Fermenting and Conditioning
Running a healthy fermentation depends on pitching an appropriate amount of healthy yeast. Beyond that, creating proper wort conditions (with respect to aeration and nutrition) and controlling your fermentation temperatures are the keys to success. How you treat your yeast is a huge factor in determining beer quality.
In this stage of brewing, the brewer must create the proper conditions that allow the brewers yeast to transform the hopped wort into beer. After fermentation, brewers may also need to set the correct conditions for the yeast to clean up some molecules produced during fermentation (esp. diacetyl).
Trub Carryover
At the end of the boil, wort has a lot of coagulated protein and tannin in it. The large, fluffy bits floating in the otherwise clear wort are called hot break. While the wort is being cooled, more protein and tannins will come out of solution. This material, called cold break, is composed of smaller clumps and gives the wort a cloudy appearance, Collectively, this — along with other solids formed in the boil — ends up at the bottom of your kettle and is called trub. You can separate this material from your wort before fermentation, but should you?
Hot and cold break can be left behind in the kettle by chilling the wort and allowing it to settle before racking the wort off the top of it and into the fermenter. Likewise, if you are using a hop jack, the hot break can be screened out prior to chilling with a counterflow or plate chiller. Some homebrewers rack their chilled wort first to a temporary holding vessel (bucket or carboy), and then later to their fermenter. This will allow all the break material — whether hot and cold break, or just cold break — to be separated from the wort. Getting rid of every last bit of break material does yield clear wort. In addition, if you were to transfer all or most of the break material to your fermenter, it would add off flavors to your beer. However, studies have shown that a small amount of trub carryover contributes to yeast health. Trub contains zinc and other nutrients as well as unsaturated fatty acids that may partially decrease the amount of aeration required. It may also serve as a nucleation site for CO2, which would lower the concentration of dissolved CO2 in your fermenting beer and in that way lead to increased fermentation vigor. As a homebrewer, your best bet is to proceed as if you planned to separate out all the trub, but then let just enough break material through to slightly cloud the wort. If you chill your wort in your kettle, let the wort sit until the break material is almost completely settled, then transfer the clear wort to your fermenter. At the end of the transfer, let a little bit of cloudy wort transfer.
If you use a counterflow chiller, let the wort sit and settle in a temporary vessel before transferring it to your fermenter. Allow enough of the cold break to transfer to very slightly cloud your wort. If you ferment in a cylindro-conical fermenter, let the cold wort sit in the fermenter for an hour or so, then dump the trub out the bottom before proceeding with aeration and pitching.
Aeration
Once your wort is chilled, it is time to aerate it. For some homebrewers, this is done simultaneously. If you have a counterflow or plate chiller with an aeration stone placed on the wort outflow side, you can aerate your wort as it flows into your fermenter. Or, as is more common, you can aerate the wort using an aeration stone and bottled oxygen or an aquarium pump. Frequently, brewers place a HEPA filter between the stone and the source of air or oxygen to ensure no microorganisms enter the wort through their aeration efforts.
Aeration is an important step, but unfortunately homebrewers do not have an inexpensive way to measure oxygen levels in wort. Dissolved Oxygen (DO) meters have come down significantly in price in recent years, but are still more expensive than the grade of pH meters most homebrewers use. Optimally, wort should be aerated so that it contains between 8 and 10 parts per million (ppm) dissolved oxygen.
Unfortunately, there is no simple way to describe how to reach a given level of aeration in a batch of wort. You can stipulate that the wort be aerated for a certain amount of time, but this doesn’t account for the flow rate of the gas or the size of the holes in the aeration stone. Even if you could measure the volume of gas pumped into the wort, you have no easy way to determine how much has been retained. This depends on a few variables, including temperature. Gas dissolves more slowly the colder the wort is (although the capacity to hold gas goes up with lower temperatures) whereas swirling the fermenter while aerating increases gas diffusion.
