How to Can Tomatoes

Capture a taste of summer with this step-by-step tomato canning how-to.

Freshly canned tomato sauce is a taste of summer that will last all year.

As summer's heat wanes, farms and gardens bring in the yearly bounty of tomatoes. When tomatoes peak, their abundance can be overwhelming, so preserving them is an excellent way to enjoy their bright flavor throughout the year.

For best results, you'll want tomatoes with a high amount of meat, such as plum or San Marzano, and you'll want them just ripe. Discard any bruised or overripe fruit or extremely under-ripe fruit.

Processing tomatoes is moderately labor-intensive, but will go much easier with some forethought and planning. In order to get a good batch of crushed tomatoes or puree, you'll need to take the following steps, which we'll break down in a photo step-by-step and below:

  • Core and score
  • Blanch and shock
  • Skin, seed and crush (or puree)
  • Stew
  • Sterilize
  • Can

In addition, you'll need some equipment. For approximately 20-22 pounds of tomatoes, you'll need seven quart-size mason jars with rings and new, unused lids; a ladle; a wide-mouth funnel; a pair of jar tongs; and a couple cooling racks set over towels. You'll also want a pressure canner.

Unlike pickles and most fruit preserves, tomatoes are comparatively low in acidity, and so must be acidified in order to be canned using the standard water-bath method. Foods with a pH higher than 4.6 can harbor botulism bacteria spores; tomatoes are generally right around 4.5, so you're playing with fire if you do not bring the acid level up. Moreover, if you add anything to your tomatoes, such as onions, garlic or basil, you are lowering the acidity further.

Water boils at 212ºF at sea level; this is not sufficient to kill off the botulism spores. By raising the pressure in the cooking environment, you raise the temperature at which water boils. By raising the pressure to 11 pounds, you raise the boiling temperature to about 240ºF, which will kill off the spores.

So, if you intend to do much canning of low-acid foods such as tomatoes, stocks or meats, you may want to invest in a pressure canner. Modern pressure canners are easy and exceedingly safe to use, and you will be able to rest easy knowing that your canned goods are free of toxins.

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