Fermentation
What’s the simplest way to improve your homebrew? Well, for most homebrewers, it’s running a good fermentation. One of the biggest factors in this is pitching an adequate amount of yeast. Pitching a single packet of yeast into a 5-gallon (19 L) batch does not give your beer enough yeast cells to efficiently ferment the wort. Beers made from underpitched worts start slower, and this slow start can leave the wort open to the growth of bacteria or wild yeast. Underpitched beers also stop fermenting at higher final gravities, resulting in a beer that may be too sweet. Finally, an underpitched wort may lead to high concentrations of esters and fusel oils, which can yield off-flavors and smells.
How Much Yeast?
A general rule of thumb for pitching ale yeast is that you need one million (1.0 x 106) cells per milliliter of wort per degree Plato. An average-strength ale weighs in at 12 °Plato (1.048 SG). So, for 5 gallons (19 L) of this beer, you would need to pitch about 228 billion (2.28 x 1011) yeast cells. You would need more cells for higher gravity beers or for larger volumes of beer.
Wyeast says its XL packs contain 40–60 billion cells and White Labs says its tubes contain 30–60 billion cells. Using the pitching rule above, 60 billion cells is only enough to pitch to 1.3 gallons of wort. You would need almost four packages of yeast to pitch to a standard five-gallon batch of homebrew. Fortunately, there’s an easy way to get from 60 billion (or fewer) cells to 228 billion (or more) — making a yeast starter.
A yeast starter is simply a small batch of beer. The yeast from this small batch is used to inoculate your main wort. In addition, if you pitch the yeast around the peak of fermentation (called high kraeusen), they will be active and healthy. Some calculations can show you how large a starter you need to raise the required 228 billion yeast cells.
At high kraeusen, yeast density reaches about 100 million cells/mL in a normal-strength beer. So, to raise 228 billion cells, you’d need 2,280 mL — just over two liters — of starter wort. To calculate the starter volume needed for any number of cells, just divide the number of cells required by 100 million (1.0 x 108) cells/mL.
Another rule of thumb relating to pitching is that the size of the starter should be at least 1⁄10 the volume of the wort. Using this rule, a 5-gallon (19-L) batch of beer would need a 0.5 gallon (1.9 L) starter. As you can see, our two estimates of starter volume are pretty close.
These numbers assume your starter wort is of average strength, around 12 °Plato (1.048 SG). They also assume your wort is well-aerated and has all the proper nutrients. And they assume that your wort is at high kraeusen and that your yeast strain of choice actually has a maximum density of 100 million cells/mL. Any deviation from these things may alter your cell count.
Without actually counting yeast cells — a procedure that requires special equipment — you’ll never know your exact cell count. However, the calculations above are a rough guideline for making an adequate starter. Even if your actual cell counts are off by as much as 20%, which is unlikely, you’ll be fine.
Materials Needed
Making a yeast starter for 5 gallons (19 L) of ale requires only the following materials: a two liter (or larger) container with cap, a fermentation airlock for the container, dried malt extract (light, unhopped), a pot and your yeast package. Glass gallon jugs, or many brewpub growlers, can also be used.
Making a Yeast Starter
Measure out enough dried malt extract (DME) to make a wort with a specific gravity of 1.040. Dried malt extract yields 45 gravity points per pound per gallon; in other words, one pound of DME in one gallon of water makes a wort with a specific gravity of 1.045. So, to calculate the amount of DME you need, take your target gravity (in “gravity points”) times the volume of your yeast starter (in gallons) and divide this number by 45. A beer with a specific gravity of 1.040 has 40 gravity points, and two liters equals 0.52 gallons, so we need about 0.46 lb. (0.2 kg) of DME to make a yeast starter for an average-strength ale.
Bring two liters of water to a boil on your stovetop, then turn off the heat. Add the malt extract and stir until completely dissolved. Expect some foaming when you add the extract. Turn the heat back on and boil the starter wort for 15 minutes. Keep the pot partially covered with a lid during the boil. When the boil is over, put the lid on and cool the starter in your sink.
