Beware Botulism

Despite the fact that science has developed accurate processing schedules for canners, there is a continuing occurrence of outbreaks of botulism largely traceable to home-canned foods. Why? Many home canners follow directions obtained from old cookbooks or handed down from their parents. Those instructions were frequently developed through hit-or-miss techniques based on little precise knowledge. Other home canners simply choose to ignore the available information.

Many unsafe canning methods continue to be practiced today because they produce a good product often enough to lull users into thinking they are safe. For instance, open-kettle canning, a method in which cooked food is simply packed into sterilized jars and sealed with no additional processing, is recommended only for jellies containing sugar, although it is still widely used incorrectly for jams, fruits, tomatoes, pickles, etc. Water-bath processing of low-acid foods has been found to be inadequate for preventing botulism outbreaks unless excessively long processing times are used—which result in a greater loss of nutrients and greater energy consumption. “Oven-canning” is another unsafe method that is periodically “rediscovered” by home canners. With this method, the temperature inside a jar is not hot enough to kill the spoilage agents. There is also the danger of the jars exploding.

Other new, unsafe canning methods are regularly developed by consumers, who try to can in appliances such as dishwashers, slow cookers, and microwave ovens. None of these processes meet the safety criteria of government agencies that regulate commercial canners.

In canning, the time required for the heat to penetrate to the center of the food in the container (the slowest heating point) is extremely important and is affected by a number of factors:

1. The size and shape of the container (quarts take longer than pints);

2. The ratio of solids to liquid (the more liquid, the faster it heats);

3. The type and size of the pieces packed in the container (smaller pieces heat faster than big chunks);

4. The amount of fat (fat is a thermal insulator);

5. The type of heating medium being used (wet steam heats cans faster than dry air).

Thus if jars are packed too tightly, or with too little liquid, an adjustment in processing time must be made.

The natural acidity of a food also makes a big difference in the time and temperature needed to safely process a food. These natural acids have the ability to inhibit or prevent the growth of many of the microorganisms that produce spoilage and disease, but the degree of inhibition depends on the type and amount of acid present. Low-acid foods must be processed in a pressure canner at temperatures significantly above the boiling point of water and for specifically recommended lengths of time in order to avoid the risk of botulism. Typical foods in this category are most vegetables, meat, poultry, fish, milk products, seafood, and soups.

High-acid foods, including almost all fruits, pickles, and tomatoes, do not require pressure canning. Even here, however, the recommended length of processing varies from food to food.

The many varieties of tomatoes now available have caused some confusion as to the method of canning required. In recent tests, so-called “low-acid” tomatoes are not truly low-acid: typically, their sugar content was higher than other varieties, which altered their taste, not their acidity. Most tomatoes can thus be processed normally. Pear-shaped and long-bodied varieties of tomatoes do tend to have a lower acid content, however. Also, as a tomato ripens the acidity lessens. Overripe tomatoes should thus be avoided; however, it is safe to process them in a pressure canner.

Following are several safeguards to avoid potential dangers from home-processed foods:

Use a properly functioning pressure canner for canning all low-acid foods, and follow reputable recommended time and temperature schedules exactly.

Have the seal and pressure gauge on the canner checked regularly to assure accurate temperature control. (Contact your local agricultural extension service office, or write Department of Food Science and Nutrition, 2218 SFLC, Brigham Young University, Provo, Utah 84602, for instructions on how this is done.)

Use the boiling-water-bath method only for fruits, fruit juices and purees, tomatoes, and pickles in acid (such as vinegar). Follow the recommended time tables exactly.

In combination foods such as stews or similar items, select processing times and temperatures for the food with the lowest acid content. A few tomatoes in canned beef stew do not turn it into a high-acid food.

Do not can overripe fruits and vegetables, especially tomatoes.

Thoroughly wash or peel any fruits or vegetables that have been sprayed with pesticides.

Do not overpack jars; allow adequate “head space” above the food.

Do not reuse sealing lids or cracked, chipped jars.

Do not use jars not designed for canning.

Do not use canned foods showing any signs of spoilage. Especially, do not taste doubtful food—it could be your last meal. “When in doubt, throw it out” (and away from children or pets). If it is a commercially canned food, put it in a safe place and call the food distributor, the public health office, or your local Federal Food and Drug Administration official. Your action could save someone else’s life.

For added safety, boil home-canned products for ten to fifteen minutes before serving whenever possible. The heat will destroy any toxin which might have formed in spite of all previous precautions. The foods most commonly involved in botulism poisonings are beans, corn, spinach, peppers, and asparagus. All should be able to stand this treatment without excess loss of quality.

Further information is available from the U.S. Department of Agriculture, from your state extension agent, or from the manufacturers of canning supplies. (Excerpted from Home Canning, a Scientific Status Summary by the Institute of Food Technologists’ Expert Panel on Food Safety and Nutrition and the Committee on Public Information)