Last weekend I canned salsa for the first time. In the past I’ve made some refrigerator salsa, so the acidity was not so much a concern, but this time there were enough tomatoes and peppers to do the hot water bath method. I wanted to know how much lemon juice to add to the salsa to make it safe. I wanted to use enough but not a huge excess. The publications about canning salsa by state extension services conveyed a sense of extreme caution: better to add way too much vinegar or lemon juice and for gosh sakes don’t ever change the non-tomato components by one iota from the recipe or grave consequences may occur.
Well I knew I did not want to use vinegar as the acid component. I like vinegar pickles, but prefer the taste of citrus in salsa. One recipe that called for vinegar cautioned that you could only exchange an equal amount of lemon juice for the vinegar, but I remember reading that lemon juice is about twice as acidic as vinegar, which seems to indicate that half the amount of lemon juice can be substituted for vinegar, something pointed out in other publications. All the extensions seem to operate from an overwhelming fear of litigation.
I did a little research. Not a lot. I wasn’t searching the peer-reviewed nutrition journals. Google was my research tool. But I did find some interesting information.
First a word about pH. It’s a logarithmic expression that represents the hydrogen ion concentration in water or aqueous medium. The lower the pH the more acidic. The important thing to remember is that something with a pH of 4 is 10 times as acidic as something with a pH of 5, and 100 times as acidic as something with a pH of 6, in other words a difference of one pH unit is actually a ten fold difference in concentration.
Vinegar contains acetic acid, usually adjusted to 5%. The acid in lemon juice is mostly citric acid. By regulation the lemon juice in bottles is required to be at least 4.5%. Both acids are weak organic acids. This means that only a small percentage of the acid molecules in a solution actually 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