The air and all foods in their natural state contain invisible mircoorganisms which if uncontrolled cause food spoilage. Proper, safe home canning procedures control the growth of spoilage microorganisms. High Acid Foods require heat processing to 212°F (100 °C) to inactivate invisible microoganisms. This is the temperature of boiling water, so it can be achieved efficiently in a boiling water canner used for the recipe specified time. The pH of High Acid Foods must be 4.6 or lower (meaning the acidity is high). Such high acid environments inhibit the growth of bacteria and other spoilage organisms. However, high acidity alone is not sufficient to preserve foods. Heat processing in a boiling water canner is still required to inactivate enzymes, yeasts and other microoganisms that can thrive unprocessed High Acid Foods.
Low Acids Foods (with pH values higher then 4.6) must be heat processed at temperatures of 240°F (116°C) for a specified length of time to destroy the growth of Clostridium bolulinum - spoilage organisms which can be present in any food. It is itself destroyed at boiling temperatures, BUT it has the ability to form hardy spores that can survive the boiling treatment. These spores thrive in a low acid environment without air - i.e. sealed jars, as they grow these spores give off a deadly toxin. Because boiling water canners cannot reach this temperature, Low Acids Foods must be processed in a pressure canner. Because Clostridium bolulinum spores do not grow in the presence of acid. HIGH ACID Foods can be safely processed in a boiling water canner.
Over the years, our understanding of food safety has grown. At the same time, new technologies and agriculture crop varieties have been developed. For these reasons, it is important to follow current guidelines for home canning instead of old recipes. While they might be family favourites, older recipes may not have been properly tested to achieve adequate heat processing method and processing times can affect the quality and safety of your final product.
High Acid Foods include fruits, fruit juices, jams, jellies and other fruit spreads, pickles, relish, salsa, chutney and tomatoes with added acid (lemon juice or vinegar). All High Acid Foods must be "heat processed" in a boiling water canner to ensure best results and safety.
Low Acid Foods include vegetables, meat and game, poultry, seafood, soups, stews, tomato-vegetables sauces and tomato-meat sauces. All Low Acid Foods must be "heat processed" in a pressure canner to eliminate the risk of bolulism.