Still, some basic guidelines can be given, and homebrewers can infer if they work or not by observing their fermentations. Generally, with a stainless steel airstone, 1 to 2 minutes of oxygenation — during which a constant cloud of tiny bubbles is coming from the airstone — should be enough to aerate a batch of beer. Likewise, 5 to 10 minutes of air (for example, pushed by a fish tank aeration pump) should get you to the minimal required level. It is possible (albeit unlikely, if you are following “normal” aeration procedures) to overaerate a batch using oxygen, but not with air.
Homebrewers need to take care to monitor their fermentations early on to see if the yeast have received adequate aeration. Adequately aerated ales should start fermenting within 24 hours. Lagers should start fermenting within 36 hours. The amount of time until a fermentation starts also depends on the yeast strain, pitching rate, wort temperature and level of wort nutrients, and therefore can vary quite a bit. Start times can be much sooner than the times given earlier, especially with healthy yeast at higher pitching rates. Incidentally, after aerating your wort, gas will begin diffusing out of solution and back into the atmosphere. For this reason, have your yeast ready to pitch immediately after aeration.
Yeast Nutrition
As well as having enough oxygen, yeast also need a healthy amount of nitrogen, vitamins and minerals. For most all-malt beers, this is not a problem. If a wort is deficient in anything, it is likely zinc. Adding commercial yeast nutrients — at a rate ranging from half to the full manufacturer’s recommended rate — during the boil will almost always solve the problem.
Pitching Rates
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 mL 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 or 19,000-mL) 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 50,000,000 cells/mL to 100,000,000 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 mL). Or, see the table on page 41 for starter sizes for three different pitching rates over various original gravities from 8 °Plato to 16 °Plato. The website mrmalty.com 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 qt. (2 L) yeast starter is optimal. Mrmalty 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 will 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. If your yeast is healthy and your sanitation is adequate, cutting the pitching rate in half (or even to one quarter) of the optimal rate can be done without too much risk. At half the optimal pitching rate, the yeast only have to multiply once to reach the same density as if they were pitched at the optimal rate.
Pitching Temperature
When you pitch your yeast, you should take care not to thermally shock them. In general, your pitching 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). Yeast are a little more forgiving when they are cold and pitched into a warm wort.
Temperature Control
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 65–72 °F (18–22 °C) and most lager yeasts work best at 50–55 °F (10–13 °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. Some times 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. 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 are inexpensive and give brewers a good idea of the beer temperature to within a degree or two.
Blow Off Tubes
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 blowoff tube. A blowoff 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.
Clean Up
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.
Aerating with Olive Oil
The fundamentals of brewing stay fairly constant, but brewing technology is always changing. Brewers constantly try new ways to make brewing better, faster or more economical. New ideas are constantly filtering into the homebrewing community and sometimes it’s fun to try out a new concept.
One such new idea arose several years ago — “aerating” your wort with olive oil. Wort aeration benefits brewers yeast because they take in oxygen and use it for sterol synthesis and unsaturated fatty acid synthesis. The idea behind olive oil aeration is, instead of the yeast taking in oxygen and converting it into these compounds, why not supply the yeast with them directly? Not only would the yeast be healthy, but the use of oxygen could be lessened or eliminated, reducing the risk of staling in finished beer. Olive oil is rich in oelic acid, a fatty acid that yeast could use or possibly also convert into other unsaturated fatty acids.
Grady Hull (of New Belgium Brewing Co, Fort Collins, Colorado) experimented with this idea and found that by adding 1 mg of olive oil per 25 billion cells in the yeast propagator, 5 hours prior to pitching the yeast, he could get unaerated wort to ferment in a similar time frame as aerated wort and produce beer that did not taste different than beer made from aerated wort.
A 2 qt. (2-L) yeast starter would contain about 250 billion cells, so you would only need 10 mg of olive oil (a tiny amount) to do this at home. Measuring that amount would be very difficult, and you don’t want to add too much as oil has a negative effect on head retention. What some homebrewers have done is dipped a sanitized needle into olive oil and swished that into their yeast starter. Shake well to disperse the oil as best you can and allow the starter to work as you normally would.
