Pitching by Numbers

It will be difficult indeed to win a contest if you are pitching yeast straight from the smack pack or dry-yeast packet. Yeast is thought by many commercial brewers to be the single most contributing factor to beer quality. A long lag phase and yeast degradation are two big problems associated with underpitching. The long lag phase can lead to contamination because it gives bacteria a chance to multiply. Yeast degradation occurs because underpitched yeast must regenerate itself several times. This extra multiplication will cause the yeast to be taxed, weak, and not suitable for re-harvest. Other side effects are increased diacetyl levels and other unwanted flavor components. With a little knowledge it is quite easy to pitch adequate amounts of yeast.

Dry vs. Liquid   

Dry yeast is a reasonable alternative to liquid yeast for making generic ales. Dry yeast is relatively inexpensive, and most packs have enough yeast so a yeast starter is not absolutely required, although it is recommended. Dry yeast fermentations generally have a relatively short lag phase, partly due to higher average pitching rates and also because they have already acquired oxygen prior to being pitched. The biggest problem with dry yeast is the number of beer styles that can be brewed.

Many commercial brewers and homebrewers have produced good beer using dry yeast. Liquid yeast has the advantage of being from a single strain, and it is less likely to be contaminated with wild yeast or bacteria. Additionally, there are endless varieties available on slants and in smack packs, which allow the brewer to create a vast array of styles much more accurately. Liquid yeast is relatively expensive and must be propagated prior to use. Because these cultures must be propagated to pitch sufficient yeast, the smack packs don’t have a huge advantage over slants. Although it is uncommon, liquid cultures may also be contaminated from time to time.

By the Numbers   

It is generally accepted by commercial brewers that the optimum pitching rate for beer is around 1 million cells for every milliliter of wort, multiplied by the gravity of the wort in degrees Plato. There are 18,931 milliliters in a five-gallon batch of beer. The number of cells needed for average-gravity beer of around 12° Plato would be calculated as: 1 million x 12 x 18,931 = 227,174,127,391

Or the amount of yeast needed is about 1 billion cells per degree of specific gravity multiplied by gallons of wort. Thus, cells needed for a five-gallon batch at 1.050 specific gravity would be calculated as follows: 1 billion x 50 x 5 = 250 billion.  Double that amount to around 500 billion for your favorite barleywine or imperial stout.  There are two ways to get enough yeast to pitch the optimum amount. One, get yeast from previous fermentations or two, propagate your culture until you have an adequate amount. Yeast in a starter will grow to roughly 50 million cells per milliliter of wort or 1.5 billion per ounce of wort no matter what the starting number of cells. To calculate your pitching rate, multiply the volume of your starter by the number of cells stated above.  A 64-ounce starter will yield: 64 x 1.5 billion = 96 billion.  A 500 ml starter will grow to: 500 x 50 million = 25 billion

According to this formula you will need a one-gallon starter to get enough cells for a five-gallon batch. The rule of thumb is a five-to-one increase when going from your starter to your fermenter — a one-gallon starter for five gallons or a three-gallon starter for a 15-gallon batch.

Yeast slurry collected (har­vested) from the end of a previous batch’s fermentation has about 1 billion cells per milliliter, or 30 billion per ounce. Calculate your pitching rate as follows: Ounces of slurry x 30 billion.  To calculate the number of cells in a cup of slurry: 8 x 30 billion = 240 billion.  As you can see from these numbers, a cup of slurry is the right amount to use on a 1.048 beer.

The average 50-milliliter smack pack of yeast has roughly 2.5 billion to 4 billion cells once it has reached high kraeusen (the peak of fermentation). This is far below optimum for a five-gallon batch of beer. Dry yeast has approximately 14 billion cells of viable yeast per gram. Dry yeast packs vary in size from five to 14 grams.

A 10-gram pack means pitching: 14 billion x 10 = 140 billion.  This is not bad, about half the optimum amount. The 100 milliliter generic ale and lager packs that will grow to twice as much yeast as the standard packs are also available. Slants of five to 35 milliliters are also available by mail and at your local homebrew shop.

Pitching the optimum amounts is easy for commercial breweries because they have plenty of fresh, viable yeast around. They also have significant capabilities for propa­gating yeast. But it may not be completely practical for home­brewers to pitch in these great quantities. Quality beer can be produced with less yeast. Always pitch as much yeast as you practically can. Pitching rates of one-fourth to one-half of optimum (one-half is a one-quart starter or one-half cup of slurry) will still give excellent results.

Practical Propagation Tips

  1. When propagating yeast, use tenfold increases. A 50-milliliter smack pack should be pitched into 500 milliliters of wort. That 500 milliliters should be put into 5,000 milliliters, and so on until you have enough. For slants a sterile wire loop should be used to inoculate a 35 milliliter test tube and then to 350 milliliters and so on.
  2. Use a flask twice as big as you need to accommodate the volume. This allows the maximum wort-surface-to-air ratio.
  3. Shake the starter well to introduce oxygen.
  4. Try to use wort with roughly the same gravity as the beer you are planning to make.
  5. For lagers slowly cool your propagation temperature from one propagation to the next until your final starter is the same temperature at which you intend to ferment the batch.
  6. A great deal of yeast is in suspension. Let the starter sit a few extra days while most of the yeast settles out. Then decant all but the last bit of beer. Swirl that beer around and pour it into your main batch.
  7. Don’t forget to torch the mouth of the flask when pouring from one starter to the next.

