Noteworthy Changes to USDA Guidelines
Two noteworthy guideline changes have occurred post World War II. Boiling water and pressure canning are currently the only recommended methods of home canning. Only high-acid foods should be preserved using a boiling water bath. Building on this point, tomatoes are no longer considered acid foods.
Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water canner.
--United States Department of Agriculture. (2015). USDA complete guide to home canning, 2015 revision: principles of home canning. Retrieved from http://nchfp.uga.edu/publications/publications_usda.html
The USDA now recommends that lemon juice or citric acid be added to tomatoes prior to processing to increase acidity (USDA Complete Guide to Home Canning, 2015). Additionally, the current USDA guidelines for home canning do not include guidelines or a processing timetable for canning in tin.
The full set of current USDA guidelines for home canning can be found here.