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Ales & Lagers
The Math
Pitching From Tubes, Packs, or Dry Yeast
Starters and Stir Plates
Repitching Yeast
There is quite a lot of information and discussion on the web and in books about proper yeast pitching rates. Maybe it is the wealth of information that makes it so confusing, but I run into many people that are completely confused about how much yeast they should pitch. Hopefully, the following won't just add more confusion, but instead it will make it a little easier to understand the math behind pitching rates and what is really required.
Ales & Lagers
The general consensus on pitching rates is that you want to pitch around 1 million cells of viable yeast, for every milliliter of wort, for every degree plato. A little less for an ale, a little more for a lager. George Fix states about 1.5 million for a lager and 0.75 million for an ale in his book, An Analysis of Brewing Techniques. Other literature cites a slightly higher amount. I'm going with Fix's numbers and that is what the pitching calculator uses.
The Math
If you're curious, here is the simple math to calculate the number of cells needed. For an ale, you want to pitch around 0.75 million cells of viable yeast (0.75 million for an ale, 1.5 million for a lager), for every milliliter of wort, for every degree plato.
(0.75 million) X (milliliters of wort) X (degrees Plato of the wort)
- There is about 3785 milliliters in a gallon. There are about 20,000 milliliters in 5.25 gallons
- A degree Plato is about 1.004 of original gravity. Just divide the OG by 4 to get Plato (e.g., 1.048 is 12 degrees Plato).
So, for a 1.048 wort pitching into 5.25 gallons you need about 180 billion cells.
(750,000) X (20,000) X (12) = 180,000,000,000
As an easy to remember rough estimate, you need about 15 billion cells for each degree Plato or about 4 billion cells for each point of OG when pitching into a little over 5 gallons of wort. If you want a quick way of doing a back of the envelope estimate, that is really close to 0.75 billion cells for each point of gravity per gallon of wort. Double that to 1.5 billion for a lager
Pitching From Tubes, Packs, or Dry Yeast
Both White Labs and Wyeast make fantastic products and you can't go wrong with either one. There are differences between their strains and each brand has pluses and minuses yet neither is better than the other across the board. Use the brand your local homebrew shop carries, if you need a way to decide.
A White Labs tube has between 70 and 120 billion cells of 100% viable yeast, depending on the yeast strain. Some cells are much larger than others and there are more or less per ml based on size. (The information on the White Labs web site stating 30 to 50 billion cells is out of date.) We can just assume there are around 100 billion very healthy yeast. You would need 2 tubes if you were pitching directly into 5.5 gallons of 1.048 wort to get the proper cell counts.
A Wyeast Activator pack (the really big ones) and the pitchable tubes have an average of 100 billion cells of 100% viable yeast. The smaller packs are around 15-18 billion cells. You would need 2 of the large packs if you were pitching directly into 5.5 gallons of 1.048 wort to get the proper cell counts. For the small packs, you'd need eleven of them!
Some exciting work has been done on dry yeast lately. Reports are coming in of better quality, cleaner dry yeast. Personally, I really prefer the liquid yeasts, but the lure of dry yeast is strong. The biggest benefit is that it is cheap and does not require a starter. In fact, with most dry yeasts, placing them in a starter would just deplete the reserves that the yeast manufacturer worked so hard to build into the yeast. Most dry yeast has an average cell density of 20 billion cells per gram. You would need about 9.5 grams of dry yeast if you were pitching into 5.5 gallons of 1.048 wort to get the proper cell counts. (Recently there have been other numbers mentioned for cells/gram of dry yeast and folks have asked me why I believe there are 20 billion cells. I've actually done cell counts on dry yeast and they're always 20 billion per gram +/- less than a billion. Dr. Clayton Cone has also stated that there are 20 billion per gram, and other folks I trust tell me that 20 billion is correct. Until I see something different, practical experience tells me this number is correct.) For dry yeasts, just do a proper rehydration in tap water, do not do a starter.
Starters and Stir Plates
One way to get the proper number of active yeast cells ready to pitch is to make a starter for your yeast. The little bit of nutrient in a Wyeast smack pack does not take the place of a starter. It is there to jump start the culture's metabolism and to act as a built-in viability test.
Starters are easy to make, especially with an Erlenmeyer flask made of borosilicate glass (Pyrex, Bomex, etc.). When making starter wort, make sure you keep the OG around 1.030 to 1.040. You do not want to make a high gravity starter to grow yeast. As a ballpark measurement, use about 6 ounces (by weight) of DME to 2 quarts of water. If you're working in metric, it couldn't be easier. Use a 10 to 1 ratio. Add 1 gram of DME for every 10 ml of final volume. (If you're making a 2 liter of starter add 200 grams of DME to the flask, then fill the flask with water until you have 2 liters total.) Add the DME and water in the Erlenmeyer flask, put a piece of aluminum foil over the top, drop in a pinch of any nutrients you desire, and put it directly on the stove burner. Boil gently for 15 minutes, then let it cool. If you have O2 handy, you should add O2 to your starter or at the very least shake it every few hours to increase the amount of O2 available to the yeast. If you have a stir plate, that works even better at providing good gas exchange and yeast growth will increase (around 2 times as much yeast as a non-stirred starter). Pitch your yeast into the starter and let it grow.
