Wednesday, June 27, 2012

Frozen Yeast Bank - Part 2

Link to Part 1

In part one we covered preparing the vials for the frozen yeast bank with glycerine and a pressure cooker standing in for an autoclave.  Part two will cover getting the yeast into the tube and the freezing process.  The better your procedure here the better the chance that you get a good culture into storage that will survive for a long time.

In this example we'll be using some washed yeast from a fermentation.  Ideally you would want to pull yeast from your starter since it hasn't gone through a fermentation of an actual beer with hops, trub, and greater chances to have infection present.  This is just what happened to be on hand for the example.  If you haven't washed yeast before look it up, it's way easy and will give you options for reusing yeast a few times in shorter windows. 

Washed Yeast from a Cake

More after the jump

 The process is pretty simple, all you need to do is fill the vials we made in part 1 pretty close to the top in the most sterile fashion you can.  Aseptic technique is obviously the best option here as it was primarily designed to handle these kind of organisms.  

Aseptic Technique

A sanitized pipette in an area with low air flow while you kept the containers closed as long as possible would be the minimum here to really even have a good shot.  You'll want to pull a thick slurry of yeast to get as many cells as you can in the vials, open the vials, fill it, then close it tightly as quickly as possible. After it's closed tightly shake it up to suspend the yeast and mix in the glycerine solution. 

After the yeast is in the vial be sure to   label it well (manufacturer, strain, collection date, and generation (how many times you've reused it)).  Then it's into the fridge for a 48 hour chill.  The ideal here is it prepares the yeast for long term storage before freezing and is intended to increase the number of viable cells after freezing.

Filled, Labeled, and Ready for the Fridge

Once the 48 hours in the fridge is up transfer the vials to the freezer. Give them a good shake to resuspend the yeast in the solution.  In the freezer you'll want to isolate the vials from the auto-defrost cycle by storing them in a small cooler full of ice packs inside the freezer.  I use a small soft side free cooler I got at some conference with cheap ice packs.

I like to keep all vials of the same strain in a sandwich bag for easy sorting

Soft Side Cooler with Ice Packs

Now that the yeast is in storage to sit for a while we'll need to have a plant to cover getting it woken up and functioning well again.  This will be covered soon in part 3.

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