Cool the wort until it is at room temperature or below. This may take 15 minutes or so. To cool the wort as quickly as possible, fill the sink with ice water and place the pot in it. Every minute or so, lift the pot out of the water and swirl the wort gently by moving the pot in a circular motion. Keep the lid on to keep out microorganisms. Swirl the ice water in the sink around and return the pot to the water. If the water is getting warm, replace it with cold water. At room temperature the side of the pot will feel cool to the touch.
During the boil and cooling, clean and sanitize your starter container and fermentation airlock. Remember that this “little batch of beer” is going into your big batch of beer, so pay close attention to cleaning and sanitation. Any contamination of your yeast starter will be magnified in your main batch of beer.
Once the wort is cool, quickly pour the wort into the container and cap the starter. Shake the starter vigorously to aerate the starter wort. Once the foam has subsided, pitch the yeast into the starter. If you keep the starter between 72–80 °F (22–27 °C), it should be ready to use in two days.
Fermenting the Yeast Starter
Treat your yeast starter like you would a batch of beer. Keep the starter between 72–80 °F (22–27 °C) while it’s growing, slightly higher than normal ale temperatures. The yeast will grow quickly and happily in this temperature range. Also, keep it away from bright light.
On brewing day, you can pitch your entire yeast starter or pour off the liquid and only pitch the yeast sediment. Pitching the entire yeast starter ensures that the yeast are active when they enter your wort. Pitching the sediment only is preferred when pitching the whole starter would dilute the color or strength of your beer. Once you’ve pitched your yeast starter, fermentation should start in 6–18 hours. You shouldn’t have any problems with fermentations that wouldn’t start or with “stuck” fermentations when using a yeast starter.
In order to provide the yeast from your starter a healthy environment to grow in, you will need to aerate your wort.
Why Aerate?
Aeration is any process that introduces air into the wort. Before fermentation, chilled wort must be aerated in order to introduce oxygen. Since air is 21% oxygen, aerating wort also oxygenates it. This oxygen is absorbed by the yeast within a few hours and is used to synthesize sterols, molecules that are important to yeast’s health.
A well-aerated wort promotes yeast health. And you can’t have a good fermentation without healthy yeast! Three indications of a good fermentation are minimal lag time, low ester levels and proper attenuation. Keep in mind that the only time you should aerate your wort is after it has been chilled and before the onset of fermentation. Oxygen is detrimental at all other stages of beer production and can contribute stale, cardboard-like flavors and smells. Before fermentation, the oxygen is absorbed by the yeast within a few hours and isn’t in contact with the wort for that long.
How Much?
The amount of oxygen your yeast requires depends on wort gravity and the amount of yeast replication that will occur. Yeast in higher-gravity worts require more oxygen. Yeast’s demand for oxygen increases steadily with gravity until about 1.060. Above this gravity, the need for oxygen increases sharply. Under-pitched worts also need more oxygen. The less yeast you pitch, the more they need to replicate before reaching a density high enough to ferment the wort. Yeast generally need between 4 and 14 ppm of oxygen for a healthy fermentation. Since the vast majority of homebrewers don’t have a dissolved oxygen (DO) meter to measure this with, you must rely on a variety of clues to indirectly gauge if aeration is sufficient.
The best way to assess efficiency is by noting the lag time until fermentation starts, ester levels in the finished beer and the final gravity of the beer. A brew that starts fast, and yields a dry and clean finished beer, received enough oxygen during aeration. A sluggishly fermenting beer that yields a sweet and estery concoction might not have received enough oxygen.
When & What to Aerate?
The oxygen you introduce into your wort isn’t the only source of oxygen for your yeast. Wort oxygen also can come from oxygen in the starter wort and, for extract brewers, oxygen in the dilution water. Also, trub (sediment) can substitute for oxygen.
If you use starter wort, aerate it well. Thorough aeration will lower the amount of oxygen you need to introduce to your main wort. Aerating both the starter wort and the main wort is a good way to satisfy the oxygen demands of your yeast.
