To make sure you end up with a safe product, it's important to get the acid right. Tomatoes used to have a higher acid count, but these days, experts say that newer varieties of tomatoes have a lower pH. In order to get the acid level to the desired point, add two tablespoons of bottled lemon juice for every quart of tomatoes you're canning. One-half of a teaspoon of citric acid will also do the trick. And don't worry about the taste because you can always add a little sugar to offset the lemon.