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How to Sterilize Autoclavable Materials at Home using a Pressure Cooker

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This Instructable will cover the materials and methods necessary to sterilize things at home using a pressure cooker as an autoclave.  I have a huge Tuttnauer autoclave at work that I use all the time but sometimes when I’m at home I feel like sterilizing things that I use for various advanced home beer-brewing techniques.  If you want to sterilize autoclavable things at home as well, then you should obtain a pressure cooker.  I use a Fagor Rapid Express because that is the one that I was given at my wedding.    
 
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Step 1: Read your pressure cooker instruction manual (safety first!)

Read the pressure cooker instruction manual thoroughly.  The manual for my model can be found here: http://www.fagoramerica.com/content/download/8799/59673/file/U40A01911.%20Rapid%20Express%20Manual%202008.pdf You are dealing with super-heated steam inside of a pressure vessel.  You and you alone are the one who is ultimately responsible for your safety and the safety of those around you.  (Special Note: The manual for my pressure cooker explicitly states that the pressure cooker is not to be used on a propane burner and that it is for stovetop-use only.  Your manual might provide different instructions about this and many other things, so it is important that you know how to operate the unit that you own.)  I use my pressure cooker indoors, on my stovetop, because that is where I can prepare a semblance of a sterile environment.

Step 2: Don't try to sterilize things for home medical/surgical purposes

My pressure cooker manual explicitly states “Pressure cookers are not to be used for medical purposes, such as sterilizers, as pressure cookers are not designed to reach the temperatures necessary for complete sterilization.”  If you use your pressure cooker for medical purposes you are taking your life in your own hands.  I have years of laboratory experience behind me and I am familiar with many laboratory testing protocols not covered in this Instructable.  Use common sense and don’t try to get your PhD in Surgery over the interwebs.

Step 3: Craft the wire mesh bottom for the pressure cooker autoclave

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In order to use your pressure cooker as an autoclave you must craft a wire standoff (similar to a cooling rack) out of steel or similar metal wire.  I made a small wire mesh out of galvanized steel recycled from an old hanging basket I got from the garage.  I made it at Tech Shop (techshop.ws) in about a minute with some wire snips, and I made another one in about a minute at home to show you how easy it can be.  Your design can be varied, but this material just has to be able to hold your autoclave contents up off the bottom of the pressure cooker pot where you will have water (according to manufacturer’s instructions).  My manual says to use 2 cups minimum when you are cooking for 10 minutes or more.  This is because the steam escapes the pressure cooker and you want enough water in there to generate steam for the desired length of time.

Step 4: Alternatively you can use a steamer basket and support trivet, or make one!

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If your pressure cooker came with one, you can use a steamer basket instead of the wire stand.  After I made my first wire metal basket contraption, I found the Fagor steamer basket that came with my unit.  This is actually ideal, since the basket is designed explicitly to hold the contents of the basket off the bottom of the pressure cooker pot.  (If you have the Fagor design, make sure that you have the ‘support trivet’ underneath the basket in order to hold it off the water.)  Your pressure cooker uses steam, which is formed when water reaches its boiling point.  The steam is what is doing the sterilization, and therefore anything sitting in the water at the bottom of the pressure pot is not being sterilized.  

Step 5: Learn what you are doing before you begin!

Learn the basics of autoclaving before you begin.  Know what materials you can and cannot autoclave (this is not covered in a lot of detail here).  Autoclaving is typically meant to sterilize laboratory glassware, metal equipment and instruments, and to kill organisms in biological waste streams.  It achieves this by saturating the contents with steam at a high pressure.  The amount of time the contents are exposed to this high pressure steam environment is also critical.  At 15 PSI, my pressure cooker produces steam at 121 degrees Celsius.  At this temperature and pressure, thermal death time for most bacteria is about 15 minutes, although some microorganisms will be capable of surviving this treatment (most likely this is the reason why Fagor doesn’t want you to run your own surgical suite out of your apartment).

Step 6: Safety first when you use your pressure cooker!

