The Science Behind Home Canning

science-behind-home-canningI recently had a co-worker ask about some home-canned bone broth I’d made up.  I figure I’d share the science behind why some foods need to be pressure canned, and why others can be water bath canned.

In a word:  Botulism.

Just so there’s no confusion, the botulism we’re discussing has two phases, so to speak.  As a spore, and as a toxin.  We care about both of them.

The spores by themselves are harmless.  I can guarantee you that virtually everyone reading this article right now has the spores on them.  They’re in the dirt, in the dust, and all over the place.

As spores, they’re harmless.  Think of them as M&M Candies.  They’ve got a hard outer shell with a treat in the center.  In this case, though, the treat is the toxin.  Put those spores in the right environment, and out come the toxins.

For the spores to release the toxins, all 3 of these conditions must be present:

  1. They must be in a low oxygen (anaerobic) environment.
  2. They must be at a temperature above 38F and below 240F.
  3. They must be in an environment of 4.6 pH or higher (low/non-acidic)

So, with canning or any food preservation method, if we can change any one of those ideal conditions, the toxins will not be released.

Let’s look at them one-by-one.


Since we all come in contact with the spores every day, the reason we aren’t all dying of botulism poisoning is because we live in an oxygen-rich environment.  The air we breath contains about 21% oxygen, so the spores don’t release the toxins even though the temperature is perfect, as is the pH of the environment.

Oxygen keeps the spores inactive.


Let’s say we want to home can some chicken thighs.  The can will become an anaerobic (low oxygen) environment, and the pH will be in the 7 range.  Both of these are perfect for the spores to release the toxins.

Our only solution is heat.  And it’s got to be enough heat.

If we water-bath can the jars, bringing them up to the temperature of boiling water – 212F – the spores won’t be destroyed.  As soon as you cool the jar to room temperature, the spores will become activated and they’ll release the toxins.

We MUST use pressure canning.  The additional pressure in the cooker will bring the temperature inside to over 240F.  This temperature will destroy the spores, thus they won’t be able to release the toxins (because they’ll be dead!).

One of the key “ingredients” in achieving the proper temperature is the altitude at which you do the canning.  Your processing time and pressure remain unchanged if your altitude is 2,000 feet or lower.  Above that altitude, water boils at a lower temperature, and you must increase the pressure inside of your canner.  The pressure must be increased for each additional 1,000 feet you go up!

You can find numbers for High-Altitude Canning in the book that came with your canner, or on the Internet.

To ensure the entire jar is sufficiently heated, different types of food will have different cooking times.  A quart of thin turkey broth may only need to be cooked for 25 minutes, but beef cubes may take an hour and a half for that same quart.

As a side note, the toxin itself is less hearty than the spores.  If the toxin reaches a temperature of 176F or more, it is killed.  For this reason, many people will boil or re-cook canned food just in case the original home canning process was flawed.

Lastly, don’t pack your jars too tightly with food, or can very dense foods (such as very thick stew or gravy).  You must ensure that the temperature of the entire jar – side to side and top to bottom – reaches the desired temperature.  Neglecting to do so might make you ill.


As noted, the spores like a non-acidic environment – like most of the world around us!

Most fruits contain enough acid so you can use a water-bath canning method instead of using a pressure cooker. That’s why when you make jams and jellies, you don’t need to use a pressure cooker.

Most vegetables have low acidity.  So, if we want to water-bath can them, we’ve got to add some acid.  In most cases, that’s vinegar.

Pickles, anyone?

Adding sufficient amounts of vinegar is important.  DO NOT JUST MAKE UP YOUR OWN RECIPES!  Take proven, safe recipes and feel free to adjust the ingredients, but NEVER adjust the type, strength or volume of acid.

A screw-up many people make is to add a vinegar that is less acidic than the type specified in the original recipe (usually substituting 4% vinegar for 5% vinegar, or adding more water than was called for in the original recipe).

Don’t mess around with this!

Acidity is also one of the reasons why babies are not allowed to have raw honey until they’re 1 year old.  The acids in their intestines are not yet strong enough to keep the spores inactive, and the toxin is released.


  • All meat and vegetables must be pressure canned.  Vegetable can be water -bath canned if they’re pickled.
  • Don’t over-pack or lessen the cooking time of your pressure-canned foods, as the temperature at the center of the jar may not have reached sufficiently high temperatures to kill the spores.
  • Fruits and pickled vegetables may be water-bath canned.
  • Don’t change a proven recipe by diluting the strength of the acid bath.

One last thought:  When you open your can of food, be sure you had a good seal and the lid doesn’t easily pull off.  Take a sniff of the contents.  If in any doubt whatsoever, throw out the jar, and get another.

I can tell you that I’ve been home canning for over a decade, and have never gotten sick – because I follow the rules.  You should too!


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