Canning Safety: The Science of Safe “Putting By”

by Jj Starwalker

I grew up in a family that gardened and, to some degree, put food by for the winter. And being old, I also grew up through the times of many changes in processing. Many of the techniques my grandmother and her friends may have used “back in the day” are still floating around out there and being promoted, despite rigorous research in more recent times, that shows us what is truly safe for home canning and the best practices for the home freezer. 

Like many who want to begin canning produce, I started with the  boiling water bath process in a huge (compared to my other pots) spatter-ware kettle. Only later did I buy an equally large pressure canner, like the much smaller (now considered “old fashioned”) pressure cooker, the size of more usual cooking pots.

Like many folks, my mom was scared of pressure cookers/canners and never used one. And I acquired my first pressure canner as a crafting tool! I was doing fiber art (batik) that used wax to resist dye, and my dyes required high heat to become color fast. It was during this process, one time, that I learned how the safety valve – included on all pressure canners – works. I wrapped each piece of wax and dye filled cloth in multiple layers of newspaper, to absorb wax as it melted and keep the pressure valve working properly. Or so I thought.

One time I was a little light on the amount of paper, I guess, because while the cooker was working and I was timing it, a very hot geyser of brightly colored water erupted toward the ceiling! It looked like the thing had blown its top. . . but it was, in fact, only the safety valve that had blown. I worked as it should. There was no “explosion” and no danger to anyone unless an extremely tall person had been standing at the stove, literally peering down over the canner. The relief valve was easy to replace and the unit is still in service with no additional follies. I am no longer using it to process fabric, however, this experience showed me “the worst case scenario,” and despite the surprise at the time, erased any pressure canning apprehension I might have had. 

Boiling water bath canning – the way most folks start – is never that dramatic. But there is much good science behind the how and why of our two canning methods. Let’s consider that method first. 

We tend to remember that water boils at 212 degrees Farenheit. This temperature is sufficient to kill harmful molds, yeasts, and some bacteria that may be found in naturally acidic or acidified foods, such as fruit and fruit products, pickles, and sauerkraut. Modern tomatoes vary in their acid levels and must have 2 tablespoons of bottled lemon juice or ½ teaspoon of citric acid per quart added, but then are safe for water bath canning as are the other products mentioned. Keep this in mind as there are still old recipes out there that do not mention this step. 

This temperature is NOT sufficient to kill the Clostridium botulinum bacteria that produces the deadly botulism toxin. Botulinum spores are on most fresh food surfaces, but because they grow only in the absence of air, they are harmless on fresh foods. The spores need the low acidity of foods like vegetables and meats and the absence of air (such as in a sealed canning jar) to germinate and produce the toxin. The spores can be destroyed only by pressure canning because this gets the food and jars to a temperature of 240 F or above for a specific period. This is why we use pressure canners, and follow the recipe, time, and adjustment instructions carefully. Salt, which may be added for taste, does not prevent spoilage. 

However, we need to think about geography and physics as well. The temperature we all remember for boiling water is, in fact, accurate only at sea level, and most of us do not live at sea level. For every 500 feet increase in altitude above sea lever, the boiling point drops 1 degree lower. The higher our elevation or altitude, the less air pressure is in play. Less air pushing down on your pot of water allows it to boil a lower temperatures the higher your elevation. We may associate this with mountainous areas, but of course it is a factor everywhere. In Omaha, Nebraska, for example (altitude a bit over 1000 feet), boiling water is actually only 210 degrees! Does that have an effect on the safety of your processing? You betcha!

If you do not know your elevation above sea level, or a quick call to your local county extension office will give you the answer. And the extension office folks can also give you exact information for timing your water-bath canning and adjusting the weight on your pressure canner. 

If you are at or above 1000 feet elevation, it is a good idea to add an extra minute to the jar sterilization time, especially for jams and other products that are water-bath canned for very short periods  And when you rev up your water-bath canner between 1000 and 3000 feet elevation for other acid foods, the South Dakota extension service says to add an extra 5 minutes of processing time.

Always use recipes approved for canning, too. The extension office once again is a good resource, as is a current edition of the Ball Blue Book. A few years ago, I wanted to put up apple pie filling and fortunately I checked the Ball Blue Book rather than using my typical recipe, which calls for flour to be added to thicken. I learned that would not be safe; best practice is to hot pack a mix of cut apples, sugar and spices, process, and then when making the pie, add the thickening agent at that time. 

When it comes to pressure canning, If you have a pressure canner with a weight, you simply use the 15 pound pressure weight above 1000 feet and leave the 10 pound weight for those of us at or below the thousand foot mark. If your pressure canner has a dial, adjust the heat so your dial rests on 11 pounds if you are at or below 2000 feet and increase above that elevation as follows:

2001-3000 12 pounds
3001-6000 13 pounds
6001-8000 14 pounds
8001-10,000 15 pounds

Let me also suggest what may seem obvious at this point: If you are just considering trying pressure canning to put up that corn, green beans, or chicken you are growing this year, get a canner with a weight. The stove heat adjustment is, indeed, much less picky. You want to hear a jiggle from the weighted gauge around 1-4 times per minute. This tells you that the pressure is staying at the correct level. You DO NOT want the gauge to constantly jiggle throughout the timed period. This would indicate that the pressure in the canner is too high. 

You can practice with a canner-load of empty jars (without lids), filling jars with water and the canner itself with as much water as recommended and following directions from the manufacturer or the Ball Blue Book for exhausting the air and bringing the unit to temperature. Start timing the process when your weight is jiggling properly. It is better if you do not have to adjust the heat up and down; my experience has shown me that causes more of the liquid to be pulled out of the jars. However, do not panic if you do loose liquid. As long as the jar seals, the food will be safe. We just eat the jars that have lost some liquid first. There may be a bit of texture change but there is always soup to be made!

And even the professionals and experts apparently have this problem. When I took a class with just such a teacher, my whole reason for doing so was to learn how to keep more liquid in the jars and, as our canner was chugging away, I asked and was quite surprised by her response. “I wish you could tell me how!” And indeed, when it was opened, some of the jars had lost some liquid. 

The Penn State extension office has these suggestions for this issue:

  • Avoid rapid fluctuation of temperature.
  • Let the canner return to 0 psi before removing the weight then let jars sit in the canner for 10 minutes before removing. 

And, if a jar does not seal, it IS safe to reprocess the contents though I usually just put it in the fridge and use it up quickly, to avoid over-processed contents. If you choose to re-process, remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time.  

Now you have canned it, keep it safe in storage. Lids should be tightly vacuum-sealed on cooled jars:

  • Remove screw bands. Wash, allow to dry thoroughly, and store for next year.
  • Wash the lid and jar to remove food residue. Rinse and dry them.
  • Label and date the jars. I use indelible marker on the lids.
  • Store them in a clean, cool, dark, dry place. Do not allow to freeze; keep out of sunlight.

    And if you are blanching food to freeze rather than canning, you might still keep your elevation in mind. At elevations of 5000′ or higher, the University of Colorado recommends adding an extra minute of blanching time. 

    Photo credit: Sigmund, Unsplash