Culturing Yeast and Using Slants - Part 3

by Bill Bunning

This is the final part of the three-part "Beer Geek Techneeks" on culturing yeast. We'll deal with obtaining a pure culture from a suspect yeast source. Part 1 dealt with equipment and supplies needed for yeast culturing as well as preparation of blank slants and part 2 dealt with how to slant yeast and prepare starters.

Preparing Plates

The first step is to prepare your petri dishes. You can either purchase plastic plates that you pour yourself or buy pre-poured plates. If you pour your own, prepare them when you make your normal slants; double your medium recipe and place the excess in a mason jar. Boil or pressure cook as described in Part 1. When it's cool enough to handle, pour the liquid into the plates (just a thin layer) quickly a leave to set. Store these wrapped (I use a zip-lock bag) in the fridge until ready to use.

Slanting Suspect Yeast Sources

If culturing from a bottled-beer source, make a small-volume starter and wait until it is fully active. WARNING! Many types of bottle-conditioned beer use either a different kind of yeast in the bottle than they fermented the beer with, or it is the same kind but has mutated, or any of several other possible complications. If you culture from a bottle of commercial beer, *taste* the small starter you make from it when inoculating your slants. If it tastes good (or at least, not bad) you are probably OK. But you should still test the yeast on a small, pilot batch of beer before committing your entire batch to it. Even when using a bottle of your own homebrew, things can happen, so I would recommend these cautions in that case as well.

The steps here are similar to those used in slanting yeast in Part 2. You'll need the prepared plate, the inoculation loop, flaming mechanism, and the small starter you prepared above. Flame the loop from handle to tip. Place in the starter and swirl. Quickly remove the lid from the plate and streak the loop back and forth over one third of the plate. Flame the loop again and streak one third of the plate again overlapping the previous streak. Repeat this procedure one more time streaking the final third of the plate. Replace the lid and store at room temperature. This whole procedure should isolate individual cells of yeast by the third streak. You'll see individual colonies growing in the final area streaked several days later. These colonies are slanted the same as if you were going from slant to slant in Part 2. All the procedures remain the same.

Maintaining Your Yeast Ranch

In no time at all you'll have several different yeasts on slants; I've got over 20! (Editor's note: Bill's refrigerator must not have any beer, or food, in it). Make sure you label each slant with type and date. You'll want to keep track of dates so you'll know when to re-slant a given yeast. Good luck and happy ranching!

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