You need to pitch an adequate amount of yeast to get your fermentation to start in a reasonable amount of time, proceed in an orderly fashion, and reach a reasonable final gravity (given the fermentability of your wort). A pitching rate of 1 million cells per milliliter per degree Plato is frequently cited as the standard rate for ales, although some sources give a lower rate. For a 5-gallon (19 L) batch of beer at 12°Plato (SG 1.048), this would be 228 billion cells. The optimal pitching rate for lagers is often given as twice this, although again lower rates can be found in the professional literature.
To accurately measure the amount of cells, you need a microscope, a special kind of slide called a hemacytometer (designed to count blood cells) and a vital stain (methylene blue). As most homebrewers do not have this equipment, most rely on pitching a given weight or volume of a yeast slurry, pitching yeast from a yeast starter of a given volume, or by pitching multiple packages of commercial yeast based on their cell counts.
For a 5-gallon (19 L) batch of moderate-strength ale, a long-standing rule of thumb has been to pitch a cup of yeast slurry. For homebrewers repitching yeast from the bottom of a fermentation bucket or carboy, this often works well because the density of yeast cells in the slurry immediately after fermentation is relatively low. This yeast sample will be liquid-like and colored with trub and hop debris that settled along with the yeast. If you harvest healthy yeast and let it settle overnight in your refrigerator, about one-third this volume (1⁄3 cup/80 mL) would be satisfactory. Yeast selected this way will be creamy to pasty in consistency. And, since the trub and hop debris will sediment in separate layers, it is relatively easy to use only yeast slurry, which will be off-white in color.
If you are making a yeast starter, you can estimate the amount of cells you will raise from a given volume of starter wort. The density of yeast in a well-aerated yeast starter would vary depending on yeast strain and other variables, but 50 million cells/mL to 100 million cells/mL is not an unreasonable estimate. If you calculate the total number of cells you need to pitch, simply divide this number by the density of your yeast starter to yield the size of the yeast starter (in milliliters). Or, see the table on page 43 for starter sizes for three different pitching rates over various original gravities from 8° to 16°Plato. This website also has a calculator that suggests a suitable yeast starter volume for a given volume and gravity of wort.
A rule of thumb BYO has used in the past is that, for moderate-strength ales, a 2-quart (2 L) yeast starter is optimal. Mr. Malty returns a value of half of this (for yeast starters initially aerated with oxygen), indicating that those calculations are based on slightly different assumptions. In reality, yeast density varies depending on yeast strain, aeration of the medium, nutrient availability, and other things. If you aren’t counting your yeast, you are relying on assumptions you can’t test. In practice, however, beer is fairly forgiving; if you raise a healthy yeast starter and are within the ballpark of the optimal pitching rate, your beer will likely be fine.
If you are using dried yeast, making a yeast starter may be counter-productive. Dried yeast has a high amount of glycogen stored in it, and making a yeast starter (especially if the starter is too small) may deplete that store of glycogen. When using dried yeast, your best option is to rehydrate the yeast in water immediately before pitching. Don’t rehydrate in wort (or any sugary liquid), as this is actually worse for the yeast.
Higher pitching rates generally lead to faster starts, quicker finishes, and higher attenuation. In addition, the amount of yeast character is lower in beers pitched at a high rate. If you are brewing a beer that benefits from some yeast character (esters, etc.), as is the case in most English and Belgian ales, pitching at a less than optimal rate can help accentuate the yeast byproducts as these are mostly formed when the yeast are multiplying, as opposed to when they are at a roughly constant number and fermenting.
Pitching Temperature and Control
When you pitch your yeast, you should take care not to thermally shock them. In general, your yeast should be within 10°F (5°C) of your wort temperature. If you brew lagers and raise your yeast starter at room temperature, cool the starter solution in your fermentation chamber (which should be set to a couple degrees below your planned fermentation temperature).
Once the yeast have been pitched, the main goal of the brewer is to maintain the temperature of the fermentation to produce the best beer. Beer yeasts grow best at temperatures above that which produces a quality beer. Most ale yeasts produce the best beer at 65 to 72°F (18 to 22°C) and most lager yeasts work best at 50 to 55°F (10 to 13°C). In some Belgian ales, fermentation temperatures are allowed to climb much higher than in English ales (up to 85+°F/29+°C).