You'll get more yeast growth the warmer your keep the starter (up to 90F +), but yeast viability at these higher temperatures start to suffer. For lager yeast, this can be a real problem in creating more petite mutants and causing poor flocculation. In general, keep the temperature of the starter in the mid 70s Fahrenheit for ale yeast and a few degrees lower for lager yeast.
It is best to keep your starter around the general temperature you're going to pitch, if you're going to pitch the complete starter into the beer. According to Dr. Clayton Cone, one of the foremost experts on yeast, the yeast should be within 15F of the wort they're being pitched into. Neva Parker, the Laboratory Manger at White Labs, suggests a maximum swing of 10F and ideally 5F. I agree with Neva 100%. Besides shocking and stressing the yeast, pitching warm yeast into a cool wort can cause many of the yeast to produce petite mutants, which will never grow or ferment properly and they can produce excessive H2S. You can add small amounts of the cool wort to the starter, to bring the temperature down gradually, but it is really better to keep everything at fermentation temperatures or below. Any time yeast senses a big drop in temperature, it tends to slow down and drop out. Any time yeast senses a warm up in temperature, it tends to get more active. Which do you think is better for your beer?
If you run your starters warmer, make sure you let them ferment out fully, let the yeast drop to the bottom, chill to the appropriate temperature, then pitch just the yeast.
Wyeast says starters reach their maximum cell density within 12-18 hours, which sounds about right to me. Some sources suggest that maximum cell densities are not achieved until 24 hours and others as much as 36, but the return on waiting that long is minimal. Let's just say that the bulk of the yeast growth is done by 12-18 hours. I like to pitch starters while they're still very active and as soon as the bulk of reproduction is finished, usually within 8 to 18 hours. This is really convenient, because I can make a starter the morning of the brew day or the night before the brew day and it is ready to go by the time the batch of wort is ready. There is no need to make a starter a week in advance, because I pitch the whole starter, liquid and all (up to a certain size of starter). Yes, you can wait longer and completely ferment it out so you don't have to pitch the liquid, but if you're going to do that, you should use a larger starter and allow the fermentation to go complete cycle over several days, chill, decant the beer and pitch just the yeast. If you're making a smaller starter, it is better to just pitch the entire active starter within about 6 to 12 hours of pitching the yeast into the starter.
I've read statements from both White Labs and Wyeast on starters and how much growth you'll get. On the White Labs web site (09/07/2005) it states, "If a starter is made from a fresh vial, one vial can be added directly to a 2 liter starter, which in 2 days will grow to approximately 240 billion cells."
Greg Doss, Microbiologist/Brewer, with Wyeast Laboratories stated on the AHA TechTalk (08/31/2005) that approximately the following was true:
Pitching a Wyeast Activator pack gives the following results within 12 to 18 hours:
- 1 liter starter = about 150 billion cells
- 2 liter starter = about 200 billion cells
- 1 liter starter, then pitched into 4 liter starter = 400 billion cells
Practical experience with both products seems to indicate that these statements are at least in the ballpark and that good results can be had as long as a starter is made. However, one would think that you could go direct to 4 liters and get pretty close to the same 400 billion without doing the first 1 liter step. If you want lots of yeast, go 2 liters on the first step, then 20 liters (about 5 gallons) and repitch the yeast from that. Why don't 1 liter starters produce more yeast, even with a stir plate? There just isn't enough food to build that much yeast, no matter how much O2 and nutrient you add to it.
Repitching Yeast
If you've brewed more than one batch, I'm sure you've noticed that there is a huge pile of yeast in the fermenter at the end. If (and that is a big 'if') you've got excellent sanitation all the way through the process and have provided proper yeast nutrition (including O2), you have a gold mine of healthy yeast ready to reuse. Of course, you don't want to reuse the whole thing. I know a number of people dump a new batch on top of the yeast cake, but you're not going to get the best beer that way. Yeast do need some growth to result in the right kind of ester profile, etc. While too big a pitch is better than too little, it is pretty easy to figure out how much you need and pitch just that.
There are about 4.5 billion yeast cells in 1 milliliter of yeast solids (solids with no excess liquid). According to Fix, in a slurry, only about 25% of the mass is yeast solids. Of course, if there is a lot of trub in there, you have an even lower percentage of yeast solids. The bad thing is that you can't tell how viable that yeast is, unless you have the equipment to properly test and count it. So this is where it gets a little bit like black magic. There are a number of factors that affect the viability of a given pitch of yeast. How old is the yeast? How stressful was their last fermentation? Have they had the proper environment and nutrients for successful reproduction or are they too scarred and tired to go on?
When the yeast is fresh and healthy off an previous batch, viability is maybe around 90%+. It goes down from there fairly quickly without proper storage and it also really depends on the strain of yeast. Unless you're going to get into testing viability, you're going to need to make some educated guesses and keep good notes on the results. This is where being a yeast psychic really helps. Start in a range of 80 to 90% viability and you probably won't be too far off. Use the Pitching Rate CalculatorTM to help figure out how much of that yeast you need. If your yeast viability is much lower than 90%, you should probably toss the yeast. If you really want to use it, you might consider pitching it in some starter wort to get the still viable cells active. When they're in solution, decant that active part of the starter into another vessel, hopefully leaving the dead cells behind.
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