Tomatoes may have that tasty zing that makes them tart and tasty. But in reality, they are not as acidic as they seem, especially when canning tomatoes.
Tomatoes have a pH value around 4.6 which makes them unsafe to can by themselves, with many varieties above 4.6. All tomatoes must be acidified with either citric acid, bottled lemon juice, or vinegar with 5% acidity in both water bath and pressure canning processing.
Without this added acid, tomatoes will likely ferment and spoil. Learn more in Preserve it Fresh, Preserve it Safe: Tomatoes.