Water Bath CanningCanning in a water bath canner is a straightforward proposition. If you can follow a recipe, you can use simple, easily available equipment to preserve a variety of foodstuffs. This post is my initial attempt to show the basic steps in water bath canning; the focus here is on the steps involved and not the recipe, though in this case the food being prepared is tomato soup.
Before getting into the technique of water bath canning, let's look at some important things to know about home canning. (If you just want to see the pretty pictures--
--scroll down.
Why do foods spoil and how does water bath canning prevent spoilage?
Foods spoil for two principal reasons: presence of
microorganisms and
enzymes naturally present in the food. Proper canning technique kills or inhibits the microorganisms and de-activates the enzymes that lead to spoilage. To accomplish this, home canning relies on airtight containers, heat and acidity.
Using proper containers isn't difficult. You simply have to lay in a supply of modern jars that accept two-piece closures. (More on that later.) Jars that have bale closures or one-piece lids are
not acceptable for modern home canning. Some of these jars are very pretty--you can use them to store dry goods, but don't put them in your canner. Also, don't reuse store-bought jars that held mayonnaise, pickles or whatever: these jars were designed for one-time use and are not safe home canning enclosures.
In short, if it doesn't take a two-piece closure, don't use it. Completely immersing the jars in boiling water long enough to heat the contents of the jars thoroughly provides the principal means for preserving the jars' contents. Modern recipes, such as those found in the
Ball Blue Book, have been specifically tested to match appropriate processing times with any given recipe.
It's helpful, at least for me, to have a basic understanding of the microbiology of spoilage microorganisms. At 40 to 139 degrees F molds, yeasts and bacteria thrive. Raising the temperature into the 140-179 degrees F range inhibits the growth of these bugs, but doesn't destroy them. Raising the temperature into the 180 - 212 deg F range destroys most of them. But note that the toxin produced by
Staph. aureus and the spores of
Clostridium botulinum are not destroyed at this temperature, only inhibited when processed at 212 deg F. To destroy these poisons, food must be heated and held at 240 deg F.......and this is temperature achievable only in a pressure canner.
So, why then are foods processed in a water bath safe and durable? The reason is that properly prepared water bath recipes are
acidic. It's the acidic environment that prevents these potentially deadly poisons from multiplying. And that's why only "high acid" foodstuffs are safe to can in a water bath canner. Foods with a pH (a measure of acidity; lower number means
more acidic) less than 4 are safe in a water bath canner. Examples include pickles, apples, peaches and pears. Foods that sit in the middle zone (pH 4-5), such as tomatoes, are also safe but require acidification before processing. This is accomplished with the addition of bottled lemon juice or citric acid. Low acid foods (pH >4.6), such as all meats, carrots, green beans and corn,
absolutely cannot be processed in a water bath.In summary, modern canning jars, sufficient periods of heating and acidity all combine together to make water bath canning a safe enterprise.
(More later.)