Extension Explores: Preserving Tomatoes

Did you know tomatoes may not be considered to be highly acidic? New tomato varieties, over-mature fruits, and tomatoes from dead or frost-killed vines may have a pH greater than 4.6. To ensure a safe acid level for boiling water canning of whole, crushed, or juiced tomatoes, it is recommended to add bottled lemon juice or citric acid directly to jars before filling the jars with the product. 

Tomatoes can be preserved by drying, freezing, or pickling, as well. They can even be used in making fruit spreads like jams, jellies, and marmalades depending on their acid level. Tomatoes are a great item to dry. They do not need to be blanched and are dried to a crisp.  

Freezing tomatoes will likely result in a soft texture and is more appropriate for cooking, such as in soups, stews, and spaghetti sauces. Tomato products, such as chili sauce and catsup, can be frozen.  

If you decide to pickle your tomatoes, commercially prepared vinegar is typically needed to achieve the required acidity. Do not change vinegar, food, or water measurements in a recipe or use vinegar with unknown acidity. Use only recipes with tested quantities of ingredients. There must be a minimum, uniform level of acid throughout the mixed product to prevent the growth of botulism. 

Other Preservation Recipes:


Frequently Asked Questions:

Tomatoes are on the borderline of being acidic enough to safely use a water bath canning process. To ensure that the risk of botulism is eliminated, tomatoes must be acidified by using commercial lemon juice, commercial vinegar or citric acid in the jars with the tomatoes or juice. Use 2 tablespoons of lemon juice or 1/2 teaspoon citric acid per quart and 1 tablespoon of lemon juice or 1/4 teaspoon citric acid per pint. Fresh lemon juice doesn’t have a consistent acidity level so only use bottled juice. You can also use vinegar but it may cause undesirable flavor changes. Use 4 tablespoons commercial 5% acidity vinegar per quart or two tablespoons per pint. The juice, vinegar or citric acid may be added to the jars and then filled with the product.

Some tomato products have tested optional directions for pressure processing as well as waterbath processing. Research shows that for some products, pressure canning will result in a high quality and more nutritious product. If there are no recommendations for pressure processing, you cannot use this method safely. Also you must still acidify the tomatoes, even when pressure processing. Some tomato products call only for pressure processing. These have too much low acid ingredients added and must be processed in a pressure canner.

Tomatoes from dead or frost killed vines may have harmful pathogens and may be less acidic leading to an unsafe product or one susceptible to spoilage. USDA recommends using disease-free, vine-ripened (preferably), high quality tomatoes. They should be firm and just at their peak ripeness. Over-ripe tomatoes are less acidic and unsuitable for canning as well.

Bring a pot of water to a boil. Cut the stem from the tomato and make an X shaped cut in the blossom end. Boil the tomatoes for 30-60 seconds and then remove to a bowl of ice water to stop the cooking and cool the tomatoes so you can handle them. The skins should slide off with little effort.

When tomatoes are cut or crushed, enzymes start breaking down the pectin in the tomato flesh that holds the cells together. Heating quickly to boiling while crushing and keeping it boiling vigorously while adding the rest of the tomatoes will help. When all of the tomatoes are added and boiling, simmer for 5 minutes – don’t overcook as that can contribute to separating. Make sure to use tomatoes that are not over-ripe as they can add to the problem and use a food mill or sieve instead of a blender to remove the skins and seeds. Separated juice is safe to use – it just doesn’t look as pretty. Shake the contents before drinking or using to remix the liquid and solids.

Sometimes canned tomatoes will have a layer of liquid at the top or bottom of the jar. This is a result of an enzyme that breaks down the pectin after the tomato is cut. To help prevent this, tear the tomatoes instead of cutting them. Don’t let them sit around after cutting or tearing. Use a hot pack method, if possible, and follow the directions carefully so you don’t overheat the tomatoes. Separated tomatoes and their liquid are safe to use, they just don’t look as good in the jar.

There are basically three types of tomatoes – round slicing tomatoes, plum tomatoes and cherry tomatoes. Slicing tomatoes have more juice and seeds while plum or Italian tomatoes are fleshier and meatier. These two types are both good for canning. Slicing tomatoes are well suited to making tomato juice or canned tomatoes. Italian or plum tomatoes are good for making sauce, salsa, catsup and purees as they have more flesh. You can mix the two as well for the best of both worlds.

Salsas are a mixture of high acid and low acid ingredients. Because of this, the proportions of ingredients and the amount of acid added are critical to the safety of the canned product. Therefore, you must use a tested recipe to make sure it is acidic enough to be safe. Choose a tested recipe that comes the closest to your home recipe. Remember that you can change the type of peppers used to adjust the heat and many tested recipes give you choices. Just don’t increase the amount of peppers, onions or garlic used. You can also adjust the dried spices/herbs (such as leaving out the oregano, or using cumin in place of oregano, etc.), but don’t increase the total amount.

Food processors can add a lot of air as they chop so it is not the best method. If there is too much air that stays in the ingredients in the jar, it could possibly cause sealing failures. So if you use a food processor, be careful not to process too long – just pulse the vegetables to chop.

Lemon and lime juice can be used interchangeably but vinegar is less acidic so cannot be substituted if the recipe calls for lemon/lime juice. Always use commercially bottled lemon or lime juice (not fresh) and commercially bottled vinegar that is at least 5% acidity (look on the label).

For more information or questions about food preservation, please contact the Family and Consumer Sciences Agent at your local county Extension office. Click here for a list of the Tennessee Extension Offices.

Trade and brand names are used only for information.  The University of Tennessee Extension and Tennessee State University Extension do not guarantee nor warrant published standards on any product mentioned; neither does the use of a trade or brand name imply approval of any product to the exclusion of others which may also be suitable.