Although tomatoes have a tangy acidic taste, precautions must be taken to can tomatoes safely as they are considered borderline between a high and low acid food. Tomatoes are one of the most commonly canned vegetables. Canning procedures for tomatoes have been handed down from generation to generation. Unfortunately, there are many canning recipes that are inadequate to kill all spoilage microorganisms. Canning recommendations for tomatoes have changed over the years so be sure you follow the most up to date guidelines when canning tomatoes.
- Do acidify the tomatoes. To assure a safe acidity level, add bottled lemon juice or citric acid to each jar before processing. Place 2 tablespoons bottled lemon juice or ½ teaspoon citric acid crystals in the bottom of each quart jar before filling with the tomato product. Use half those amounts when canning in pint jars. Other tomato products should also be acidified. Some catsup and barbecue sauces do not need to be acidified because they contain large amounts of vinegar that provides the needed acidity. If in doubt, add the lemon juice or citric acid.
- Do process long enough. Processing times have increased over the years. Processing time depends upon the method of pack and added ingredients. Follow the recommended process time, adjusted for altitude, for the product you are making. Tomatoes may also be processed in a pressure canner. Tomato products with added vegetables or meat require pressure canning.
- Do match the type of tomato to the product being canned. Regular tomatoes work well for juice and canned tomatoes. Paste, plum and Roma varieties are good for making sauce, salsa, catsup, and purees. The two types can be mixed.
- Do follow the same directions for canning low-acid or yellow tomatoes as are recommended for regular tomatoes. Although low-acid tomatoes don’t taste as sour, their acidity is masked by the natural sweetness of the variety; no changes are needed in the recipe for safety.
- Do use high quality tomatoes. Some growing conditions may cause the tomatoes to be unsafe to can even when the tomatoes look fine. Tomatoes with blight and those from dead or frost-killed vines may be lower in acidity and are more likely to carry bacteria.
- Do not can using the open kettle method. Do not just heat the tomatoes or tomato product and pour the hot product into the jars, add the lids, and wait for the lids to “pop” without any further processing. Even though the jars may seal, the contents inside the jars have not been heated adequately to destroy harmful spoilage organisms. Other unsafe methods include oven canning and the use of so-called canning powders. Aspirin belongs to this last category.
- Do not make up your own recipes for canning salsa, spaghetti sauce, pizza sauce, etc. All these foods have added low acid ingredients for flavor. Use up-to-date scientifically research-based recipes.
- Do not add extra low acid ingredients to canning recipes. Do not add extra peppers, onions, garlic, etc. for flavor in a tested recipe. Measure ingredients accurately—don’t use the extra onion to finish off what is cut up. Add no more than three cups in any combination of finely chopped celery, onions, carrots, and pepper for each 22 pounds (the amount for seven quarts) of tomatoes when making tomato-vegetable juice blends. It is safe to reduce or omit a low acid ingredient such as garlic in a tomato sauce recipe.
- Do not add thickeners or milk to tomatoes or tomato products before processing. Add the ingredients to make stewed tomatoes or tomato soup when you are ready to serve them.
Use the most up-to-date information on canning tomatoes along with recipes for salsa, spaghetti sauce, catsup and other tomato products. For more information on Home Food Preservation visit the web site at: https://food.unl.edu/food-safety-preservation
Submitted by Andrea Nisley, UNL Extension Educator, Dawson County
Source: https://extension.psu.edu/canning-tomatoes-dos-and-donts, Martha Zepp, Penn State Program Assistant