Fruit and Vegetable Acidity
High acid foods Lemon Juice Cranberry Sauce Gooseberries Rhubarb, Dill Pickles Blackberries Applesauce, Strawberries Peaches Raspberries, Sauerkraut Blueberries Sweet Cherries, Apricot Tomatoes Low acid foods Figs, Pimentos Pumpkins Cucumbers Turnips, Cabbage, Squash Parsnips, Beets, Green Beans Sweet Potatoes Spinach Asparagus, Cauliflower Carrots Potatoes Peas Corn
Holding of produce to be canned for long periods in warm summer temperatures gives bacteria ample time to multiply into vast numbers, thus increasing the chances of spoilage.
For example, one cell can multiply into a billion cells in just 15 hours of holding under favorable conditions.
Salt and spices added to low-acid canned products in amounts recommended do not appreciably alter processing time. Salt may slightly lower the heat resistance of some micro-organisms but not enough to present a problem if omitted for dietary reasons.
Fats and oils, if added, may reduce the rate of destruction of bacterial spores. Spores of C. botulinum have been known to survive beyond all reasonable expectation when heated in oil suspensions. Thus, adding oil or fat to a product being canned could be dangerous and is not recommended.
The type, consistency, and piece size of food and how it is packed in the jars are important factors which affect processing time. In preparing jars of food for the pressure canner, follow directions carefully. Do not use jars larger than the directions specify.
Determining the safe processing time for a food product involves two important steps.
First, the rate of heat penetration is measured by finding the spot in the jar that takes the longest time to heat. This is referred to as the “cold spot”. Times will depend on the type of food (squash vs tomato juice) as well as how it is prepared (whole kernel vs cream style corn).
The second step is done in a laboratory. A known amount of some live bacterial spores is put into the “cold spot” of the jar of food. These jars are then heated and the amount of time needed to kill the spores is determined.