Learn how canning preserves food in this mini training series on safe canning at home and start canning confidently to preserve your homegrown fruits and vegetables this season.
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How Canning Preserves Food
In last week’s training, I talked about Clostridium botulinum which is the bacteria that causes botulism which can be a deadly form of food poisoning caused by a toxin these bacteria produce.
So here’s the thing, this bacteria forms both spores and live growing cells. The spore is like a suit of armor for it. It only opens when the conditions for the bacteria to grow are just right.
Clostridium botulinum just straight-up love to grow and multiply in an environment that is moist or wet, low acid, low oxygen, and at a temperature that we find most comfortable although these suckers will thrive at 40 – 120 F or 4 to 48 C. This temperature is ideal for most microbes though and is called the Danger Zone.
Now the Clostridium botulinum spores are on most fresh food surfaces but because of the oxygen in the air, they are harmless on fresh foods. So how do you try to remove them?
Nerdy Joke Alert!
As a chemist, there’s a long-standing joke between chemists and microbiologists. If you gave a chemist 2 beakers and asked if they are clean, the chemist would be looking for a sign of residue or build up on the glass. They’ll say yeah it’s clean. Give the beakers to a microbiologist the answer is usually it’s never clean! Because bacteria, yeasts, and molds are on everything all the time.
How to remove microbes?
We want to reduce the numbers before we start preserving our food. The first thing other than washing our hands and work surfaces is to wash the fruits and veggies. This washes some of the microbes away.
Peeling root veggies, skinning tomatoes, and peaches or blanching for hot pack recipes are other ways that a recipe is helping to remove bacteria that make food go bad.
The canning process and the time we have our jars in the canner is what kills the largest expected number of bacteria, yeasts, and molds.
The Danger Zone
Now the Danger Zone is 40 – 140 F or 4 – 60 C and this is where those microbes grow fast and thrive.
At temperatures lower than this say in the fridge or freezer, it slows down the growth of microbes but doesn’t kill them.
Now above 140 F or 60 C is where we start to kill these microbes and we need to be above 180 F or 82 C to start making some headway in killing them.
So at sea level water boils at 212 F or 100 C. Your water bath canner
Your pressure canner gets up to much higher temperatures because of the pressure it creates.
So a pressure canner
Temperature isn’t the only way to stop Clostridium botulinum. Using acid is another method and it is the acid and heat working together that is how water bath canning works for acid foods. Low acid just means that there isn’t enough natural acidity to prevent these bacteria from growing.
Acid vs Low Acid
You might be familiar with the term pH to measure the acidity in something. I know I talk about pH for soil testing so if you’re a regular here then you might remember me talking about that!
Lower numbers on the pH scale mean that it is acidic, higher numbers mean it is an alkali or basic and 7 is neutral. So water is neutral, vinegar is an acid and baking soda is an alkali.
If you ever did that experiment as a kid putting vinegar on baking soda and see it bubbling and frothing and getting hot, that’s a neutralization reaction happening.
Some acid is naturally in foods, it’s what makes them taste sour. So lemons and limes have natural citric acid in them. Apples have citric acid and malic acid naturally in the fruit and vinegar is acetic acid.
The acid stops the growth of bacteria. Acid foods are things like
- jam or jelly
- fruit butter
- sauerkraut,
- fruit in syrup
Low acid foods are things like fresh beans, cucumber, squash, corn, asparagus, beets. This is why you have to pickle these vegetables or pressure can them.
Low acid foods are also
- chicken
- beef
- elk
- fish
All of which need to be pressure canned.
Acidifying Tomatoes
Tomatoes and figs are borderline cases. The level of acidity can vary depending on the ripeness of the fruits, how they have been stored, and even the variety. You will see that in most recipes with tomatoes and figs, they add lemon juice or citric acid. This is to make sure there’s enough acid in there to be able to use a water bath canner.
What about altitudes?
So canning is preserving food by making the environment uninhabitable for yeasts, molds, and bacteria. In particular the Clostridium botulinum bacteria.
Bigger jars have longer processing times to make sure that the temperature is hot enough all the way through the jars to kill the microbes. If you live above 1000 ft above sea level, then water boils at a lower temperature.
Lower temperatures are not as good at killing bacteria so the processing time in a water bath canner
Read more
Safe Canning Basics (week 1 of the mini training series)
Canning Equipment You Really Need (week 3 of the mini-training)
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