This is part 2 of the three-part "Beer Geek Techneeks" on culturing yeast. We'll learn how to slant yeast and prepare starters. Part 1 dealt with equipment and supplies needed for yeast culturing as well as preparation of blank slants. Part 3 will deal with obtaining a pure culture from a suspect yeast source.
Inoculating Blank Slants With Yeast
So now you have a bunch of slants. At this point, the procedure depends on what the source is for your yeast to be cultured. You may be starting from another slant or a packet of Wyeast or Yeast Lab starter. I'll describe doing it from a packet of Wyeast, and then comment on variations used for other sources.
The assumption here is that you are culturing from a pure source, like a Wyeast packet or a slant that someone sends you. If you are culturing, say, from a bottle of commercial bottle-conditioned beer, extra steps are required to isolate pure cultures (bottling strains are rarely pure). This is much more involved, and will be described in Part 3. A couple days prior to slanting the yeast, smack the Wyeast packet just as if you were going to prepare a starter. You can use the packet for a starter when your finished making your slant. When the packet is swelled, you should lay out your working area in an organized way to minimize having to get up and down, reach long distances for things, etc. Wash your hands thoroughly and begin. Have your slants, inoculation loop, flaming mechanism, and yeast packet ready to go.
Shake the Wyeast pack well, and then open it using standard procedures. Flame the inoculation loop from the handle to the tip. Stick the tip into the Wyeast packet and swirl it in the liquid. Remove it from the packet and place it near your flame source, but not in it. The air is sterile within several inches of the flame. Open your slant, flame the opening, and smear the loop over the surface of the growth medium. Flame the opening and replace the cap. That's it. Use the remainder of the packet and make up a starter. Make sure you label the slant.
When done, leave the slant out at room temperature for a week. Within a couple of days you will see a cloudy film on the slant surface, and a few days later it will develop into a milky white layer about a mm thick. You'll need to "burp" the slant every other day due to CO2 evolving from the yeast growing on the slant surface. No big deal - just bleed the gas out by cracking open the cap for a moment. After the week is over, store the slant in the fridge, where it will keep for at least 6 months in a perfectly viable condition. I've used them over 1 year old.
From Slant To Slant
When 6 months is nearly over, I reculture the strain by doing the above to another slant, but using an "old" slant as the source instead of a Wyeast packet. Otherwise the procedure is identical. Reculturing in this way does not increment the strain generation-number because the yeast have not made enough copies of themselves for mutation to occur. You can also culture yeast from a friend's slant this way.
Making A Starter From A Slant
Before starting, be sure to let your slant sit out for about an hour so it can slowly come up to room temp from fridge. The yeast will be stepped up in three steps: from slant to 10 ml, from 10 ml to 50-75 ml, and from 50 -75 ml to 500 ml. When making a lager I step up to 1000 ml (and it takes an extra day). Here's where the mini-yeast starters come in handy. They're 10 ml of sterile wort in a vial. If you don't have these, you can make mini-starters in test tubes ahead of time. Again, you'll need the inoculation loop, flaming mechanism, yeast slant, and starter. Flame the loop from handle to tip. Open the slant and flame the opening. Scrape some yeast on to the loop (try cooling the loop on some exposed growth medium first). Plunge the loop into your mini-starter and shake off the yeast. Remove the loop and replace the top.
It will take about 2 days for this yeast to propagate. This is stepped up to 50 -75 ml and then to 500 ml. It only takes about 1 day for each of these steps. I use about 1 tbsp malt extract for the smaller volume and 4 tbsp for the larger volume when preparing my starters. After 4 days, you're ready to pitch.
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