Yes, yes, yes, I know. The tomato can jam entries should be posted next week, not this. Consider this post a safety intervention.
The most important thing to consider when you’re canning is whether your item is a high-acid or low-acid food. Most food-born pathogens, including yeasts, molds, and most bacteria, can be killed at 212°F. Botulism, on the other hand, that nasty, invisible, food-borne pathogen we’ve all heard so much about, can survive up to somewhere around 238°F. (It’s actually the spores that can survive, but never mind that). But because botulism can’t grow in a high-acid environment, you can still can high-acid foods in a water-bath canner that will never get about 212°F. For low-acid foods, you need the additional heat of a pressure canner, which can take you up to 240°F at 10 pounds of pressure at sea-level. Let me repeat: you can’t get botulism from high-acid foods.
With me so far? Good.
Here’s the thing. Tomatoes aren’t quite a high-acid food. They’re borderline. Older varieties, many heirloom varieties, and most of the varieties that you’re likely to grow at home, frequently are. Newer varieties—particularly the kinds of tomatoes that you tend to find in grocery stores—aren’t necessarily acidic enough. It’s therefore recommended that you add 1 T of lemon juice per pint jar, or 2 T per quart jar, of tomatoes to make them safe for water-bath canning. If you don’t like lemony tomatoes, you can always pressure can them.
Which brings us back to sauce. I’ve seen many, many recipes over the years that say that you can can tomato sauce in a water-bath canner. Many of these are in reputable books that I trust, like Anna Thomas’s The New Vegetarian Epicure. But let’s think about this for a minute. Here you are, taking tomatoes, already a low-acid food. You’re adding olive oil, probably some onions, maybe some garlic, and a handful of herbs. All of these things are low-acid foods. With each one, you’re raising the pH (lowering the acidity) of your product. The more experienced I get in my own canning, the more and more queasy I am about all the people who are going to excitedly put up jars and jars of tomato sauce in a water-bath canner this summer.
The Ball Blue Book does include a version of tomato sauce safe for water-bath canning, but it contains 1 T lemon juice per pint jar and a processing time of 35 minutes (for pints). I can only conclude that if you’re not adding something more acidic than tomatoes (like lemon juice or vinegar) to your sauce, it’s probably not safe for water-bath canning, and certainly not at the 20 minutes usually recommended in non-canning cookbooks. Ergo, you might want to try pressure canning it.
Here’s one version of a meatless tomato sauce appropriate for pressure canning, using the processing times recommended by the National Center for Home Food Preservation.
Canned Tomato Sauce
10 pounds tomatoes, peeled, cored and roughly chopped
1 large onion, chopped
6 cloves garlic, minced
1/4 c. olive oil
A big handful of basil—about a cup or so—chopped
Salt to taste (I use about 1 T kosher)
1) Heat the oil is a large, wide stockpot. The wider the stockpot, the faster your sauce will cook. Sauté the onions and the garlic in the oil until they’re translucent.
2) Add the remaining ingredients and bring to a rapid boil. Turn the heat down to medium and cook until the sauce has reduced by about half. This may take several hours, depending on your stove and your pot. If you want, you can run the sauce through a food mill at some point, but I find that if you cook it long enough, the tomatoes break down on their own.
3) Meanwhile, prepare 4 to 6 pint jars (and maybe a half-pint just in case) and an equal number of lids. Transfer the hot sauce to clean jars and adjust two-piece lids. Process pints for 20 minutes, quarts for 25 minutes at 10 pounds pressure in a steam-pressure canner.
For step-by-step instructions on pressure-canning, please see this post. Please note: I find I get a better seal, and am less likely to lose the contents of my jars, if I wait a long, long time after the pressure’s dropped before opening the lid. Like, hours.