SEPTEMBER 9, 2000: Charlotte, NC-- Believe it or not, as recently as just five years ago most smaller breweries were using dry yeast in packets as their house yeast for all their brews, and only a few breweries with traditional training in microbiology were able to culture yeasts. But with the introduction of high-quality commercial yeast strains in liquid packets and on gelled growth media (malt agar) the brewing culture has grown into its own. Today, professional brewers culture, re-culture and repitch their specialized yeast strains with every batch...

While all this may seem well beyond the capabilities of the average homebrew, you'd be surprised to know how easy it truly is!

The objective of culturing yeast is to isolate a single cell from a beer or culture that has the characteristics desired for the beer style you are brewing, and culture this cell to reproduce enough additional cells to pitch into a new batch of beer.

But before we get into the actual procedure let's answer a basic question: "What exactly is yeast?" Well, yeast belongs to the plant group fungi, which also includes mushrooms and molds. Yeast products fall under the broader definition of direct-fed microbials, or DFMs, and most contain the species Saccharomyces cerevisiae. Put simply, it's a Fungus! Yeast is often referred to as the oldest plant cultivated by man. It's been used for thousands of years to make bread, wine and of course beer.

So what's involved in culturing?

Well, before we begin actually culturing our yeast strains, we should prepare slants that will "house" our yeast. Yeast strains on properly prepared slants can last up to 6 or more months in the fridge, and of course could be recultured indefinitely.


1. Bring 1 cup (about 250 ml) water to a boil. Remove from heat, add 15 grams of dried malt extract, and stir till dissolved. Put back on the heat and boil for 5-10 minutes to ensure sterility. Remove from heat.

2. Pour a packet of gelatin into this "wort" and stir till COMPLETELY dissolved. Now pour this mixture into as many of your pyrex vials/test-tubes as you can; a small funnel is useful for this step. Fill the vials about 1/4 full-- do NOT fill them all the way up. Keep at least one vial empty for use in the next stage--see below. I typically fill about 15-20 vials with these amounts.

3. Now place the pyrex vials mentioned above inside a large pot that has a lid (like your brewkettle). Place your partially filled slant vials in the dish. Here is one place where having the flat-bottomed vials makes life easy--just stand them up in the dish. (If you have test-tubes, you will need a rack to stand them in, and the rack must sit in the pyrex dish. If your rack is the right shape and size, you may be able to omit the pyrex dish.) Put a couple cm of water in there--be sure it doesn't come up over the lip of the pyrex dish. Increase up the heat so that this water boils, and keep it boiling for 10-15 minutes.

4. Now turn off the heat. Once cool enough, put the caps on the vials firmly. You are now out of the woods as far as sanitation is concerned.

5. Now you must cool the vials while placing them at an angle of about 40-45 degrees. When you do this, the surface of the still-liquid-but-cooling gelatin + malt extract will of course stay horizontal. Let the vials sit like this for 24 hours, after which time the gelatin + malt will be as solid as it gets (which is still a bit soft and yielding--ideal for this purpose). After cooling, the surface of the medium is at the angle you propped the box-top up to--that is, it's "slanted", hence the name. These are now ready to be "inoculated" with cells of your favorite yeast.


Next time, we'll look at the next step in our yeast culturing adventure!

Click here to read Yeast Culturing Part II: Innoculation


References:
CULTURING YEAST AND USING SLANTS
by Dave Draper, Sydney, Australia, January 1995

Yeast Culturing Kit Components

Starter tube: A small, capped tube containing 10-15 mL of sterile wort. Used for expanding culture size. Flask: A laboratory-grade (Erlenmeyer) flask, narrow at the mouth but wide at the bottom, usually 1/2 or 1 L in size. Used for expanding culture size. Slant: A small, capped tube containing solidified (gelled) sterile wort. Used for long-term storage of yeast samples. Yeast is often supplied on slants when purchased. Plate: A short, wide, covered dish to be half filled with solidified wort. Often called a petri dish. Used for isolating yeast colonies and short-term storage. Inoculation loop: A metal rod with a wire extending axially from one end. The end of the wire forms a small loop. Used for transferring small amounts of yeast. Growth media: Dry malt extract plus nutrients to feed growing yeast. Many specialized media are available, but this combination is easy for craft brewers to obtain, store, and use.