)
The reason we specified pitching rate is because it is the starting point for predicting total yeast growth. The rate of yeast growth is most dependent on temperature, and the total amount of growth is limited by the aeration and wort nutrients. By controlling the starting point, the available nutrients, and the rate, you control the outcome — the flavor of the beer. The idea of yeast growth management to ensure batch-to-batch consistency also applies to successfully duplicating an award winning beer from a recipe — match the fermentation to match the beer.
Pitching rates for ales
Ale styles that are noted for their yeast character, like English ales and Belgian sour ales, should be pitched to the low end of the window, 0.375–0.5 billion cells per liter with fresh yeast. Ale styles that are regarded as having a more balanced or light yeast character, such as Southern English brown ale, dry stout and extra strong bitter should be pitched more to the middle at 0.50–75 billion cells per liter. Ales that are considered to have a very clean character, like American pale ale, blonde ale and Northern English brown ale should be pitched at the 0.75–1.0 billion cells per liter. Of course, some yeast strains have a very assertive character all on their own, and therefore the pitching rates for these yeasts tend to be on the upper end in order to prevent the fermentation going hog-wild and funky. Examples include weissbier, saison and witbier.
Pitching rates for lagers
Pitching rates are typically doubled for lager styles, so the corresponding rates for estery lagers such as Kölsch and California common are 1.0–1.125 billion cells per liter. More typical lagers having a clean or low yeast character like Dortmunder export, Munich dunkel, and American lager have rates of 1.125–1.5 billion cells per liter. Very clean styles like Munich helles, Vienna lager, Bohemian and German Pilsner can use even higher rates, 1.5–1.75 billion cells per liter to produce the best results.
It may be surprising to note that strong beers like old ale, Russian imperial stout, Baltic porter and barleywine use relatively middling to high pitching rates. You may think that these beers have a lot of flavor, and they do, but the flavors are typically clean, estery flavors, rather than funky off-flavors. Strong beers need clean fermentations to prevent the off-flavors from overwhelming the beer.
Of course, most homebrewers do not actually count their cells. (This requires a microscope and a hemacytometer. See the December 2003 issue of BYO for more information.) Instead, we rely on estimates of how many cells should be in a given volume of starter wort. For example, according to the pitching rate calculator at mrmalty.com, a 2-quart (2-L) yeast starter, pitched with one pack of yeast and given a single shot of oxygen, should yield roughly 230 billion cells. In practice, estimating cell counts from starter volume yields good beer, but brewers should be aware that it does leave room for considerable variation in the actual cell count.
To consistently brew good beer of any style, the brewer needs to understand yeast behavior, and manage that behavior to his purpose. No single pitching rate is right for every style of beer. The pitching rate is just the tip of the iceberg. The rest of the ‘berg is the yeast health, the nutrient resources and the fermentation environment.
John Palmer is Brew Your Own’s “Advanced Homebrewing” columnist.