The most common way to maintain proper fermentation temperature at a homebrew scale is to place the fermenter (bucket, carboy, or stainless-steel fermenter) in an environment that is a few degrees colder than the planned fermentation temperature. Sometimes this simply means placing an ale in a cool spot in the basement. Other times it means placing the fermenter in a fermentation chamber made from a freezer or fridge and an external thermostat. At high kräusen, the environmental temperature may need to be lowered an extra degree or two to keep the desired temperature constant. Likewise, the temperature may need to be raised to the target fermentation temperature near the end of fermentation as yeast activity slows.
Because the beer may not exactly match the surrounding temperature, you should have a temperature probe monitoring the beer itself if possible rather than the environment. A stick-on thermometer affixed to the fermenter is a common way to measure beer temperature as it ferments. These are not very precise, but they are inexpensive and give brewers a good idea of the beer temperature to within a degree or two.
Vigorous fermentations can produce so much kräusen that it rises and pushes out of the fermentation vessel through the airlock. One solution to this is to affix a blow-off tube. A blow-off tube will also remove some of the bitter compounds that get pushed up by the kräusen and cling to the inside of the tube or are expelled into the water lock. If you are brewing a malt-focused beer, this can help you achieve a smoother bitterness. In contrast, if you’re brewing a double IPA, you might not want to lose those compounds. If you expect a vigorous fermentation, choose a fermenter with a headspace volume that will let you retain or blow off the kräusen, according to your desires.
After any fermentation, but especially lager fermentations, the yeast may need to mop up excess diacetyl. Don’t ever rush to separate the beer from the yeast the minute that fermentation is complete. If you are using a diacetyl-prone yeast, don’t rack the beer off the yeast until you’ve sampled it and confirmed that the diacetyl is gone.
At some point every homebrewer faces the dilemma of a dreaded stuck fermentation, which is when the yeast ceases activity before all of the fermentable sugars are converted into alcohol. If you are careful and take steps to keep your yeast happy and healthy, however, you can avoid getting “stuck.”
A stuck fermentation is often the result of one of three common conditions: improper fermentation temperature conditions, unhealthy yeast (or not enough healthy yeast cells), or a lack of oxygen.
Yeast can be fickle under the wrong temperature conditions, and more specifically they don’t like to be too cold or too hot. Yeast suppliers provide temperature guidelines for each of their yeast strains, which are ranges that they have determined in their laboratories as the temperatures that the yeast are able to grow and thrive without going dormant or dying, while producing the best beer. When you’re brewing a batch of homebrew, be sure your fermenter is kept in an area that doesn’t get too cold, which is a common reason for a stuck fermentation. When brewing beer styles that need to be kept on the cooler side, such as lagers, keep a close eye on the temperature inside your fermenter. If your fermentation starts to slow or stop, you can try warming things up a few degrees by moving your fermenter to a warmer area or with an electric heat wrap around the fermenter to get things moving again.
One of the most important steps for brewing any beer should always be pitching enough healthy yeast. Without enough healthy cells, the yeast can struggle and even decide to quit. If you are brewing anything with a higher-than-normal gravity, or anything that needs to ferment at a cool temperature, it’s a good idea to build up a healthy population of yeast a day ahead of pitching with a yeast starter, or at least pitch more liquid or dried yeast than the recipe might call for.
If you have experienced a stuck fermentation, depending on where you are in your fermentation (take measurements with your hydrometer), you can try repitching more yeast. If fermentation stops near the beginning or middle of fermentation, you can pitch another full dose of yeast. If the fermentation stops near the end, try pitching a smaller amount of yeast—about a pint of yeast as a starter. You can also try adding yeast nutrient to be sure the yeast is healthy. Another trick is to kräusen the beer by adding some beer that is in the high kräusen stage of fermentation (36 to 48 hours after pitching for most beers). The rule for kräusening is to add 10 percent of the fermenter volume, or 0.1 part kräusen to 1 part beer.
In addition to temperature constraints, yeast need oxygen. Aerate your wort well before pitching the yeast, which many beginner brewers do by letting the wort splash when transferring it into the fermenter followed by vigorously shaking their fermenter. A more fail-safe method of aeration, however, is to invest in a simple aeration stone setup.
More from Big Book of Homebrewing
- Guide on Closed-System Racking Methods
- How to Clone Beer Recipes
- Advanced Dry Hopping Methods
- Hopping Methods for Homebrewing
Excerpt from Big Book of Homebrewing, by Brew Your Own, published by Voyageur Press. Copyright © 2017 by Quarto Publishing Group USA Inc. All rights reserved.