Tomatoes, because of their acid content, have in past years been considered safe to can in a boiling-water bath. However, in recent years hybridizers have developed "sub-acid" varieties that may be on the borderline of acid content where the safe canning of tomatoes or their juice is concerned. to be on the safe side, taste the juice before canning it. If it lacks tartness, be sure to add enough lemon juice to re-create the characteristic pleasant tomato sharpness.