A guide to home canning

Home fruit canning has long been one of the most popular forms of preserving food. Shelves lined with a bountiful array of colourful preserves present a satisfying and attractive sight. Doing it correctly will provide you with food through the winter and save you from potential illness .

The basic principle of food canning is simple: food is heated to a high enough temperature and for a sufficient time to kill disease-causing bacteria and stop the spoiling action of the enzymes. The food is then stored in sterile, airtight containers in order to prevent microoganisms from re-entering the food and causing further contamination. All food naturally contains acid; the higher the acid level, the better the food's ability to stop the growth of dangerous bacteria.

Meats and most vegetables are so low in acid that they cannot be safely canned at home. Low-acid foods can often harbour heat-resistant, disease-causing bacteria and need to be processed at temperatures of 115°C to 120°C (239°F to 248°F).  As this cannot be reliably achieved in home kitchens, health authorities strongly recommend against the home canning of vegetables (unless they are first pickled), and of meat, poultry and seafood because of the risk of poisoning. Pickled vegetables can be safely canned at home because the addition of vinegar will raise the acidity of the vegetables to a point where food-poisoning bacteria will not grow.

High-acid food, such as pickled vegetables and most fruits, can be safely processed at the temperature of boiling water, 100°C (212°F), because surviving bacteria will not grow under acid conditions. Pears and tomatoes are not as acid as other fruits, and when they are to be home canned the acidity should be increased by the addition of ascorbic acid (vitamin C) or lemon juice.  Seven to 20 millilitres (1 1/4 to four teaspoons) of ascorbic acid or 15 millilitres (one tablespoon) of lemon juice for every 500 millilitre (two cup) jar should do the trick.

There are many benefits of canning, but don't overlook the importance of doing it right and that comes with knowing the acidity of the foods you want to preserve.