Friday, August 9, 2013

Canning salsa and pH

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 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 addition 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.

So here’s how I made my batch of canned salsa, which is how I’ve made it in the past except for the acid component.  First I processed the tomatoes, Supersonic and San Marzano, getting 8 cups of pulp.  Red sweet peppers were grilled and then de-skinned and de-seeded.  From that I got 2 cups of pepper pulp.  This stuff would be fantastic layered on hummus.  The two pulps were combined in a blender.

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).

I hope you find this information useful.    

1 comment:

Anonymous said...

Very useful; thank you.

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