Making a Yeast Starter
What’s the simplest way to improve your homebrew? Well, for most homebrewers, it’s increasing their yeast pitching rate. Pitching a single packet of yeast into a five-gallon 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 microorganisms such as 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. If everything else has gone right, these flaws may not be overpowering — and thousands of homebrewers brew decent beer this way — but pitching the correct amount of yeast can turn a so-so beer into a suh-weet one. The first step towards pitching the right amount of yeast is to know how much yeast you need.
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. I calculated that number like this: (1.0 x 106 cells/mL/° Plato) x (12° Plato) x (19,000 mL) = 2.28 x 1011 cells. You would need more cells for higher gravity beers for larger volumes of beer. See the table below for recommended starter sizes for a variety of worts.
Wyeast says its 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. (This number assumes your yeast package isn’t more than a couple weeks old and hasn’t been mishandled at any point. Cell counts decrease with time and mishandling. Not refrigerating the yeast will result in low cell counts.) 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 to raising the correct amount of yeast cells, a starter also ensures that your yeast cells are healthy. If you pitch the yeast around the peak of fermentation, also called high kraeusen, they will be in good shape. Some quick back-of-the-envelope 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. For example, five gallons of barleywine at 24° Plato (1.096 SG) would require 456 billion cells. For this you’d need (4.56 x 1011) ÷ (1.0 x 108) = 4,560 mL (about four and half liters).
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 five-gallon (~19 L) batch of beer would need a 1.9 L starter. As you can see, our two estimates of starter volume are pretty close. When I brew a five-gallon batch, I split the difference and make a two-liter starter.
You should take the above numbers with a grain of salt, perhaps even an entire salt lick. As the car commercials say, your mileage may vary. 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 at high kraeusen. Any deviation from these things, or a host of other things, may alter your cell count.
Without actually counting yeast cells — a procedure that requires a microscope, a special dye (methylene blue) and a piece of equipment called a hemacytometer — you’ll never know your exact cell count. You will also never know what percentage of your cells are alive and what percentage are dead. 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 five gallons of ale requires only the following materials: a two liter (or larger) container with cap, a fermentation lock for the container, dried malt extract (light, unhopped), a pot and your yeast package. The new three liter soda bottles, made of PET plastic, work well as a yeast starter container. A #6.5 stopper — the stopper that fits a standard carboy — also fits the three liter bottle. Glass gallon jugs can also be used, although for most you’ll need a smaller size stopper.
Making a Yeast Starter
Measure out enough dried malt extract (DME) to make a wort with a specific gravity of 1.048. Dried malt extract yields 45 gravity points per pound per gallon; in other words, one pound of DME in a 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 1.048 has 48 gravity points, and two liters equals 0.52 gallons, so we need [(48 points x 0.52 gallons) ÷ 45 points/lb./gallon = ] 0.55 lb. DME. So you will need about a half a pound 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. If you’d like your starter to have hops in it, just like your beer will, add a few hop pellets. Keep the pot partially covered with a lid during the boil. When the boil is over, put the lid on and cool the starter wort in your sink.
Cool the wort until it is at room temperature or below. This may take 15 minutes or so. Don’t cut corners on this step or you may negate all your efforts by killing your yeast. 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 with your hand 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. You can also use a sterilized thermometer to measure the temperature of the wort. The wort should ideally be under 80° F, but over 65° F, before pitching.
During the boil and cooling, clean and sanitize your starter container and fermentation lock. 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. A sterilized funnel can be used if you have troubles pouring. Quickly 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, it should be ready to use in two days.
The yeast starter’s fermentation may not be as vigorous as a normal beer fermentation. Remember that a fresh package of commercial yeast has enough cells to ferment 1.3 gallons of beer, over twice the volume of a two-liter starter. To reach high kraeusen, the yeast will only have to replicate roughly two times. In a normal batch of beer, the yeast must divide three or more times to reach high kraeusen. Thus, the vigor of your starter fermentation may be less than you expect given your previous experience with beer fermentations. This is especially true of smaller yeast starters.
Some optional things to do when making a yeast starter are adding yeast nutrients or aerating with pure oxygen. If done properly, either will increase your cell counts. There are many different yeast nutrients available (White Labs and Wyeast have their own kinds, for example). Follow the manufacturer’s directions and don’t overdose your yeast on nutrients. Likewise, if you aerate with oxygen, be careful not to overdo it. An easy way to aerate effectively without using pure oxygen is to refrigerate your yeast starter overnight, then shake vigorously when it’s cold. More oxygen will dissolve in cold liquids than warm, so you’ll get a little benefit from the refrigeration. When you’re ready to pitch to your starter, place the starter up in a sink full of warm water to quickly heat it to room temperature.
Fermenting the Yeast Starter
Treat your yeast starter like you would a batch of beer. Keep it away from bright light, even if you didn’t add hops. Avoiding skunking isn’t the reason for shielding a yeast starter from light. Ultraviolet radiation from sunlight causes mutations in laboratory yeast, and this affects their growth. Keep the starter between 72–80° F while it’s growing, slightly higher than normal ale temperatures. The yeast will grow quickly and happily in this temperature range.
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. (For more on this, see “The Great Homebrew Debates” in the January 2001 issue.) For high-gravity beers, pitch only the yeast sediment from the starter.
Once you’ve pitched your yeast starter, your fermentation should start in 6–24 hours. Variation in start times depends on the number of healthy yeast cells in the wort, the level of wort aeration, yeast strain and wort temperature. Once I began making yeast starters, I never had any more problems with fermentations that wouldn’t start or “stuck” fermentations.
