|
|
|
 |
|
|||
|
|
|
|||||
|
|
||||||
|
|
|
|
|
|
|
|
|||
|
|
|
|||||
|
|
||||||
|
|
|
Yeast: Scale Up and Pitching Rates
by David Allaben, 1997
In order to have a healthy and optimal fermentation, it is necessary to pitch the correct amount of yeast. Pitching rates not only affect fermentation, but beer flavor as well. Pitching is governed by a number of factors including wort gravity, wort constituents, fermentation temperature, degree of wort aeration, yeast strain, previous yeast history and proper attenuation and fermentation characteristics. Pitching the correct amount of yeast ensures a minimum lag period and rapid fermentation. As a result, wort pH drops quickly which decreases possible bacterial growth. In general, pitching rates vary from ten to twenty-five million cells per milliliter or one million cells per degree Plato. Ten million cells per milliliter is considered an optimal level because it results in many yeast strains reproducing four to five times. The number of yeast cells pitched per milliliter is usually increased in proportion to an increase in wort gravity.
Many small scale brewers pitch yeast by weight or volume. The standard rule is one pound of yeast slurry per U.S. barrel (0.5 ounces per gallon or about 14 grams per gallon). This rule is inaccurate because yeast slurries can vary in their consistency from thin to thick. Although it depends on volume, a thin yeast slurry often has a low yeast cell concentration. A thick yeast slurry may appear to have a high concentration of yeast cells, but it may be primarily composed of trub and other particulate matter. Therefore, the correct pitching rate, either by weight or volume, requires calculated dilution of the yeast slurry based on an estimate of cell concentration. In other words, it is essential to know the ratio of dead cells to live cells in a yeast slurry. Ideally, pitching rates are based upon the concentration of live yeast cells. Estimating the viability of cell population is accomplished using methylene blue staining by slide culture or plate counting. A concentration of approximately ten million live cells per milliliter in 12 degree Plato (S.G. 1.047) wort is normal.
Most liquid and dry yeasts available to homebrewers need to be scaled up. Propagation should be done in batches of increasing size until the correct amount of pitching yeast is obtained for fermentation. Yeast reproduction is performed at slightly higher temperatures than the intended fermentation temperature. This helps ensure proper beer flavor besides accelerating growth of new yeast. Also, it is best to scale up yeast in starters similar to the actual wort to be fermented as a final product. This will prevent high yeast mortality rates which will impede slow and weak fermentations. In addition, there will be fewer dead yeast cells to influence or taint beer flavor. Typical wort gravities for scale up range from 10-16 degrees Plato (S.G. 1.039-1.062).
Depending on the various characteristics of a particular strain of yeast, the addition of yeast nutrients may be beneficial for healthy growth. Also, during stages of scale up, intermittent aeration must be used to stimulate yeast growth. Agitating the yeast slurry is usually unnecessary because evolving carbon dioxide maintains the yeast in suspension. A good starting point would be to pitch 50 ml. of yeast slurry into 100 ml. of sterile wort contained in a 200 ml. flask. Repeat by pitching into 250 ml. of sterile wort in a 500 ml. flask and finally pitching into 500 ml. of sterile wort in a 1000 ml. flask. This will result in approximately one liter of yeast slurry. For most 5 gallons (18.9 liters) wort fermentations, for gravities between 12 and 15 degrees Plato (S.G. 1.047-1.058), pitching about one liter of yeast slurry is usually adequate. Higher gravity fermentations will require larger volumes of pitching yeast. On the other hand, pitching too much yeast will create a supply and demand problem with available oxygen, sugar and nutrients. As a result, yeast autolysis may occur which affects beer flavor in a negative manner.
In conclusion, optimal pitching rates require knowing actual live yeast cell concentration in a slurry. The most inexpensive and easiest method homebrewers can use to determine yeast cell concentration in a slurry, is to use a microscope and hemacytometer. A microscope is an expensive investment. On the other hand, a hemacytometer kit can be purchased from a hospital supply company for a fairly reasonable cost of $45.00 to $55.00. Investing in expensive equipment, in order to know the exact concentration of yeast cells in a slurry, is not necessary to obtain healthy fermentations. Remembering to scale up the yeast to about one liter, for pitching into five gallons of wort, will usually ensure a short lag period and vigorous fermentation with proper wort aeration.
Bibliography:
- Allen, Fall, "The Microbrewery Laboratory Manual: Part 1: Yeast Management," Brewing Techniques, Vol. 2, No. 4, July/August 1994.
- Briggs, D. E., Hough, J. S., Stevens, R. and Young, T. W. (1981). Malting and Brewing Science,Volume 2: Hopped Wort and Beer. St. Edmundsbury Ltd. Great Britain.
- Burch, Byron (1992). Brewing Quality Beers. Joby Books. Fulton.
- Hardwick, William ed. (1995). Handbook of Brewing. Marcel Dekker, Inc. New York.
- Forget, Carl (1988). Dictionary of Beer and Brewing. Brewers Publications. Boulder.
- Noonan, Gregory (1986). Brewing Lager Beer. Brewers Publications. Boulder.