Harvesting and Handling    

If you want to use a yeast slurry from a previous batch, the yeast can be harvested from the top or from the bottom of the primary fermenter. Many commercial microbreweries harvest from the bottom. They generally transfer the yeast with a hose from one fermenter to the next.

At Midnight Sun Brewing Co. in Anchorage, Alaska, yeast is har­vested from the top. The feeling is that the yeast is in its peak state while it is on top of the beer prior to falling back into solution and settling to the bottom. Further, this allows harvesting of only yeast and not other trub (a layer of sediment consisting of proteins, oils, and tannins that have settled out during chilling). Top harvesting is a traditional and age-old method. Although few breweries in America top harvest, it is a fairly common practice in Europe. Top harvesting, according to some experts, has a higher risk of contamination than an enclosed hose transfer.

Remember, as in any other brewing practice, sanitation is paramount when harvesting yeast. Thoroughly sanitize any instruments that may be used to harvest.  To bottom harvest wait until fermentation is completed. This should be no more than a week for ales and two weeks for lagers. Then carefully rack the beer off the yeast. If you are using an open vessel, scoop the best looking yeast from the bottom of the fermenter, but be careful to avoid any trub. (Not an easy thing to do!) If you are using a carboy, slosh the last bit of beer around to get the yeast into solution, torch the mouth, then dump the solution into a container or another fermenter.

Commercial breweries have enough of a yeast depth (three feet or more) on the bottom of their fermenters that they are able to select only the middle layer of yeast. This layer holds the healthiest yeast and very little trub. Unfortunately for homebrewers, the depth is too thin to distinguish practically. So when you harvest from the bottom, you will inevitably pick up trub along with the yeast. This is not a major concern but is certainly not ideal.

When top harvesting, timing is critical. As fermentation nears completion, yeast will begin to form a thick layer on top of the beer. This layer will become very dense as fermentation is completed. You must harvest the yeast immediately at this stage as the yeast will begin to drop back to the bottom after another day or two. Skim the yeast off the top and place it in a sanitized container.

A note about top harvesting: Some ale strains do not settle to the top and are therefore not suitable for top harvesting. One such strain is Wyeast 1084 (Irish ale) strain. When this strain reaches terminal gravity it goes straight to the bottom. Also, lager strains cannot typically be harvested from the top.

After the Harvest   

The ideal situation is that the yeast you harvest goes straight into another batch. Yeast starts to go downhill fast, and pitching the yeast immediately is a huge part of successful yeast management. This may not be practical for one brewer, but a group of brewers may be able to help each other out with this type of program.

Things to consider when repitching: How many times can yeast be repitched? There is no right answer to that question. Yeast can, in theory, be regenerated indefinitely, and some commercial breweries do that. However most commercial brewers don’t go much more than 10 generations. Keep in mind that most commercial breweries have the capability to monitor their yeast for bacterial levels, wild yeast, and overall viability. Also, some strains are hardier than others and therefore are better suited for multiple generations.

One simple method of assuring that your yeast has not been compromised is to learn its behaviors. Every yeast strain has a unique set of behaviors. If these behaviors start to change, it is time to reculture. These behaviors include smell (during and after
fermentation), taste, speed and level of attenuation (how fast and how fully the beer ferments), and overall vigor of fermentation. Any changes should be noted. For instance wild yeast contamination may cause a more vigorous fermentation than normal.  These are some of the things to monitor, but basically you need to get to know your yeast and then you will be able to tell if it is functioning normally. If you do see something strange, it might be time to reculture.

As a rule of thumb for most homebrewers, if strict sanitation procedures are followed incon­junction with these recommendations, then five generations could be achieved; much more than five would be risky.

Yeast Storage   

  1. While yeast ideally should be used immediately after harvest, it is possible to store it. Here are a few guidelines:
  2. Store the yeast as close to 32° F as possible.
  3. Store it in a container that will let pressure escape; a small amount of fermentation will continue.
  4. Cover the yeast with beer. The alcohol and low pH of the beer will help keep bacteria away.
  5. Don’t store yeast for more than a week. In general 10 percent to 25 percent of yeast will die every week, and the vitality of those that survive will drop considerably.
  6. Warm yeast slowly after storing to avoid shock.

Notes and Disclaimers   

Keep in mind that each additional step in the brewing process provides another opportunity for contamination to occur. Think about that when harvesting and growing yeast. Consider whether you have the know-how, equipment, and time to do yeast starters. The great strength of smack packs is that there is little possibility of contamination. This should be weighed against the possible gains of harvesting your own yeast.

The yeast cell numbers stated here are averages, and conditions and strains may have slightly different cell densities. The only sure way is to get a microscope and count your cells prior to pitching, but this is typically not practical for homebrewers.  Like other brewing advice, this information was compiled as a tool for gaining more consistency and quality in your brewing. If you are happy with your beer, then don’t change a thing.

Issue: August 1998