Tomato

Tomato is a fruit that cooks like a vegetable. For preserving, the important fact is acidity, not the salad course.

NCHFP, adapting USDA home-canning guidance: low-acid foods have pH above 4.6 and include all fresh vegetables except most tomatoes. Although tomatoes usually are considered an acid food, some now have pH slightly above 4.6. If they are to be canned as acid foods, they must be acidified to pH 4.6 or lower with lemon juice or Citric acid ¹. Properly acidified tomatoes may use a boiling-water process. Do not guess the acid amount. Use the tested recipe. Lemon juice here is a safety ingredient, not salad dressing. Craft: Canning and Acidity.

Jam. U.S. jam identity lists tomato (and yellow tomato) among Group I fruits ². That is a labeling standard for a fruit preserve, not a kitchen obligation to make tomato jam. Kitchen jam is Jam.

What to notice. Smell at the stem; a ripe tomato smells like tomato, not like refrigerator. Watery collapse is texture, not acidity. pH is a meter or a tested process, not a tasting note.

Food safety before you put tomatoes in a pantry jar.