At home, the only reason I would autoclave something is if I wanted to culture pure yeast from an unfiltered beer and needed to sterilize culture medium, equipment, and wort for yeast starter cultures, etc. (this will be covered in a follow-up Instructable).  For my home autoclave purposes I use laboratory-grade Pyrex (borosilicate glass, available at Amazon) and stainless-steel materials only.  Sometimes I will wrap utensils or the open tops of glassware in aluminum foil, as aluminum foil is also autoclavable and really handy for autoclaving purposes both at home and in the lab.  Mason jars (Ball) can also be used in much the same way they are used for pressure canning.  Just know that you should NEVER screw down the lid of any jar inside of an autoclave or pressure cooker.  If you autoclave regular glass you also run the risk of having said glass explode (glass can shatter this way when heated and cooled rapidly).  This is also a reason why I won’t use Pyrex bakeware that isn’t laboratory grade.  Pyrex bakeware is made of cheaper soma-lime glass: cheaper for them to make but also more prone to shattering when heated and cooled rapidly.  When you use mason jars or home-grade Pyrex just realize that this risk is there.  I have never had a problem with this but I always cool my autoclave down extremely slowly.

Step 7: Cook with pressure

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Now let’s actually use the pressure cooker.  According to my manual, I placed two cups of water in the bottom of the pot, then the support trivet and steamer basket, followed by a mason jar (filled halfway with water and granulated ‘sugar-in-the-raw’), lid and band, and stainless steel spoon wrapped in aluminum foil (instrument proxy).  I put the lid on and pressure cooked according to the manufacturer’s instructions for 15 minutes using the two-burner method (electric stove).

Step 8: Wait patiently and drink a beer

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You should not leave the pressure cooker unattended.  A watched pressure cooker will in fact boil, and hopefully not explode since you take care of your pressure cooker lid, seal, and emergency release valve according to manufacturer’s instructions.  During your 15-minute wait you should enjoy a delicious Michigan-made craft beer.  Here I have chosen Bell’s Oarsman Ale.  Cheers!

Step 9: Release the pressure safely and slowly

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After the 15 minute sterilization time is up you should allow the pressure cooker to release the pressure by the slowest method possible.  I used the natural release method of pressure release as indicated in my Fagor instruction manual.  I let the pressure cooker sit off the burner on the stovetop for an hour and a half (closed to maintain a sterile environment inside) to let it come back to room temperature as slowly as I could so as to not have a problem with the glass mason jar.

Step 10: Drink more beer while you wait

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During your 1.5 hour wait you should pour yourself a delicious home-brewed beer.  Here I have chosen one of my personal favorites: a Nägerbomb Nelson Sauvin Double IPA.

Step 11: Your instrument and jar is now sterile

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Now you have sterilized instruments and liquids.  Store them where they will not be disturbed (be creative).  To be sure your pressure cooker autoclave is working properly you should spore test it after 10 hours of use.  

In a follow-up Instructable I will show you how you can use the skills learned from autoclaving spoons and sugar water to autoclave lab-grade Pyrex petri dishes and solid culture medium for yeast and bacteria culture.  I will walk you through the process of culturing a pure strain of brewer’s yeast from an unfiltered/unpasteurized bottle of Michigan craft beer, for use in your own homebrew experiments.  Also, check out Tech Shop!!  This place rocks!!!
Jaepheth6 months ago

What's with the jar of urine? Brewing some Bud Light?

thebobr.8 months ago

so i'm waiting for that petri dish version :)

NonDisjunction (author)  thebobr.7 months ago
This one?: http://www.instructables.com/id/How-to-make-solid-culture-medium-plates-for-yeast-/

Or the one where I try to teach sterile/axenic techniques for yeast culture? Because if that's the one I referred to in the "solid culture medium" Instructable, then it is in the works in the next few weeks or so. I got a bottle of "Mischief" from 'The Bruery' today, which is a low-alcohol, unpasteurized, unfiltered beer made only with their house Belgian strain, so I figured it would be an awesome one to show people how to culture from.