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When canning tomatoes safely at home, be sure to use tested canning recipes that add citric acid, vinegar or lemon juice. Tomatoes are a medium-acid food with a pH in the range of 4.5–5.2. You must use a tested canning recipe that has determined the amount of acid to add in order to increase the acidity by lowering the pH to a safe level below 4.6.
The Science behind pH
When ingredients dissolve in water (H20), they produce ions H+ and OH–. If there are more hydrogen (H+) ions, the solution is acidic. If there are more hydroxyl (OH–) ions, it is alkaline or basic. Therefore, acidity is determined by measuring the potential hydrogen, abbreviated pH. The pH can range from 0 to 14. A pH of 0 to 6 is acid, 7 is neutral, and 8 to 14 is alkaline. Lemon juice has a pH of 2 to 3. Tap water has a pH near 7 (neutral), while rainwater is around 6 (slightly acidic) and seawater is around 8 (slightly alkaline). Baking soda solutions have a pH around 9, and household ammonia has a pH of 11 to 12. High-acid foods have a relatively low pH of 4.6 or less.
You cannot determine a safe acidity level by simply testing the pH when canning tomatoes or any other food; you must use tested canning recipes. For one thing, the pH of a food fluctuates from one variety to the next (plum tomatoes versus beefsteak tomatoes, for example) and within a specific type (green, mature, ripe, or overripe tomatoes).
In addition, something called water activity (aw) influences the canning process. Water activity is not the same as water content. Aw is a measurement of the free water on a molecular level, or the water that is available for food-borne microorganisms to grow. Other ingredients in canning recipes such as sugar, salt, liquids other than water (tomato liquid), and the ratio of solids to liquid also influence water activity. This is why tested canning recipes are the only safe way to can foods at home.
Citric acid, vinegar, and lemon juice
The following acids are available to increase the acidity (lower the pH) in tested canning recipes when canning tomatoes safely at home:
- Citric acid powder (food grade) is available from suppliers selling natural foods, nutritional supplements, and candy-making supplies. Vinegar or lemon juice may be more readily available.
- Commercial vinegar (5 percent or higher acidity) is available in any grocery store. You must use commercially bottled vinegar with known acidity strength of at least 5 percent (50 grain).
- Bottled lemon juice (a type of citric acid) is readily available in grocery stores. You must use commercially bottled lemon juice because the pH of fresh lemon juice fluctuates.
Which acid is best for canning tomatoes?
Last year, I canned pint jars using each of these three types of acid, with and without sugar. In all jars, I used the same batch of locally grown, field-ripened San Marzano tomatoes. This paste-type tomato is very sweet when fully ripe. For each type of acid, I found that the canned tomatoes without sugar tasted slightly better. Added sugar tended to overwhelm the tomato flavor making the canned tomatoes a less than optimal ingredient in some recipes.
Canning tomatoes with citric acid powder produced very good tomatoes with no competing flavors. As expected, tomatoes canned with commercial cider vinegar and bottled lemon juice had altered flavors. Note that the flavors mellow (like most pickled or acidified products) after 4 to 6 weeks, so it’s best to test your canned products after several weeks. After a few months, the tomatoes acidifed with cider vinegar had rich tomato flavor and pleasing acid balance. Tomatoes canned with bottled lemon juice had a lemony undertone that seemed to compete with, rather than balance the tomato flavors.
When using canned tomatoes in cooking, I found that lemon juice didn’t work as well as cider vinegar with many of the dishes I like to prepare, such as marinara sauce, chicken cacciatore, lamb moussaka and other dishes from central and east Europe. If you prepare more Latin American dishes or tomato sauces to serve with seafood, you may find lemon juice more acceptable. However, the flavor of bottled lemon juice is not comparable to fresh. Overall I, and many traditional canners with whom I have spoken, find the flavor of bottled lemon juice objectionable and the last choice for increasing acidity when canning tomatoes. By contrast, citric acid powder is the preferred choice because it contributes no off flavors.
If you are unsure whether to use lemon juice or vinegar, can a few jars with each type of acid. To counteract lemon or vinegar flavors, or simply as a matter of preference for your style of cooking, you may also add sugar. Since the acid and sugar are added to the jar before filling with tomatoes, it is easy to experiment when canning tomatoes at home to determine the type of acid that works best for you.
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