Canning: A Modest Miracle

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The Two Basic Strategies

Water-Bath Canning

There are two basic ways to process preserves: water-bath canning and pressure canning. The appropriate method is determined by the level of acid in the produce. All low-acid foods such as vegetables, meats, poultry, and seafood must be processed under pressure at 240°F. Without this intense heat a harmful bacteria, called Clostridium botulinum, can survive and thrive. The growing spores produce a deadly yet insidious toxin, since the food gives no obvious indication of spoilage. Luckily the spores do not grow in high-acid foods such as tomatoes, pickles, and fruits. It is safe to process these in a boiling water bath at 212°F. The equipment needed for this is either a traditional water-bath canner or a metal pot that is three to five inches deeper than the jars. The pot must have a tight-fitting lid, a flat bottom, and a wire rack for lifting jars out and for preventing them from touching the bottom (or each other) or falling against the sides of the canner. You can also use a steam-pressure canner for waterbath canning provided it is deep enough. And the pot should be no more than four inches wider than the stove's burner.

Pressure Canning

It may be best not to buy a used canner made before 1970 because it may lack vital safety features and finding replacement parts could pose a headache. Pressure cookers should come equipped with a jar rack, a dial pressure gauge, an automatic vent/cover lock, a steam vent, a safety valve, and a lid that turns to lock into place.

Now, do you want to use the hot pack method or raw pack method of filling the sterilized jars? One rule applies to both techniques: Only use produce in its prime. Fruit and vegetables will be preserved but not improved by canning.

In the raw pack method, the food is packed raw into the jars, then covered with a boiling hot liquid. Generally, raw packed fruits and vegetables shrink during processing and they should be packed tightly. There are exceptions however, including corn, peas, and lima beans which actually expand during processing and so should be packed loosely.

With the hot pack method, the food is preheated in liquid, packed hot into hot jars, and then covered with hot cooking liquid. Hot packing is superior in the sense that more air is forced out of the food tissues. There should be enough liquid to fill in around the food as well as to cover it.

After packing each jar, remove air bubbles by poking a plastic spatula or plastic knife between the food and the jar. Metal utensils could scratch the glass. Move the spatula up and down while rotating the sealer. As air is released, the level of liquid may subside and you'll need to add more fluid. The head space is the space between the top of the food and the lid, and it is essential for the expansion of food during processing and to enable a vacuum to form. If too little space is left, some of the food may pump out during processing. If too much space is left, air may remain in the jar after processing. Either could mean a poor seal. The required amount of head space varies according to the food, so it is important to follow the recipe.

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