Why Process Tomatoes?

Tomatoes cooked for the week's upcoming meals do not need to be pasteurized. As long as they are eaten within a few days and kept cool in the refrigerator, they do not pose a risk of harmful bacteria growth. However, when you cook tomatoes in summer or fall for storing during winter, typically referred to as home-canned tomatoes, processing is necessary to prevent the growth of bacteria during storage. A certain amount of heat applied for a particular amount of time kills the bacteria in the tomatoes and prevents them from growing.

Preparing Your Batch

Beginning with firm, healthy tomatoes and clean jars is vital to safely processing your tomatoes. Wash the tomatoes in cold water and then remove the skin. Dipping a tomato into boiling water until the skin cracks and then plunging it into ice water makes quick work of removing the skin. The cold water causes the skin to split, allowing you to slip it off the tomato. Cutting the tomato into halves or quarters gets them ready for canning. Wash jars, covers and lids in hot soapy water. Presterilizing the jars before canning fruits or vegetables that are processed in boiling water for at least 10 minutes is not necessary, says the National Center for Home Food Preservation.

Let's Start Packing

Packing the cut tomatoes into the jar is the easy part. Simply fill the jars and pack the tomatoes in tightly, because they will shrink in the processing. When it comes to tomatoes, acidity is as crucial as heat in ensuring food safety. Add 2 tablespoons of lemon juice or 1/2 teaspoon of citric acid to each quart jar to acidify the tomatoes and make them safe for processing and storage. You may wish to add a pinch of sugar to offset the tartness of the lemon juice. Adding 1 teaspoon of salt to each quart jar gives the characteristic canned tomato flavor, but you can use less salt or omit it if you prefer. Cover the tomatoes with tomato juice or water, leaving 1/2 inch of headspace. Check that all tomatoes are fully covered with liquid.

Choosing a Canner

Canners come in two basic varieties: boiling water bath canners and pressure canners. Regardless of canner type, the water needs to be boiling to process the tomatoes. Where they differ is in the required processing time. Pressure canners also require a pressure setting given in pounds per square inch, usually referred to as PSI. Pressure canners allow you to process foods faster and more efficiently than traditional boiling water bath canners, but they are not necessary for tomatoes. If you do not have a canner, a large kettle that provides enough room to accommodate your jars with an additional 1 or 2 inches of water at a full, rolling boil can be used.

Getting It Done

Place the jars in the canner and cover with water, allowing 1 to 2 inches of water above the jars. Processing tomatoes in boiling water bath canners takes 85 to 100 minutes, depending on your altitude. Altitudes of 1,000 feet or lower require 85 minutes, while those above 6,000 feet require 100 minutes to adequately process tomatoes and ensure safety. Processing tomatoes in a weighted-gauge pressure canner takes 40 minutes if set at 5 PSI for altitudes up to 1,000 feet or 10 PSI for altitudes above 1,000 feet. Pressure settings for dial-gauge pressure canners range from 6 to 9 PSI, depending on the altitude, when processing for 40 minutes; 6 PSI is recommended by the National Center for Home Food Preservation for up to 2,000 feet, with 1 PSI added for every additional 2,000 feet.