Canning Vegetables: How It Works

Enlarge Image Image Gallery Canned pickles lived down in the root cellar back in the day. iStockphoto.com/Rich Hill

I vividly remember the first time my grandmother took me to “the cave” to retrieve a couple of jars of pickles. We lived on a hundred-year homestead, and who knows which generation had dug the root cellar and created the little hill in Grandma’s front yard. The door didn’t angle up that far, and after you opened it, you went down a few crude stairs and still had to dip your head low to get in. To my young eyes, it seemed to be a door into the past. The rock-lined chamber was cool and smelled like dirt and age. Shelves lined both sides, and, though we never ate their contents, some of the jars had to be left from generations past. By the time I was around, Grandma only used the shelves closest to the door. We grabbed up a couple of jars of dill pickles (I also remember how we hoarded the last jar of Grandma Pickles). As I carried them back to the house, I wondered how it was that we could put food in a cave that wasn’t nearly as cold as our refrigerator and not have it spoil.

Now I know more about the process and the science that makes canning a safe way to preserve food. The canning process mitigates several factors that can make food inedible. It removes oxygen that reacts with food; destroys enzymes that cause food to break down; prevents the growth of bacteria, molds and yeasts; and forms a vacuum in the jars that keeps liquid in and air and microorganisms out.

But how does it work? Well, the heat processing is the most important part. The filled jars are closed with metal lids and screw bands, and placed in the canner. Then you heat them up, and the stuff in the jar starts to expand and push some steam and air out of the headspace (even with tops on the jars and tightly screwed rings, the pressure is enough to let gasses escape). After the elapsed heating time, when the jars begin to cool, the food contracts again, increasing the pressure inside the jar and pulling the lid down (pop!) to form a “vacuum seal.” This seal keeps the bad stuff out and the good stuff in.