Yesterday I got an inquiry about canning tomatoes and acidity. This is a topic I hear a lot in conversations with home canners. People tend to think of tomatoes as a highly acidic fruit/vegetable when in fact they most definitely are not. Tomatoes ride the dividing line between high acid and low acid and there is no way for the home canner to know which side of the line the tomatoes they are working with that day fall on. Some sources say the lower acid tomato is a result of breeding a tomato for sweetness and palatability. But if sweetness indicates low acid then there are plenty of sweet heirlooms out there that must be low acid. My personal theory is better testing methods show that tomatoes have been a fairly low acid fruit all along.
To safely preserve tomato products using a boiling water bath to process the jars, the tomatoes must be acidified. This is accomplished by adding vinegar, bottled lemon juice, or citric acid to the canning recipe or directly into the jars. If you are making a recipe like salsa or tomato chutney where the recipe already calls for the addition of vinegar or bottled lemon juice, then your tomatoes will already be properly acidified. If, for example, you are raw packing something like plain diced tomatoes then you must add acid to the jars in order to process them safely. In this case you would add 1 tablespoon per pint or 2 tablespoons per quart of bottled lemon juice or citric acid directly to the jars before packing the tomatoes in them. This also holds true for tomato sauce and tomato paste. I've noticed that this amount of lemon juice does not alter the flavor in a lemony way but does seem to brighten it a bit which is a good thing. You may have noticed my repeated hammering of bottled lemon juice. This is because fresh lemons, like tomatoes, vary in their acidity from lemon to lemon. Bottled lemon juice has a constant reliable acidity.
Be aware that the above does not hold true for products like spaghetti sauces where you've added other low acid vegetables and spices like onion, garlic, bell/hot peppers, celery, etc. Every low acid vegetable addition to the tomatoes brings the overall acidity of the sauce down that much more, rendering it unsafe to process by boiling water bath. These kinds of sauces need to be processed in a pressure canner or frozen. If not, they become a perfect environment for the development of botulism neurotoxin and you just don't want to go there. That stuff will paralyze and kill you in no time.
Ok, I hear you. "Granma/Mom always did it this way and we never got sick." Actually, someone probably did but it got passed off as "a touch of the flu" or "a bit of a bug". Most healthy teens and adults can shake off mild bacterial food poisoning and never realize that's what it was. Unfortunately it's the very young, the elderly, and those with suppressed or damaged immune systems that will suffer the most. Don't take chances. If Granma, Mom, and food science had known then what food science knows now they would have canned differently. Back in the day dentists pulled teeth without anesthesia but I'm betting you don't want to do that now just because Granpa did. So please. Be safe. Acidify your tomatoes before water bathing and pressure can or freeze your mixed vegetable-tomato sauces.
For the most up to date information on canning tomatoes and tomato products please go to the National Center for Home Food Preservation's tomato specific page: