Tips for Canning Tomatoes
Choose the Best: When picking tomatoes, choose the freshest you can find. Ones that are locally grown tend to be the most flavorful. Choose tomatoes that are fully ripe, smell fruity, and feel heavy. Make sure they don't have any bruises or cracks because those can harbor hidden mold or bacteria that can ruin your processed jars.
We tested all manner of tomatoes during our recipe development and liked plum (often called roma) tomatoes the best. They taste sweet with a strong tomato flavor and are the perfect combination of juicy and sturdy. Their size makes them easy to pack into jars and they really hold their shape well. Big beefsteak tomatoes were less juicy, with a firmer structure. They took longer to break down in recipes, so we ended up cutting them into smaller pieces. We also found beefsteaks to be less flavorful than plum tomatoes. Heirloom tomatoes encompass so many types that it is hard to generalize about them. We did not test our recipes using either grape tomatoes or cherry tomatoes.
Acidify the Jars: Tomatoes vary widely in natural acidity and ride the pH borderline (4.6) between high and low acid foods. In order to ensure a safe pH level for boiling water canning, we add bottled lemon or lime juice or vinegar to the hot jars before adding the tomatoes to up their acidity. (We use bottled juice, which has a consistent pH, instead of fresh lemons or limes, which can vary in acidity from lemon to lemon.)