Pitching Rate Experiment

All homebrewers realize that healthy yeast makes great beer. Our microbial friends need to have adequate oxygen and plenty of nutrients to ferment a batch of beer to completion without any off-flavors. Another aspect to yeast, besides their health, is the amount of yeast added to unfermented wort. This is also known as the pitching rate.

This number is important because if you add too little yeast they will tend to grow more, produce more off-flavors such as diacetyl, and not ferment your beer adequately. Adding too much yeast could lead to yeast autolysis flavors, a thin and watery mouthfeel, and lack of desired flavors. There is a sweet spot that all brewers (pro and amateur) aim for to excel in fermentation. Pitching rates are measured in yeast cells per ml per degree of Plato and there are differing opinions as to the ideal number. If you find different pitching rate numbers the most important thing to do is to determine your own pitching rate on your system as this could vary between different brewing systems. An average pitching rate that is cited is:

1 million yeast cells / ml of wort / º Plato

This number can change depending on several factors:

  • Ale versus lager. Lagers generally need more yeast to ferment adequately, and the pitching rate could jump to 1.5 million cells/ml/º Plato. Some ales, such as mild ales, may need less.
  • Gravity. Higher gravity beers need more yeast to ferment the high concentration of sugars (above 1060). Again, 1.5 million cells/ml/º Plato is a good choice for high gravity ales.
  • Fermentor Geometry. Different vessels can alter how yeast grow and ferment. For example, fermenting in a tall cylindrical vessel puts more pressure on the yeast so a slightly higher rate is needed. The opposite would be true in a plastic bucket.

So how do you determine the number of yeast cells to pitch? Lets do some some simple math. Lets say you have a 5 gallons of 1.035 wort and we selected a pitching rate of 0.8 million cells/ml/º Plato. First convert gravity to Plato scale. Divide the last two gravity points by four to get that number:

º Plato = 1.035 gravity / 4

Plato = 8.8º

Next, figure out how many millimeters are in 5 gallons of wort. Since 1 gallon has around 3785 mls that would be 18,925 mls. Now multiply all the numbers together to get your total number of cells needed to ferment the batch of beer:

Cells needed = (pitch rate) * (mls of wort) * (º Plato)

=(800,000) * (18,925) * (8.8)

= 133 billion yeast cells

One important thing I want to mention is the need for a starter. Realizing that we need 133 billion yeast cells for a 5 gallon batch of low gravity beer, the 100 billion cells provided by WYeast packs, for example, is not enough yeast cells to pitch. Lots of homebrewers stick to the rule of “above a gravity of 1.050 is when you make a starter”, but this might not necessarily true and why I always make a starter for my beer. Go by the numbers and not by conjecture.


Now this is all well and good, but what does that mean for my beer? Personally, I’ve never tasted a beer that was underpitched or overpitched. Will there be off-flavors? If there are, what kind? What will be the kinetics of fermentation? This sort of experiment has been done before, but I really wanted to see the results first hand. Moreover, I will be blind tasting the results at a yeast class that I am teaching at my local homebrew shop, Brooklyn Homebrew.

The experiment will based on the following criteria:

  1. Beer style: Blond ale. I chose a blond ale because any off flavors will have little to hide behind.
  2. Yeast strain: Wyeast 1056. I had a tough time choosing the yeast. I could have gone with a very expressive yeast to detect changes in flavor. I eventually went with a popular strain that many brewers use for their batches.
  3. Every sample will be treated the same and the test batches will be fermented in 1 gallon batches. The original gravity of the beer will be 1.046 and only one hop will be used (see recipe below).


  1. Control pitch: 0.75 million cells/ml/º Plato – 32.9 billion cells.
  2. Overpitch: 2.5 million cells/ml/º Plato – 112.8 billion cells.
  3. Underpitch: 0.1 million cells/ml/º Plato – 4.7 billion cells.

The recipe (4.5 gallon batch to be split into 1 gallon fermentors):

  • 7 pounds 2-row Pale malt
  • 1 pound of Carapils

Mashed in at 153ºF for one hour. Raised temps to 168ºF for mashout and recirculated as usual. First runnings at 1.063. Second runnings at 1.018. Collected a total of 5.9 gallons at 1.034. Boiled for 60 minutes:

  • 0.5 oz of Centennial at 60 minutes.
  • 0.5 oz of Centennial at 0 minutes (flameout).
  • Whirlfloc and yeast nutrient at 15 minutes.

Cooled to 62ºF and pitched the amount of yeast noted above. I did not oxygenate the wort in any sample. Fermented cool (62ºF) until fermentation slowed then slowly ramped up to 70ºF.

On December 14th the class is scheduled and I will post the results, including flavor and fermentation profiles. I’m also thinking of polling the students to see how each beer scored.


Results are posted here…


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10 responses to “Pitching Rate Experiment

  1. You are the man. Great test. Wanted to do something like this forever. I’ll be in your debt!

  2. Chris M

    I think this is a great experiment, but not oxygenating the yeast in any way? Won’t this put the yeast in a stressful situation and make each batch taste off? Why not give each 30 seconds of O2?

    • Yup. I decided to leave this variable out. The wort actually picks up quite a bit of oxygen when I whirlpool and transferred to the 1 gallon carboys. I feel that giving oxygen might actually cover up any off-flavors that might come from differences in pitching rates.

      Having said that, you are right about the oxygen. I think my next experiment will be to add oxygen at different rates to observe the effects.

  3. Hey Jason,

    I work at Brooklyn Homebrew—I was the guy that was wondering if you could bank some East Coast Yeast for him. That offer is still valid—though with the caveat that from time to time I may present you with a yeast slant to inoculate. If you’re still interested let me know, I’ll be at the yeast class. I currently have in my possesstion: Old Newark Ale, Saison Brassiere, Farmhouse Brett and Scottish Heavy from East Coast Yeast…


  4. Jolly Rodger

    I know in this case it works… but I’m kind of saddened that a site called Brew Science is using the Gravity/4 rule of thumb approximate to convert to Plato instead of the full calculation.

    1.070 for example is 17.5P with the “Divide by 4” rule but it’s actually 17.08P

    • Well don’t be too saddened. 🙂

      While yes my blog does portray an accurate scientific presentation of brewing beer, keep in mind that my readers include novice and expert homebrewers alike. Too many specifics and details could potentially confuse some readers.

      Therefore the divide 4 method is a decent approximation and can be found many homebrewing books. But you are correct it is not the most accurate way to convert specific gravity since SG and plato are not linear.

      {Plato/(258.6-([Plato/258.2]*227.1)}+1 = Specific gravity

      This of course takes into account standard temps that are kept constant.

      I will still advocate the the divide by 4 rule. Works for me in a pinch.

  5. Pingback: Pitching Rate Experiment: Part Deux | Brew Science – Homebrewing Blog

  6. Pingback: Pitching Rate Experiment Part Deux: Results | Brew Science – Homebrewing Blog

  7. Alex Weaver Crocker

    I understand wanting to control variables like oxygen but anyone using your experiment as reference will oxygenate their beer. You will not know how over- and under-pitching affect actual beer, only how they affect un-aerated beer.
    Badass experiment though.

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