There are many methods and means of canning tomato sauce, and cooks would be wise to do their research and a bit of planning before beginning. Some of the best tips include starting simple, removing the tomatoes’ skins and seeds and using a pressure cooker to avoid potential contamination. There is no defined "right" way to go about canning tomato sauce, however. Cooks often add spices, other vegetables and even meats to their sauces before canning them, and some people like the taste and texture that are lent by seeds and skins. If the tomatoes are acidic enough, water bath canning usually also works just fine.
The first thing a cook should do when canning tomato sauce is to select the tomatoes. To be good for sauce, tomatoes should be ripe and deep red but still firm to the touch. Most recipes call for several bushels’ worth of tomatoes, which makes canning sauces a good way to get rid of bountiful garden harvests. Gardeners who have huge tomato crops often boil the fruits down to sauce as a way of preserving them through the winter or as a way of minimizing garden waste. Purchasing enough tomatoes to make a sauce, unless from a local farmer’s market, often negates any cost savings and can be quite expensive.
Recipe selection is another essential part of canning tomato sauce. The most basic recipes involve little more than blanching, blending and peeling the fruits, then simmering them down, often with just a pinch of salt. More complex recipes require cooks to combine the tomatoes with other vegetables, such as squash or eggplant; to add ground meat; or to mix in certain amounts of herbs and spices. Most of the time, the simmering phase of canning tomato sauce is quite lengthy — often several hours or more — and spice proportions must be adjusted accordingly.
Cooks also must decide whether they want to remove the tomatoes’ skin and seeds. Recipes are largely split over whether these steps are necessary. It usually is much easier to simply leave the fruits whole, but skins and seeds often lend a tart flavor to the finished product. Cooks can remove the skins through blanching, which is basically a boiling water bath, then can strain out the seeds by sieving or running the boiled fruits through a food mill.
Another tip for canning tomato sauce relates to how the cans are actually processed. Most fruits that have high acid concentrations can be canned without issue in a boiling water bath. Boiling water baths use prolonged exposure to boiling water to sterilize and seal jars that are used for preserving foods. Tomatoes usually are considered to have moderate acid levels. Cooks who elect to use the water bath method usually add a bit of lemon juice or vinegar to the sauce to ensure that its acid levels are high enough for safety.
Otherwise, cooks should go about canning tomato sauce in a pressure canner. Pressure canners work best for low-acid fruits and vegetables because it uses heated pressure — not just temperature — to seal and sterilize the canning jars. Food safety experts never recommend open kettle canning for tomato sauce, unless it is to be consumed immediately. The open-kettle process is generally frowned upon for long-term canning at home but can be particularly risky for bottling food that has moderate to low acid content, as tomatoes do.