Like many other hobbies or techniques, home food preservation has its own distinctive vocabulary and jargon, which can be mystifying to the unsuspecting novice. Water-bath canning and pressure canning are relatively easy terms to understand, but cold-pack canning can create confusion. It is not an alternative to pressure or water bath canners, but a way of preparing the food to be canned.
Regardless of which canning method or type of canner you use, there are a few fundamentals you must observe. Select only the best and freshest produce for canning, as ripe as you would choose for eating. Sterilize your canning jars and lids before using them, and use brand-new lids every time to ensure a good seal. Check the lip of each jar for chips or abrasions that might keep them from sealing, and always wipe the rims after filling your jars. Leave one inch of air space at the top. Tighten the lids finger-tight before loading the jars into the rack of your canner.
Water-bath processing is the simplest form of canning. Pack the food into your sterilized jars, seal them and cover them with hot water. Boil the water as directed in your recipe, which heats the contents of the jars to a food-safe temperature. When the jar cools, the air in the jar contracts and forms a vacuum, sealing it from outside air. Water-bath canning is only safe for foods with a high level of acidity, expressed as a pH of 4.6 or lower. This includes most fruit, pickles and vegetables such as tomatoes.
Prepared foods, meats and other foods with low acidity can't be preserved in a water-bath canner, because acidity will kill botulism spores but boiling temperatures will not. A pressure canner creates an artificial environment that forces water above its normal boiling temperature, killing the spores and making the food safe for long-term storage. This is done by locking down the canner lid to make an airtight seal, and using pressurized steam to raise the canner's internal pressure by 10 psi or more, as needed. After processing the food, you cool the pot until you can safely break the seal.
The cold-packing technique is one way to prepare foods for canning by either the water bath or pressure-canning method. Cold-packing means that you do not cook the food in its packing liquid before placing it in the jars. Cold-packed foods are more prone to spoilage and discoloration than hot-packed foods, but advocates of cold-packing feel it results in a fresher, more natural flavor. Check the USDA's home canning guide, or another authoritative source, before cold-packing any fruit or vegetable. Not all produce can be used with this method.
Hot packing is a more widely used technique for preserving foods. It simply means that the foods being preserved have been poached for a few minutes in their syrup before canning. Hot-packing helps eliminate air from the fruit, leaving it less prone to spoilage. Hot-packed foods are also less prone to floating in the jars, which raises the ends of the food out of the syrup or brine and leads to discoloration.