I agree with Greene. A work around if you are growing your own tomatoes is to plant high acidic tomatoes. Many of the newer hybrids are specifically bred to be low acid, which is what prompted the addition of an acid. You might try adding powdered citric acid rather than lemon juice, although I don't know if/how that affects the flavor. The other option is to pressure can your tomato juice, in which case there is no need to add any additional acid.
