This section is from the book "Everywoman's Canning Book", by Mary B. Hughes. Also available from Amazon: Everywomans canning book; the A B C of safe home canning and preserving.
This great resistant power of spores in vegetables makes the Intermittent or Fractional Method of canning the method chosen by some experts, especially for peas, corn, Lima beans, and string beans, on which bacteria are most likely to be found. Some practical housekeepers, who have been most successful in their home canning, are inclined to look with disfavor on the intermittent process, because of the additional labor involved in processing the jars three successive days. This method, they feel, discourages rather than encourages much canning. It is, however, widely used in the South, and it is well for every woman interested in canning her garden surplus to know how to do the intermittent method, since some years the bacteria are particularly virulent, and spoilage after careful processing would indicate that the intermittent sterilization would be best for that particular kind of vegetable.
The principle of the intermittent method of sterilization is that spores not killed after the first processing will be less resistant and will probably be killed after the second day's processing, and that after the third day's processing there is very little chance of their living to do harm to the product.
Partially seal the product in the usual way and process one hour. Remove jar from container, seal, and set aside for twenty-four hours. On the second day, lift the spring of the clamp and set the partially sealed jar back in the processing bath, and process again for one hour. Remove, seal, and set aside for another twenty-four hours. On the third day, repeat this process. The jar is then ready to seal and put away.
Fruits are never subjected to the intermittent method.
It is absolutely necessary to follow accurately the time given in the tables for processing, if success is to be assured. It is quite common to hear the amateur say, in reporting failures, that she processed the beans thirty minutes, just as long as she ever cooks them for the table, and they spoiled. Thirty minutes will cook tender beans and make them edible, but it takes three hours' continuous cooking to kill the spores living in them, and to prepare them so they will keep all winter. Do not compare the time you would cook your product for the table with the length of time it needs processing. Follow your time table if you wish for success.
Once sterilized, if for any reason the jar must be opened, even though the cover is lifted only for a second, be sure and place again in the sterilizing bath and process for ten minutes, otherwise organisms enter with the air, multiply rapidly, and spoilage results.