ion of lemon juice brought the pH down to a range between 4.14 to 4.30, well below the 4.6 threshold. It looks to me like the 4 tablespoons of lemon juice per pint used by the U of Georgia group is gross overkill.
But there’s a fly in the ointment here. The NDSU paper never said how much lemon juice was added in the study. They recommend adding one tablespoon per pint, but their own recipe in the same paper which makes 16 pints of salsa calls for 2 cups of lemon juice which works out to 2 tablespoons per pint.
I went back to the acidification curves from the Georgia paper, where they added lemon juice in increments to tomatoes, peppers and onions and then tested the pH. Interestingly they never did an acidification curve for the salsa, just the components so I considered the tomato pulp the same as salsa. To compare apples to apples I converted everything to percentages. A tablespoon of lemon juice per pint works out to 3.12%. In the Georgia study 5 mL lemon juice in 100 mL Roma pulp, the lowest amount tested, works out to 4.8% lemon juice, about 1.5 tablespoons per pint. That amount produced a pH of 4.2. A salsa with a pH of 4.2 is 2.5 times as acidic as a salsa with a pH of 4.6. Looking at the titration curves and estimating the lines between the points it looks like a tablespoon per pint gets the acidity to a pH of about 4.4. To be solidly on the safe side it seems to me that a minimum amount of lemon juice should be 1.5 tablespoons per pint. The take-home message is this: there is considerably more acidity in a tablespoon of lemon juice than in a pint of tomatoes.
That leaves one other question: Do you really have to use the bottled lemon or lime juice? The canned salsa recipes stress that the bottled version has a consistent acidity, whereas real lemons or limes will not, so you should always use the bottled lemon juice, according to them. Of course real limes or lemons will taste better. To answer that question there is a post Real Lemon versus ReaLemon by Linda Ziedrich, who has a chemistry background. She titrated some ReaLemon made from concentrate and found that it tested at 4.9% citric acid, exceeding the federal standard of 4.5%. She also found a study that tested the acidity of lemons from Florida and California, and the range was 4.53% to 7.3%, all of them exceeding the standard. Then she titrated a supermarket lemon: 6.0% acidity. It looks like a fresh lemon will provide at the least the acid content of the bottled stuff, and even if one lemon or lime is below the standard it is very likely that by using several lemons or limes the average acidity will be greater than the standard. Given that one and a half tablespoons of bottled lemon juice brings a pint of salsa to a pH of about 4.2, even if a lemon or lime was a bit below the standard 4.5% acidity there’s enough margin of safety that it would have little effect.
I cut up two medium onions and sweated them at low heat in a pan coated with a little oil, long enough to remove the pungency and added them to the mix. Garlic, parsley, and oregano were chopped and added. For hot peppers I used one jalapeno and one serrano, deseeded, for each pint. I also threw in a tobasco for the batch.
Then there’s the acid component. I like the taste of lime in salsa. Two limes gave up 3 oz of juice. The batch produced six pints of salsa, so 3 oz works out to 1 tablespoon per pint, the amount recommended by the NDSU publication. Just to be on the safe side I added 1.5 oz of the bottled lemon concentrate, for 1.5 tablespoon per pint. Anyway that’s how I made the salsa. In a few days there will be enough tomatoes for another batch. This time I’ll buy more limes and use more serranos. (Those are refrigerator okra pickles next to the salsa).