Frozen Yeast Bank - Part 1

To go along with the last post on yeast starters I thought it would be a good idea to introduce the frozen yeast bank.  This is not only a good way to always have plenty of yeast strains on hand but is also a very good way to save money if you brew quite often.  A standard liquid yeast strain is around $8.00 USD and even then those should normally be grown up in a starter anyway.  This method allows you to purchase one of those liquid cultures and use it many times.  As a side benefit you can keep limited edition strains that are sometimes released on hand when they are no longer commercially available.  Lets start with the equipment required. Flyguy's Homebrew Talk Frozen Yeast Bank Thread thread is where I got a ton of information on how to do this and it's a great read.


Equipment:
-Small glass vials with a good sealing top that can be frozen and stand up to the heat of a pressure cooker (cynmar lab product 115-27910 is what I selected)
-Some pipettes (I got mine on amazon.com), the ones I purchased were graduated plastic 4ml
-Some food grade glycerine (also amazon.com)

Glass Vials from cynmar and the glycerine from amazon

-Something to hold the vials when they are in the pressure cooker, I used a beaker for most of these but you can also use several glass mason (canning) jars.

Vials staged in a beaker

-A pressure cooker, 15psi so that you can get up to 250°F is recommended. Read the instructions and follow them as these things can be very dangerous if used incorrectly.

-A small cooler and ice packs to go in your freezer.  A soft side small lunchbox sized cooler is a great option. This is to keep the yeast temperature stable during the auto defrost cycles most freezers have. (covered in part 2)

Optional (i.e. cheaper) Version - During each step in the process I'll  highlight a cheaper alternative if possible.  It likely won't be as clean (relatively speaking it will be the difference between sanitized and sterilized) but should work with the caveat that you will have a slightly higher chances of something going wrong due to an infection.

Procedure after the jump:




Procedure

Step 1 - place glycerin in the vials measured to approx. 10-20% of final volume to be placed in the vial before freezing.  I'm using 24ml vials and planning to leave about 4 ml of head space for expansion when freezing which leaves 20ml to be placed in the vial.  so I'll be placing 2-4ml of glycerin in each vial to be sterilized.

Placing Glycerin in the vials with my handy 4ml graduated pipette

All vials staged with a measured dose of Glycerin

Cheaper Option - Instead of the glass vials simply get some smaller centrifuge vials from amazon or save your old yeast vials if you're using white labs.  I've also heard you can purchase 2L soda bottles that haven't been expanded yet that are very similar to the white labs containers (search for baby soda bottle).  Going this route simply Star San (or some other no rinse sanitizer) the vials and boil/cool your glycerin with some water to sanitize it.  Then measure out to the vials and you're ready for yeast.  Word of warning that boiling temperatures can't kill all wild yeast spores but will get most of them, same goes for Star San.  Your odds are actually pretty good here but as always you can do better.

Step 2 - After the vials all have their dose of glycerin you'll want to loosely put the lids on each vial so that they can pressurize/depressurize during the sterilization process.  Then place the vials in some beakers to keep them upright during the sterilization process. 

Vial with the cap loosely on and glycerin dose in the bottom

Vials in Beakers and Mason Jars ready for sterilization

Cheaper Option - If you don't already have beakers on hand and don't want to spend the extra money to order them pick up some mason jars that are made for canning at the grocery (see the far left container in the last image).  


Step 3- Get your pressure cooker ready per it's instructions, please read them and follow them.  While pressure cookers are very safe if used correctly there can be great consequences if used incorrectly.  Standard autoclave temperatures are around 252 degrees F and a pressure cooker at 15psi should be around 250 degrees so it's almost equivalent.  This is the magic temperature to kill pretty much anything that might be on the glass or in the glycerin.  You'll want to come up to temperature/pressure by following the instructions for your cooker (many require some amount of venting before placing the weight on the outlet fitting) and then start timing.  You'll want 15 minutes at temp/pressure so start your timer, if it drops below that for some reason during the process restart the timer.

Vials in the Pressure Cooker (Top) and the pressure cooker steaming away (Bottom)

After your 15 minutes is up just kill the heat on your pressure cooker and let it sit until there's no pressure left.  This is an important part of the process as it keeps temps higher than 212 degrees F for quite a while as the pressure cooker cools down and the pressure lessens.  Quick cooling by running water over the outside of the cooker or pulling the weight off the outlet valve can not only hurt your sterilization efforts but can be dangerous/damage your cooker.  Cooling usually takes 1-2 hours depending on ambient conditions.

Step 4 - Remove the vials and tighten the caps.  After the cooker has cooled open it up and remove the vials one at a time tightening the cap on each as you remove it.  They are now sterilized and sealed so they should store for a very long time if everything went well.  I place these in the fridge for use later whenever I make a starter of a new strain or the last vial of a strain in the bank.

That's it for making the storage vials, part 2 will cover placing yeast in the prepared vials and the freezing and storage process.   

Update: Part 2 Is Up 6/27/12