cleaning up a yeast strain

I'm working on building up a yeast library (on slants) and am about to brew with some Rochefort yeast. I've read that breweries will, after 6 generations or so, "clean up" the yeast, or re-isolate it.
Does anyone have any information regarding how to go about doing that?
Thanks!

Saccharomycetaceae said:

I'm working on building up a yeast library (on slants) and am about to brew with some Rochefort yeast. I've read that breweries will, after 6 generations or so, "clean up" the yeast, or re-isolate it.
Does anyone have any information regarding how to go about doing that?
Thanks!

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Most breweries keep the pure strain under cryogenic storage these days. I'll assume that isn't an option for you.

I think the method you are shooting for involves diluting a sample of the yeast and streaking it on plate. You then select a colony on the place (grown from a single cell since you used a very dilute sample) that you believe is healthy and is the yeast strain you want and grow that up.

To clean up a strain I would recommend that you first make a plate with healthy yeast. If you do not know what they look like under a microscope then you might not be able to determine which ones have mutated. Here is how I do.

Make a vial mini starter from a slant of healthy yeast and take a vial of 6-10 gen yeast. Streak a couple plates and label them healthy, old, or whatever. Streak old with old yeast, new with new only.

Once the colonies are blooming view them under the microscope and see which ones look the best and closely resemble the healthy plate. Carefully pick off the ones you think are the best with a loop and make a mini batch of beer with it. If you succeeded you can then go on to brew a bigger batch. If not then try again.

Since this is so time consuming you might want to take sample A, B and C and determine which one is best.

Mateo said:

Make a vial mini starter from a slant of healthy yeast and take a vial of 6-10 gen yeast. Streak a couple plates and label them healthy, old, or whatever. Streak old with old yeast, new with new only.

Once the colonies are blooming view them under the microscope and see which ones look the best and closely resemble the healthy plate. Carefully pick off the ones you think are the best with a loop and make a mini batch of beer with it. If you succeeded you can then go on to brew a bigger batch. If not then try again..

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Awesome,
thanks so much for the advice.
One more question though, I was given some agar to work with, but recently noticed that the jar says for bacteriological use only. The yeasts seemed to grow out just fine on the plates, but what type of agar blend have you found to be best? I'm assuming malt extract agar would be ideal...

I use a powdered agar that I mix with wort that I sterilize in an autoclave (pressure cooker) to form the plates.

In place of agar you can use agar-agar or worst case scenario plain gelatin. The agar for bacteria might have a special nutrient added to it. If it works fine you should have no problem, but if you find that it has something toxic to yeast pitch it. Sometime the agar has a toxin added to kill certain things and allow others to grow.

agar-agar can be purchased from oriental food markets.

In place of wort you can use dried malt extract.

When I brew I collect about 8 pints of wort to use for everything else. Starters, yeast work, spiese, etc. DME is expensive. I strain out the trub and autoclave the canning jars.