Fundamentals of Pressure Canning – What is it, How does it work and Why do it?

We’ve had a lot of questions around pressure canning in recent months so I thought it was time to get back to the fundamentals and chat about the basics of pressure canning.  Let’s start with a definition:

Pressure canning is a technique of home food preservation that uses special equipment to process food at a higher temperature than ‘normal’ waterbath canning.  It is primarily used to can meat and vegetables without the need for adding high acid (like pickles).

We use a pressure canner (it’s similar but different from a pressure cooker) to can peas, asparagus, stock, soup and more.  The process makes these things shelf-stable so they produce homemade versions of canned vegetables and stocks found in the produce aisle.  The homemade version can be a superior product as you can choose the ingredients (including lowering or emitting salt and avoiding preservatives alltogether).

How does Pressure Canning Work?

Let’s start by discussing how water bath canning works: jars are covered in boiling water and processed for a certain amount of time.  Water can only reach 100 degrees Celsius (212 Fahrenheit); the boiling water raises the temperature of the ingredients within the jar to kill bacteria.  You use this technique for high-acid foods as they are safe to process at these temperatures.

Pressure canning uses less water (the jars aren’t covered) as it traps steam inside a pressurized container to raise temperatures beyond 240 degrees Fahrenheit.  This allows us to preserve food that otherwise would require acid to be added to it.

What are the advantages of Pressure Canning?

There are several advantages to pressure canning, including:

  • It maintains the flavor of the ingredients (peas and asparagus taste almost as fresh as they were canned).
  • It uses less water and less energy (debatable) than water bath canning.
  • It allows you to store items on the shelf without needing the space, budget or energy for a freezer or cold cellar.
  • As mentioned above, you can control the ingredients that are added to your preserved foods.

What are the disadvantages of Pressure Canning?

  • It requires specialized equipment that can be expensive (most canners are around $100) that can also be difficult to store.  I use my pressure canner (without the lid) as my pot for waterbath preserving so it sees lots of use.  We’ll share guidelines on what to look for when buying a canner later this week.
  • It can be intimidating.  The equipment is more technical than most preserving equipment and there’s less people who do it which makes it a little more difficult to learn/ wrap your head around.  Once you do it a few times you’ll find it’s no more difficult that ‘normal’ canning.  If you’ve never used a waterbath (i.e. to preserve jam, pickles or tomato sauce), you may want to start there.
  • The texture changes.  Vegetables will be softer as they are fully cooked (although every preserving method changes the texture of the original product in some way).
  • Canners take maintenance (for example, a dial-gauge canner should be checked yearly for accuracy)

What’s the Basic Process?

I’m skipping a few details here (this isn’t a how-to) but it’s a good general overview:

  1. A few quarts of water is added to the canner and turned on high.  You add enough water to ensure that it won’t run out when boiling during processing but not enough to cover the jars.
  2. Vegetables/ product is heated and added to clean mason jars.
  3. Rims of the jars are wiped clean, lids attached and placed in the canner (generally before boiling as you’ll see in the resource article below).
  4. The lid is secured on the canner and it’s ‘brought up to pressure’ meaning that the water is brought to a full boil and the pressure accumulates within the pot until a certain temperature is met.  Canners let you know that temperature has happened by by a dial or a weight that wiggles in place.
  5. Food is processed for a specific time (which has to be adjusted if you’re at extreme altitude).
  6. Heat is turned off and the canner is allowed to cool before removing the lid and the jars.
  7. Jars are cooled for 24 hours before being placed on a shelf or in a larder.

Other Resources

There’s a great (and detailed) walk-through of pressure canning and pressure canning safety at the National Center for Home Food Preservation.

Closing

What questions do you have about pressure canning?  We’ll be sharing more about it this week as well as featuring a big round-up on pressure canning in the weekly newsletter this weekend (you can sign up here).

Comments

  1. Does the temperature of the water in a pressure canner increase as well as the temperature of the steam? Do the water and steam reach the same temperature inside the canner? It seems like they should, or else the jars would heat unevenly. I made the mistake (a not uncommon one, I would imagine, for those proceeding to pressure canning from waterbath canning) of filling my pressure canner with water above the jars for my first few pressure canning attempts several years ago. I ended up throwing the jars out when I realized my mistake, but now I find myself wondering if the jars actually would have reached the same temperature covered with water as they were, as they would have with a canner filled with just a few inches of water. Any insight?

    • Hi Carter,

      I’ve done the same thing! My words here are more opinion than insight – I don’t know enough to know if it heats up beyond 212 but my guess is that it does not.

      My Father was a firefighter and he would remind me that water was only capable of getting to 212 degrees, not matter how hard your boiled it. After that, it turned to steam. Of course the pressure could change that but I would think it would just make more steam. But, even if it did increase the temperature of the water, it would still be a gamble to eat. A canner with a tonne of water would still produce steam and reach pressure shortly after boiling (the limited space for steam to fill would fill quickly). The steam would be much hotter than the water (it would heat slower than the steam which releases as soon as it hits 212) so the time it would take to get the entire kettle of water to 240 would be difficult to know/ control/ tell.

      Now I want to google a bunch of things! :)

      j

  2. jacqui baiardo says:

    Please remember to let those venturing in to pressure canning that the canner must be allowed to let steam off before you put the pressure weight in place. Most recipes will say to let the steam escape through the vent pipe for 10-15 minutes PRIOR to placing the weight in place and adjusting to the required pressure. The process can be intimidating, but is so worth it. You do get accustomed to the whole process after just a few tries. My husband and I fish for trout much of the summer and the freezer will only hold so many frozen trout. I pressure can the fish in 8oz jars and use it just as I would canned tuna or salmon!

Leave a Reply