The Science of Canning

Brain Food 101 is a Savvy Eats series in which you may not be familiar – here is a brief overview. Brain Food 101 posts make the science behind food easy to understand and accessible to everyone. I recently completed my undergraduate degree at the University of Wisconsin-Madison, where I received a degree in Biological Systems Engineering – Food and Bioprocessing. Essentially, my curriculum was half Chemical Engineering classes and half Food Science classes. Though I decided to pursue writing rather than a technical engineering career, I can still put my education to good use by writing Brain Food 101!

Brain Food 101: The Science of Canning

At the beginning of the summer, I wanted to give canning a try, but I hesitated. I was afraid that I would somehow poison myself and Dan by doing it wrong. I’ve heard from several others that this is a concern, and I’ve done a lot of reading/research on the science end of it. I am much more confident now that my canned goods are going to be safe for my pantry. Phew!



As frightening as it may seem, microorganisms such as yeasts, molds and bacteria are naturally present in all foods. A natural level of these microorganisms will not harm us, but when left unchecked, these yeasts, molds and bacteria can grow, change, and multiply to poisonous or even toxic levels. A less dangerous form of spoilage is quality loss. The canned food may be off-color or lose flavor. Before using any canned food, check for these signs of spoilage. If any are present, throw it out!

  • Bad odors
  • Lid that bulges out
  • Lid that comes off easily/doesn’t have a tight seal
  • Mold
  • Air bubbles moving towards the top (stationary air bubbles are fine…you just didn’t get them all out before you sealed the jar!)

The Three Factors

There are three things that can cause canned goods to spoil:

  1. Microorganisms can grow.
  2. Quality of food can drop because of enzyme activity. Enzymes are proteins that speed up chemical reactions, and occur naturally in all fruits and vegetables.
  3. Recontamination due to outside air getting into the jars.

Beating the Big Three

Luckily, we can prevent all three of these factors from becoming an issue by properly canning our food. For all canned vegetables that aren’t being pickled, a pressure cooker is absolutely necessary to insure safe food. For now, let’s focus on acidic foods such as pickles, jams, and fruit preserves. Do NOT use this science to can carrots, green beans, etc.

Savvy Tip: These methods will be safe for canned tomato products, as long as you add some lemon juice or other acid. Tomatoes are right on the border of foods safe to can using the boiling-water method, so make sure you add the amount of acid called for in the tomato recipe, and you’ll be good to go!

Most canned foods contain enough water to support microorganism growth, but by heating the food and adjusting the oxygen content and pH (acidity) of the food, we can keep problems at bay. Heating Heating food to 212*F (the boiling point of water) will kill most molds and yeasts, as well as some bacteria. It will also deactivate most of the enzymes that would otherwise hurt the quality of the food.

Savvy Tip: Older canning recipes may instruct you to simply invert the jars, rather than processing them in boiling water. However, this is no longer considered safe by modern food safety standards! As a result, a boiling-water canner should be used to process all jars of fruit preserves and pickles.


Boiling-water canning

pH Adjustment Even with heating the food, some bacteria and spores can still flourish in your jars. And we don’t want that, now do we? To remedy, make sure that your food has enough acid in it to prevent this growth, which puts its pH below 4.6. Though you could get some pH testing strips, the best and most reliable way to make sure your recipe is acidic enough is to base it off of a tested and approved recipe found in a cookbook such as the Ball Complete Book of Home Preserving or my personal favorite,Canning for a New Generation by Liana Krissoff. For safe modifications you can make to a recipe, check out my first post on canning. Vacuum Seal The final and most important part of the canning process to prevent unsafe food is creating a vacuum seal. Here’s how it works (and why you MUST leave some space between the food and the lid when you fill the jars): When the jars are processed in a boiling-water canner, the heat causes the food to expand (which is why you need head space!). This creates a pressure that pushes air out of the jar.


As the jar cools back down to room temperature, the pressure inside the jar drops. Eventually, the pressure outside of the jar is higher than that inside the jar, which pushes the lid in. This is when the seal really forms.Because of the vacuum, there is no oxygen for the microorganisms to use for growth. And because of the seal, no outside air can get in to recontaminate the food.

Savvy Tip: Jars need to be left largely undisturbed for 24 hours after processing. However, after an hour of cooling, press down on the center of the lid. If there is any movement at all, such as the edges flexing upwards, the seal did not form and the jar should be refrigerated immediately! Otherwise, leave the jars alone for the rest of the day!

So to sum it all up:

Picture 30.png

Note: These are not ALL of the factors that go into whether or not a home-canned recipe is safe.  You must also consider things such as water activity and density.  Only use tested canning recipes when preserving your own food.


  1. says

    I’ve never done canning by putting the cans in a big bowl of boiling water. I use the inverted jar method for jam and it’s worked so far – although I’m not canning them for a long period of time. For pickles I just heated the jars before filling them with hot pickle. They have always sunk in by themselves. It’s interesting how procedures and recommendations change over time and from country to country.

    • Julie @savvyeats says

      I do all my canning per the USDA recommendations, but I know procedures vary by country!

  2. says

    Thanks for sharing this great information! I went to visit my family this weekend and came home with pounds of jalapenos and banana peppers – I’m going to try to pickle and can them today…this was really helpful!

  3. Kelley says

    Wow! It was like I was back in Food Sci Class. Except for this time, I paid attention the whole time 😉

    Maybe it was because you weren’t sitting next to me in class or we didn’t have our laptops in front of us to Gchat?? 🙂


  4. Erin T. says

    Yes! So, as long as I maintain acidity at or below 4.6pH and waterbath can for 10 minutes/half pint or 20 minutes/pint (and check seal and all that) I should be good? Is there anywhere you know of that delves deeper into the science of food preservation without having to take more college classes?
    I have a B.S. in Microbiology and another in Medical Technology, but we never went over food science. I have been canning for a number of years and wanted to can a dried fig compote with port. I haven’t been able to find the acidity threshold and the local extension college ‘experts’ (referred to by the USDA) aren’t very expert at all. I can’t find any exact recipes (I can find figs different ways and I can find other mixed dried fruit recipes with port, but not dried figs with port. Argh.)

    • Julie says

      Acidity plays a big role, but so does water activity/the density of what you are canning. For instance, you can never can something like pumpkin butter, even if you make it acidic enough.
      Also, 10/20 minutes isn’t always enough time…and sometimes it is more time than you need!

      Normally, I’d suggest college extension experts to learn more. I know here in Ithaca, we have a master preservation course you can take from them. Perhaps check out the college extension people in nearby areas?

      I would guess that you could substitute in figs for the mixed dried fruit in the tested recipes you have looked at, BUT I can’t say for sure without looking at the recipe and studying it a bit more, and therefore don’t feel comfortable recommending it.

      • Dave Maxx says

        Is pectin a food acidifier or just for gel production and if not how does it react with sugar to gel??? Is there a ratio? I dont like the recipes fot 7 cups of sugar to 3 cups of fruit…I may as well snort maple syrup

  5. says

    My husband & I put up 164qts of Green Beans & 45qts of Beets!! It makes you feel so good
    to see your awesome veg, looking sooooooo pretty in there jars. In the winter they taste so much better then, what you buy in the stores.