pH’s role in home canning

All food to be preserved at home through home canning must be processed either via water bath canning, or pressure canning.

Which technique is used is determined by the pH of the food product, which we’ll explain in a second. If a food is low pH, then it’s water-bathed; if it’s high pH, then it’s pressure-canned. That’s why pH matters in home canning.

What is pH?

pH is a scale that measures how acidic something is (or isn’t).

There’s disagreement about what pH actually stands for or means as an acronym. Most people agree that the H stands for hydrogen, but people debate whether the p stands for “power” or “potential.”  ^[1] United States EPA. What is pH? https://www.epa.gov/acidrain/measure/ph.html Accessed March 2015.

How does the pH scale work?

The scale was invented in 1909 and refined in 1924.

It appears befuddling at first glance because

(a) it’s based on 0 to 14, instead of 1 to 10 as might appear to be more user friendly; and

(b) the lower the number, the higher the acidity, which is what we care about in water bath canning.  0 is the most acidic; 14 is the least acidic. So it’s the opposite — low equals high.

The neutral middle is 7. Pure, distilled water is very close to 7.

And, the pH scale is a bit like the Richter earthquake scale, in that a teeny increment matters.

In the pH scale, 3.0 is 10 times more acidic than 4.0.

“The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6.”  ^[2] Ophardt, Charles E. Virtual Chembook. Elmhurst College. Acids and Bases: pH Scale. Accessed March 2015 at https://www.elmhurst.edu/~chm/vchembook/184ph.html

So a difference between, say 4.3 and 4.6 is actually more significant than the .3 difference might seem.

That was a lot to take in, but don’t worry about it: the next section has the actual take-home information for you to absorb.

What is the pH number to memorize?

The middle ground of pH, for other sciences, might be 7.0, but forget about that: for the science of home canning, the middle ground is 4.6. “A low pH, below 4.6, will prevent the growth of potentially deadly spoilage bacteria in canned foods.”  ^[3] McGlynn, William. Choosing and using a pH meter for food products. Oklahoma State University Extension. FAPC-117. Accessed March 2015 at https://fapc.biz/files/factsheets/fapc117.pdf.

4.6 is the magic number for us to memorize. Anything below that is high acid and can be water-bathed. Anything above that is low-acid and has to be pressure-canned.

In evaluating a canning recipe for water bathing, you just have to learn to think, how far below 4.6 will the end product be. The further the better.

Pickles are quite low pH food products: “The pH of acidified cucumber pickles is typically between 3.4 and 4.1.”  ^[4] Bredit, Frederick, et al. Use of Linear Models for Thermal Processing of Acidified Foods. In: Food Protection Trends, Vol. 30, No. 5, 2010. Pages 268–272. Accessed March 2015 at https://www.foodsafety.wisc.edu/business_food/files/Lethality_TimeTemp.pdf

It’s also important that a recipe to be water-bathed has something about it that drives that low pH level into each and every piece of food in the jar: either simmering, or heat treatment of the jar to help drive the vinegar into the pickles quickly, etc.

Why is 4.6 cited as the magic number?

In home canning, the main focus is on Clostridium botulinum. The reason is two-fold: (a) it really is the most dire concern of all, and (b) on the bright side, if you’ve covered for botulism properly with the addition of a heat treatment, you’ve covered for everything.

4.6 is the pH point below which Clostridium botulinum spores can’t produce the botulism toxin. So in home canning, if a recipe can render them ineffective with a low pH, then the resultant food product can be water-bathed. If you can’t render them ineffective with a low pH, then you have to pressure can the food product to kill the spores off outright. Boiling water temperatures won’t kill the spores; pressure canning temperatures will.

“The pH 4.6 value was based on data showing that spores of Clostridium botulinum will not germinate and produce neurotoxin at or below pH 4.6.”  ^[5] Bredit, Frederick, et al. Use of Linear Models for Thermal Processing of Acidified Foods. In: Food Protection Trends, Vol. 30, No. 5, 2010. Pages 268–272. Accessed March 2015 at https://www.foodsafety.wisc.edu/business_food/files/Lethality_TimeTemp.pdf

In effect, commercial canners aim for a pH lower than that, of 4.3 or lower, to give themselves a safety margin.  ^[6] Harris, Linda J. and Sheryl Yamamoto. Master Food Preserver Slide Presentation. University of California Cooperative Extension. 9 October 2014. Accessed June 2015 at https://ucanr.edu/sites/ucmg2014conference/files/200476.pdf . Page 18

So is a low pH enough for complete food safety?

No. Note that in the previous section it reads, “if you’ve covered for botulism properly, you’ve covered for everything.”

Part of the properly is that low-pH food products must also be heat processed.

People have got e. coli and salmonella from apple cider and orange juice. Even though the pH of those products is usually between 3.5 and 4.0, that acidity hasn’t helped. ^[7] Breidt, Fred. Determination of 5-Log Reduction Times for Food Pathogens in Acidified Cucumbers during Storage at 10 and 25 C. In: Journal of Food Protection, Vol. 70, No. 11, 2007, Pages 2638–2641. Accessed March 2015 at https://www.foodsafety.wisc.edu/business_food/files/JFP_coldpack.pdf

That’s why open-kettle canning — just slopping hot jams, relishes, and pickles into a jar and whacking a lid on and waiting for a seal — isn’t safe.

The combination of acidity and heat is what does the trick to make low pH products safe. Boiling water level temperatures won’t kill botulism spores, but those temperatures deal handily with everything else. And the acidity deals with the botulism spores. So it’s a nice combo.

Boiling water treatment of the jars also kill off invisible moulds inside your jar, which don’t mind acidity and can grow invisibly, raising the pH of your product above 4.6 and allowing those botulism spores to spring to life.

Pressure canning brings on such extreme temperatures that the pH of a product doesn’t matter: the temperatures alone kill everything. But, we don’t pressure can everything. When the pH is acceptable, we water bath it. The reason is for reasons of food quality and palatability. Pressure-canned jams, relishes and pickles would be safe, all right: they’d be safe if only because no one would want to eat them. No one would touch them with a ten-foot pole.

An additional role of pH in home canning

pH plays another role in home canning, too: getting classic pectins to gel for jams and jellies:  the classic (‘high methoxyl’) pectin molecule ‘will not gel above a pH of 4.6. It’s actually quite a bit lower than that.’  ^[8] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. At 44:00. Accessed January 2015 at https://nchfp.uga.edu/multimedia/video/nchfp.wmv [Ed: between 2.8 and 3.6 ]

Why you don’t have to worry about pH in recipes

You don’t have to worry about pH in your home canning if you use tested recipes from trusted, science-based authorities that take that all well into account.

If, however, you are using recipes from authorities which are “experience-based”  or “ain’t no one died just yet” based, rather than “science-based”, working with proven pH in recipes is another good reason to switch sources for your home preserving recipes.

People do see pH being mentioned, however, and today’s home canners want to know why. This page was for those curious minds.

 Reference

USDA. Approximate pH of Foods and Food Products. April 2007.

pH Values of Common Foods and Ingredients. Clemson University.