Acid in canned food is expressed as pH value. Foods having a pH of 4.5 or lower are called high-acid foods and those with a value of 4.6 or higher are termed low-acid foods.
Since few bacteria thrive in acids, their destruction is less difficult in fruits than in vegetables (with the exception of tomatoes).
Botulism is a deadly poison caused by a toxin from the growth of spores (seeds) of the bacteria, Clostridium botulinum. These spores will produce a deadly toxin in low-acid foods in the absence of air (oxygen) inside a sealed jar. Therefore, the spores must be destroyed by processing under pressure at 240° F. The length of time has been determined by scientists for each individual food.
Clostridium botulinum will not grow in foods with a pH of 4.5 or lower, so high-acid foods may be processed safely in boiling water at 212° F.
Low and High Acid Foods
Low-acid vegetables Asparagus Beans—snap or shelled Beets Carrots Corn Potatoes Pumpkin Squash Sweet potatoes High-acid fruits and vegetables Apples Apricots Berries Cherries Grapefruit Peaches Pineapple Rhubarb Tomatoes
Yeasts, mold and non-spore forming bacteria are readily controlled by processing at 212° F.
Most canning equipment and supplies may be purchased at hardware stores, housewares departments, and from mail order companies. Jars and lids are available in many retail stores.
Canning Jars
Select standard canning jars made of tempered glass that can withstand high temperatures. The manufacturer’s name or symbol in glass will identify the product. With careful handling, jars last an average of about 10 years. Avoid using antique jars because there can be hair-line cracks not visible to the eye, causing jars to break.