Most of the time, making a yeast starter is going to improve your batch of homebrew. In some cases, you can get away with pitching one homebrew-sized package of yeast, presuming the yeast is fresh. Earlier, I posted a chart of likely outcomes when a single liquid yeast package — containing around 100 billion cells (i.e. White Labs tubes and Wyeast XL smack packs) — is pitched. Today, I’ve included a chart for dried yeast sachets, which have somewhat higher cell count. However, there is a difference between getting away with something, and having something work optimally. And running the best possible fermentation is necessary if your goal is to brew the best possible beer. In this article, I’ll explain what I believe to be the best way to make a yeast starter.
Let’s start with the things that most knowledgeable brewers would agree on. Firstly, use fresh malt extract (or malt). Using stale malt extract will definitely add color to your beer, even if you only pitch the yeast sediment. The yeast will be surrounded by amber beer and this can darken pale beers. It can also add stale flavors and aromas to your main batch of beer. Adding hops is optional, but likely won’t make any discernible difference in the finished beer unless you hop your American Pilsner starter as if it were a double IPA. And obviously, use water that tastes good and is potable, and treat it for chloramines, just as you would your brewing liquor for a full batch of beer.
Use a pitching rate calculator for an estimate of how much starter wort you’ll need. If you want to get more character from a yeast strain, you can lower the volume of the starter and ferment the main batch of beer towards the high end of its fermentation range. Lowering the volume of the yeast starter to about three-quarters of it’s optimal volume is a good place to start when experimenting with this. Even if you halve the volume, the yeast only need to replicate once to reach the optimal pitching density.
If you want to the strain to ferment cleaner than it normally would, increase the volume of the starter and ferment the main batch in the cooler end of the yeast’s range. Doubling the starter size will clean up almost any yeast noticeably.
Make low-gravity starter wort to raise the yeast. The exact specific gravity is not important, only that it be low enough not to stress the yeast. Anything in the SG 1.020 to 1.040 range will work fine, especially if you aerate starters in the high end of this range very well.
Ferment the starter warmer than you will ferment the beer. The idea behind a yeast starter is to quickly raise a sufficient amount of yeast. The starter beer will be discarded (except for the volume that the yeast sediment is suspended in), so if turns out overly estery, it is not a problem. Fermenting yeast starters at 75–80 °F (24–27 °C) works well for most ale strains. Occasionally swirling the starter, or having it stirred continually on a stir plate, will increase your cell counts.
You want the yeast you raise to be as healthy as possible, so aerate the starter well (more on that below) and add a pinch of yeast nutrients. Don’t overdo the nutrient addition — just add the recommended dosage for a normal batch of beer, or slightly more.
The Best Way
As I mentioned in the first part of this article, you really only want one thing from a yeast starter — yeast. But, there are plenty of things you don’t want. These include off flavors, off odors, and unwanted color. Most of all, you do not want the yeast starter to be a source of contamination for your main batch of beer. Avoiding this is what guides my recommendations for making the best yeast starter.
There are a variety of ways to make a yeast starters, but you can separate them into categories. In one category, you boil the starter wort, cool it and transfer it to the starter vessel, then pitch the yeast. In the other category, you boil the starter wort in the starter vessel, cool the wort and pitch the yeast. I’ve done it both ways, but strongly prefer the second way as it lowers the chances of contaminating your starter wort by eliminating one transfer step.
If you use a clean Erlenmeyer flask or media bottle made of borosilicate glass, you can heat and cool your starter wort in your starter vessel. They will stand up to being heated on your stove (use low or medium low heat; it’s all you’ll need). Or, they can be heated in a microwave. Remember that hot glass looks exactly like cool glass and wear heavy thermal gloves when handling a hot flask. Borosilicate glass can be plunged into a ice water batch to cool the contents of the flask or bottle. This should yield sanitary wort, and you don’t need to transfer it to another vessel.
Regular flint glass will shatter if heated and cooled suddenly. If you have a starter vessel that can withstand having just boiled wort into it and also survive cooling, that would be a good option. The heat from the wort should minimize any chance of contamination during the transfer. In any case, the wort must be cool before pitching the yeast.
When cooling the starter vessel, reduce contamination from airborne bacteria by fitting the opening with a foam stopper. As you cool the starter, the volume of air above the wort will cool and shrink in volume. This will pull air into the vessel. Having a foam stopper in place will reduce (slightly, it’s nowhere near a sterile filter) the amount of airborne bacteria pulled into that space. Insert the foam stopper before you heat the wort and cover the starter’s opening with aluminum foil. Sterile cotton can take the place the foam stopper. Or, you can put a couple thicknesses of clean paper towel over the starter vessel, cover that with aluminum foil, and hold the cover in place with rubber bands. At a minimum, cover the opening with aluminum foil.
There are several ways you can aerate a starter vessel. If you can cap it (with a sanitized cap), you can shake the vessel to force air into the wort. The best way, however, is to inject oxygen with an aeration stone. If you shake the starter to aerate it, you force whatever airborne bacteria are in the vessel’s headspace into the wort. (The same goes if you use a whisk or similar methods.) If you inject air oxygen that has been pushed through a sterile HEPA filter, you do not force any bacteria into the starter wort. In fact, the positive pressure from the incoming gas will displace some of the air in the flask, lowering the level of airborne contaminants (although to what degree is an open question). This, of course, assumes the aeration stone and tubing is sanitary. I always boil mine before using it. In addition, when you open the vessel to insert the aeration stone, you are briefly exposing it to outside air. Replace the aluminum foil — and foam stopper, if you’re using it — as quickly as possible before you turn on the air or gas.
Pitching the Yeast
Once you have sanitary wort that has been aerated, you need to pitch your yeast. You should seek to pitch as soon after aeration as possible, as gas will diffuse out of solution over time. Obviously, you don’t want to undo everything you’ve done so far by contaminating the starter when pitching the yeast. One way to protect against this is to swab the yeast container with sanitizing solution, swab the exterior rim of the starter vessel with sanitizing solution (or flame it, if you have a gas stove or butane lighter), then make the transfer as quickly as possible. I find it helpful to shake the tube or smack pack immediately before the transfer to get all the yeast in solution. Once the transfer is complete, quickly re-close the starter container with a (sanitized) stopper airlock (or the equivalent). The less time you leave the starter vessel open, the less chance airborne contaminants have to fall into it.
So basically, my recommendations are based on minimizing the amount of contamination to your starter. If you boil the wort in the starter vessel, you save the step of transferring wort from the vessel it was boiled in to the starter vessel. If you sanitize the outside surfaces of the starter vessel (around the rim) and yeast package, you minimize the chance of contamination from surface bacteria. And finally, if you work quickly — minimizing the amount of the starter wort is exposed to air — you limit the amount of airborne contamination.
If you aren’t set up to make a yeast starter this way, don’t worry. It is better to make a yeast starter than not, even if you have to chill the starter wort and pour it into a gallon jug (or whatever you’re using for a starter vessel). It is better to make a yeast starter than not, even if you have to aerate the starter by shaking. It is better to make a yeast starter than not, even if you can’t make the full volume recommended by a pitching rate calculator. (Try to make at least half that amount, though.)