y dissociate, which is when the positive hydrogen ion breaks off the molecule. A strong acid like HCl will completely dissociate in water. What they all have in common is the production of hydrogen ions.
What is a hydrogen ion? Well a hydrogen atom is the simplest atom, just a proton and an electron. When the positive hydrogen ion breaks loose from a citric acid or acetic acid molecule, it is simply a proton breaking loose from the acid molecule, leaving behind its electron. It's believed that a loose proton attaches to a water molecule to form a hydronium ion, H3O+. That is the essence of acidity in water - protons breaking off the mother molecule and attaching to water. Of course chemists have made the concept of acidity much more complicated with esoteric theories that are mainly useful to chemists but that’s another story. The essential concept of acidity is what is useful here. Acidity has profound effects on the chemical processes of life.
Both acetic acid and citric acid are weak organic acids. Citric acid is stronger than acetic acid, in other words a greater percentage of citric acid molecules will give off a proton compared to acetic acid. Which is why you don’t need as much citric acid as acetic acid to achieve a certain acidity.
So what acidity is a safe acidity for boiling water canning? Boiling water will not kill some bacterial spores, but sufficient acidity will keep them from growing. From what I’ve read the safe threshold is a pH of 4.6 or less. Somewhere somehow somebody came up with that number, and I'm guessing that there is a built in safety margin with that pH. Tomatoes have a pH of around 5, not quite acidic enough, so some acidifying agent must be added.
Of course all the canning recipes published by the state extensions will caution you that the other ingredients in salsa – peppers, onions, herbs and spices – are not as acidic as the tomatoes, so YOU MUST NOT DEVIATE FROM THE PROPORTIONS IN THE RECIPE or the salsa may not be sufficiently acidic (and you may get sick or worse). I sensed a lot of overkill here, and my experience as a bench chemist informed me that small changes in the non-tomato ingredients in a salsa that is predominantly tomatoes will have little effect on the acidity of the salsa. Surely someone with some math skills and a bit of common sense can keep their salsa at a safely acidic level.
I found a study called Safe acidification of salsa for home boiling water canning from the Foods and Nutrition department at the University of Georgia. For some reason this group seemed to believe that it was necessary to bring the pH of the salsa to around 3.8, which requires 4 tablespoons of lemon juice per pint. Some quick calculations on the old Texas Instruments scientific calculator told me that a pH of 3.8 is over six times as acidic as a pH of 4.6, the threshold pH. I guess they wanted to make sure that the salsa would be sufficiently acidic no matter what ingredients were used. Their recipe called for 200 g tomatoes, 200 grams other ingredients and 60 milliliters lemon juice (4 tablespoons) which works out to about a pint of salsa. Their research showed that you could use the ingredients in any combination, even leaving out the tomatoes, and the addition of that much lemon juice produced a pH of about 3.8. In other words, nearly all the acidity in the salsa came from the lemon juice.
This publication by the North Dakota State University extension, Why add lemon juice to salsa before canning? has a lot of useful information. First of all they tested the acidity of different varieties of crushed tomatoes. The pH ranged from 4.95 to 5.20, but most tomatoes were about pH 5. Then they tested the pH of salsa made from the same varieties before and after the addition of lemon juice. Guess what, the pH of the salsa before adding lemon juice was actually slightly lower than the pH of the tomato pulp. Not much lower but lower, indicating that the additional ingredients really have little effect on the acidity (onions and peppers are also slightly acidic but not as acidic as tomatoes). The addit