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"Unless you are sure that these safeguards were *observed, a margin of protection is added by boiling the canned low-acid food hard for 15 minutes to destroy any hidden toxins (corn and greens require 20 minutes), and stirring to distribute the heat.  If the food foams unduly or smells bad during boiling, destroy it completely so it cannot be eaten by people or animals."

Excerpt taken from -  PUTTING FOOD BY

Canning and Botulism

Low acid foods do not need to be boiled before eating if you are certain the foods were canned according to instructions stated in the 1994 USDA Complete Guide to Home Canning or another reputable resource. If you have any doubts about the canning methods used, boil the food before eating.

     Boil spinach and corn for 20 minutes;
     Boil meats and poultry for 20 minutes; and
     Boil all other products for 10 minutes.

Because bacteria are killed when exposed to a specific temperature for a set time, foods heated in the microwave would have to be heated for the times listed above. There is no research to indicate that this time can be decreased. Most foods begin to lose quality when microwaved for the times listed above. Also, due to the uneven heating pattern of microwaves, cold spots could develop and so all toxin is not destroyed.

Because boiling fish vigorously would destroy the texture, botulism toxin in fish was developed:

     1. Open the jar and, if there are no signs of spoilage, insert a meat therometer so that the tip is as near as possible to the center of the contents.
     2. Cover the jar with foil. Insert the therometer through the foil.
     3. Put jar in an oven preheated to 350 F for 30 to 40 minutes or until the therometer registers 185 F.
     4. Let the hot jar stand at room temperature for 30 minutes to complete the heat treatment. Do not speed the cooling. At the end of 30 minutes, serve hot or refrigerate for use later.

By Angela M. Fraser