The exact time depends on the kind of food being canned, the way it is packed in jars, and the size of jars. Although tomatoes used to be considered an acidic food, some are now known to have pH values slightly above 4.6, which means they are low acid. To safely can them as acidic foods in a boiling water canner, you must add lemon juice or citric acid. When canning, you must know your altitude. Do not use process times recommended for canning food at sea level if you live at an altitude above 1,000 feet.
Water boils at lower temperatures as altitude increases.
Lower boiling temperatures are less effective for killing bacteria. You must increase either the process time or canner pressure to make up for lower boiling temperatures. Most boiling water canners are made of aluminum or porcelain-covered steel. They have removable racks and fitted lids. The canner must be deep enough so that at least one inch of briskly boiling water will be over the tops of jars during processing.