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:
- Beer style: Blond ale. I chose a blond ale because any off flavors will have little to hide behind.
- 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.
- 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).
- Control pitch: 0.75 million cells/ml/º Plato – 32.9 billion cells.
- Overpitch: 2.5 million cells/ml/º Plato – 112.8 billion cells.
- 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.