Retaining a bit of trub in the bottom of your fermenter decreases the need for oxygen. Trub doesn’t contain oxygen, but it helps yeast with sterol synthesis. Still, homebrewers should attempt to minimize the amount carried over from the kettle as too much trub can cause off-flavors.
The efficiency of aeration depends on wort gravity, wort temperature and aeration techniques. Higher-gravity worts hold less oxygen at saturation. Unfortunately the yeast in higher gravity worts need more oxygen. So, for higher-gravity beers, you should aerate until the wort can’t hold any more oxygen.
How you aerate your wort is also important. The techniques fall into three general classes: shaking, splashing and injecting. When choosing your aeration technique, you should consider your yeast oxygen needs, starter size and the temperature of the wort.
Method One: Shaking
Shaking a container of wort will aerate it. Shaking is not very effective for large volumes of wort, but works great for smaller volumes.
If you use your foot for support, you can rock a carboy back and forth vigorously until you have raised some foam. Be sure to get a good grip on the carboy and work up to speed slowly. A sloshing wort wave can jerk or twist the carboy from your hands.
Though a great way to get exercise, shaking is not the most effective way to aerate a carboy. The top of the wort will pick up air, but the bottom doesn’t get any.
The starter wort can be refrigerated overnight before it is pitched. When it’s cold, a vigorous shaking will aerate the starter wort far beyond what shaking a carboy full of room-temperature wort will. The agitation is greater and the wort will hold more oxygen at the lower temperature.
Method Two: Splashing
You can aerate your chilled wort by splashing it around. This is more effective than shaking. But there are also greater opportunities for the wort to become infected when splashing wort in the open air.
To do this, you’ll need two sanitized buckets and, optionally, a large sanitized kitchen strainer. Pouring wort back and forth between two buckets will aerate it. Get a friend to hold a kitchen strainer above the receiving bucket. Straining the wort will further agitate it.
When splashing, care must be taken to avoid contamination. Don’t transfer the wort between buckets in the same area where you crushed your grain. Grain dust harbors all sorts of bacteria and wild yeast that will spoil your wort. Also, don’t touch the inside of your buckets or the wort itself. Remember to brace the receiving bucket. An empty bucket can tip or slide suddenly when hit with a “wort waterfall.”
Method Three: Injecting
The most effective way to aerate your wort is by injecting air or oxygen directly into it. Injecting minimizes the chances of contamination.
The most common homebrew aeration set-up includes an aquarium pump, tubing with an in-line filter and an aeration stone. An aeration stone can also be connected via tubing to an oxygen tank.
Injecting air or oxygen into wort is simple. First, sterilize the tubing and aeration stone (usually made of a porous stainless steel). As the chilled wort transfers to the fermenter, air (or oxygen) is bubbled through the aeration stone and dissolves into the wort. Continue bubbling air through the stone until a layer of foam covers the wort. This takes 5 to 15 minutes.
The filter takes out dust and microorganisms in the air that could potentially contaminate the wort.
If a high level of aeration is desired, pure oxygen can be used. However, high levels of oxygen can over-stimulate the yeast, resulting in foul-tasting fermentation byproducts.
If you’ve pitched an adequate amount of yeast and aerated your wort properly, all you need to do is ferment your beer.
Fermentation
On brew day, you make your wort. Then you unleash an army of yeast cells to turn the wort into beer. During primary fermentation, your main goal should be to maintain a relatively constant temperature, within the yeast’s recommended range, and let the fermentation proceed without distrubing it. You should also protect your fermenting wort from strong light. Many homebrewers perform a “secondary fermentation” of their beers. Secondary fermentation simply means the beer is racked to another vessel (a secondary fermenter) and allowed to age prior to packaging.
The advantage to secondary fermentation is that the beer is racked off the trub and yeast from the primary fermentation. If the beer remains in contact with these materials for too long, it may pick up off-flavors. A second benefit is that it helps the beer clarify faster. Some homebrewers use secondary fermenters that are smaller than their primary fermenter to minimize the headspace in the secondary.
