Why add lemon juice when canning tomato sauce?

Why add lemon juice when canning tomato sauce?

When I published my article on making tomato sauce with the assistance of the Instant Pot, a number of people commented that I had left out the lemon juice. They referred me to this slightly misinformed warning article.  More to the point, the USDA recommends adding 1 Tb of lemon juice per pint of sauce.

In fact, the USDA, on a site hosted by the University of Georgia, explains that the pH of canned tomato sauce must be at or below 4.6 to prevent the growth of botulism. This sounds like really good advice, but we have been canning tomato sauce for over 30 years without adding lemon juice, and no one has had any ill effects.

The pH value is a measure of the acidity of a solution, here of tomato juice, and the lower the pH the higher the acidity. Thus foods having a pH of 4 are more acidic than those with a pH of 5. This Is a logarithmic scale, so foods with a pH of 4 are ten times as acidic as those with a pH of 5.

So we decided to look into this a little further. It turns out that we aren’t the first. The University of North Dakota Ag Extension in 2007 looked into the pH of a number of popular tomato varieties that you might use in making salsa. They measured the pH of the tomatoes, of the salsa and of the salsa with lemon juice, and found that only the salsas with added lemon juice had a pH below 4.6.  These were grown in Williston, ND and probably in a greenhouse, so the pH values might differ from the garden and in warmer states.

More recently, in 2010 Heflebower and Washburn at Utah State measured the acidity of juice from a number of popular varieties, including Celebrity and Rutgers, finding that the pH varied from 3.92 to 4.32. Clearly sauce from these varieties need not be further acidified. They also found that pH didn’t vary much based on maturity of the fruit, nor on whether a new or an heirloom variety was tested. However, since tomato sauce may vary with the mixture of fruits you use as well as the weather conditions, they suggest that it would not be unwise to continue to add lemon juice.

However, since 2010, there have been a number of tomato varieties bred especially for flavor, and we decided to test the pH of the 8 varieties growing in our garden. We used a THZY portable pH meter, calibrated with the supplied buffer solution. We squeezed juice out of part of each tomato and filtered it through a coffee filter into a freshly washed glass, rinsed with distilled water.

Here are our pH readings

Opalka * 4.62
Lemon Boy 4.45
Garden Gem ** 4.30
Amish Paste * 4.14
Fourth of July 4.14
Better Boy 4.11
Cloudy Day 4.11
Garden Treasure ** 3.91

* Heirloom
** Recently developed

The Opalka variety has been a reliable paste tomato for years, but obviously if you use it, you must add lemon juice to your sauce. If it is only one of many, you may not need to. Note that Lemon Boy, a mild yellow tomato is the only other one even close to a pH of 4.6.

Professor Klee at the University of Florida developed the Garden Gem and Garden Treasure varieties (**) with significantly a improved taste, and we recommend them highly.

What about the sauce?

The pH of the final sauce will be influenced by the ingredients you add: in our case onions and spices. We tested the pH of sauce from 2015 and from last week. They were 4.18 and 4.33 respectively, and thus perfectly safe. Interestingly, it was the 2015 sauce that used Opalkas. This year they haven’t done well and none was in last week’s batch.



You would probably be safe without adding lemon juice, but a tablespoon of lemon juice will make a substantial change in the pH and in your safety. We found that adding a tablespoon of lemon juice to a pint of distilled water reduced its pH from 5.86 to 2.96. Thr scientists at the University of North Dakota found that the lemon juice reduced the pH of salsa by only 0.3 pH units. And don’t worry about the possible sourness: one or two teaspoons of sugar will easily cancel it out. In the batch we made yesterday, we added 1 Tb of lemon juice and 1 tsp of sugar to the bottom of each pint jat.

What is pH? [a sidebar]

The concentration of acid, or specifically of hydrogen ions (H+) in a water solution can vary from 100 (or 1.0) to 10-14. Since this is hard to write down, we usually refer to the concentration by the exponent of 10 or 0 to -14. And, in order to make this more convenient we remove the minus sign, so pH values run from 0 to 14. Neutral pH (neither acidic nor basic) is pH 7, where the concentration of hydrogen ions and hydroxyl ions (OH) are equal.

So if we want to write down the acid concentration equivalent to a ph of 4.6, that means 10-4.6 and that is the same as.00002512.

And what are the units of this concentration? It is moles per liter, where a mole is the molecular weight of an element in grams. A gram of hydrogen ions (or of hydrogen itself for that matter) is one mole. It turns out that a mole of any element or compound has the same number of particles (ion, atoms or molecules) and that number is called Avogadro’s number. While Avogadro proposed it, it was first calculated by Loschmidt to be 6.02 x 1023 particles.



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