A canning tomato is a variety of Solanum lycopersicum specifically developed for preservation rather than for fresh consumption. Selecting the correct tomato type significantly influences the final quality, texture, and safety of the preserved product. Choosing a tomato with the right physical and chemical properties ensures that the resulting sauce, paste, or whole peeled tomatoes retain their desired characteristics through the high-heat processing of home canning. This choice prevents a watery final product and ensures safe, long-term storage.
Physical Traits That Matter
Canning tomatoes are typically paste-type varieties distinguished by a dense internal structure, setting them apart from slicing tomatoes. These paste tomatoes possess thick walls and a significantly higher ratio of flesh to gel material and seeds. This results in a low water content, measured as a high percentage of total solids. When cooked, this low moisture content means the tomatoes require less time to reduce into a thick sauce or paste, making the preservation process more efficient.
The structural integrity of the canned product is supported by the fruit’s natural chemistry, particularly its pectin content. Pectin is a complex polysaccharide that acts as a cementing agent within the cell walls. Paste tomatoes often contain higher levels of pectin, which helps the fruit maintain its shape and prevents mushiness during the heat-processing stage. Selecting firm fruit is also important, as firmness is linked to a higher drained weight and better overall texture in the final canned goods.
Acidity and Safe Preservation
The most important consideration in canning is the food safety risk posed by the bacterium Clostridium botulinum. This microorganism thrives in oxygen-free environments, such as a sealed canning jar, when the acidity level is too low. The growth of C. botulinum spores is inhibited when the food’s pH level is 4.6 or lower, which is the established threshold for safe preservation using a boiling water canner.
Tomatoes are naturally considered an acid food, but they are classified as a “borderline” case because some varieties can have a pH slightly above 4.6. For this reason, current food safety guidelines recommend that all home-canned tomatoes be acidified to ensure the pH remains below the critical threshold. This is achieved by adding a measured amount of bottled lemon juice or citric acid directly to the jars before sealing and processing. The required addition of acid overrides natural variations in the tomato’s acidity, providing a reliable defense against the germination of C. botulinum spores.
Recommended Tomato Types
The best choices for canning are those varieties that consistently deliver the desired structural and chemical profile for preservation. Roma tomatoes are a widely recognized standard, valued for their dense flesh, small size, and balanced flavor profile, making them highly versatile for pastes, sauces, and dicing. Another highly regarded choice is the San Marzano, an Italian heirloom variety known for its elongated shape, thick flesh, and minimal seed count, which is particularly prized for making smooth, whole-peeled canned tomatoes and sauces.
Amish Paste is a large, meaty heirloom variety that offers a rich, complex flavor that intensifies upon cooking. Like the other paste types, it has fewer seeds and less water content than traditional slicing tomatoes. These recommended varieties consistently exhibit the low moisture and dense texture necessary to produce high-quality, non-watery canned goods, ensuring a satisfying final product.