Acidity / low pH is what makes water bath canning safe

Heating your filled and covered jars in a hot water bath (or steam canner) kills off a lot of things. But it won’t kill botulism spores; the food will just never get hot enough. But, if the food is high enough in acidity, you don’t have to kill them. High acid levels (levels below 4.6 pH) prevent botulism spores from germinating and giving off their deadly toxin as a by-product. So appropriate heating kills off most things (such as campylobacter bacteria, listeria, salmonella, moulds, etc), and acidity suppresses the rest.

What does the acidity do to any botulism spores?

Just remember, high acid == low pH. A bit confusing at first, for sure.

When people talk about acidification of a recipe, they basically mean “lower-pH-i-fication”, but “acidification” is far easier to say.

To be clear: the acidity just prevents any botulism spores from doing anything harmful. It doesn’t kill them off:

 Clostridium-botulinum type A and type B spores were stored in tomato juice (pH 4.2) and citric acid-phosphate buffer (pH 4.2) at 4, 22, and 32 degrees C for 180 days. The spore count was determined at different intervals over the 180-day storage period. There was no significant decrease in the number of type A spores in either the tomato juice or citric acid-phosphate buffer stored for 180 days at 4, 22, and 32 degrees C. The number of type B spores did not decrease when storage was at 4 degrees C, but there was an approximately 30% decrease in the number of spores after 180 days of storage at 22 and 32 degrees C.”  ^[1] T E Odlaug and I J Pflug. Effect of storage time and temperature on the survival of Clostridium botulinum spores in acid media. Appl Environ Microbiol. 1977 July; 34(1): 30-33

Don’t overboil pickling liquids

Home canning author Linda Ziedrich notes,

Take care not to boil pickling liquid for a long time, since acetic acid evaporates faster than water does. Boil pickling liquid only as long as the recipe calls for. Then, if you’re not ready to use the liquid immediately, remove the pan from the heat and cover it.”  ^[2] Ziedrich, Linda. The Joy of Pickling. Boston, Massachusetts: The Harvard Common Press. 2009. Page 11.

The Putting Food By authors advise the same,

Avoid boiling for a long time any vinegar solution that is to be used as the canning liquid for pickles, because the acetic acid in vinegar is rather volatile, and it will lose its ability to keep stored pickles safe.”  ^[3] Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 316). Penguin Publishing Group. Kindle Edition. Note: Healthy Canning occasionally cites Putting Food By. This doesn’t constitute an endorsement of the entire book per se.

How strong are the various acids in home canning?

The National Center for Home Food Preservation says,

Commonly used acids in home canning are vinegar and lemon juice. Lemon juice is more acidic than vinegar, but has less effect on flavor. Use only vinegar that is at least 5% acid and use only commercially bottled lemon juice. If you wish, you may safely substitute an equal amount of lemon juice for vinegar in recipes using vinegar. Do not substitute vinegar for lemon juice.”  ^[4] Ingredients for Salsa Recipes. Accessed March 2015.

Washington State Extension says,

Lemon juice is more acidic than vinegar, but has less effect on flavor. Use only vinegar that is at least five percent acid and use only bottled lemon juice. If you wish, you may safely substitute an equal amount of lemon juice for vinegar in recipes using vinegar. Do not substitute vinegar for lemon juice. This substitution will result in a less acid and potentially unsafe [product.]”   ^[5]Hillers, Val and Richard Dougherty. Salsa Recipes for Canning. Washington State University Cooperative Extension. Accessed January 2015 at https://extension.usu.edu/utah/files/uploads/Recipes-Cooking-Canning/Salsa%20Recipes%20for%20Canning.pdf.

Bottled lemon juice packs quite a punch. Researchers found that “60 ml lemon juice per pint safely acidified full pint volumes (263-304 g) of onions, green peppers, or jalapeños alone to below pH 3.82 … Using the correct amount of bottled lemon juice (¼ cup per pint) full pint volumes of either onions or bell peppers are safely acidified. This helps provide a safer recipe despite the possibility of consumer error.”  ^[6] B. A. Nummer, M. Thacker, E. M. D’Sa, and E. L. Andress . Studies on safe acidification of salsa for home boiling water canning. Paper 33C-9. Presented at the Institute of Food Technologists Annual Meeting, Las Vegas, NV, July 14, 2004. Accessed March 2015.

Be wary of rice vinegars: most are less than 5%, though you will find one or two that are 5% or higher. Check the labels to make sure you are using a 5% or higher ones.

Wine vinegars are usually higher than 5%, but, again, check the label to make sure.

We’re stuck in a rut…..

The current realm of tested recipes is pretty stuck in the rut of white and cider vinegar, lemon and lime juice to lower pH.  But there are also other things in the world, such as tamarind which, with a pH of 3.0, is lower than cider vinegar (3.1)  ^[7] USDA. Approximate pH of Foods and Food Products. April 2007. Accessed March 2015 at https://foodscience.caes.uga.edu/extension/documents/FDAapproximatepHoffoodslacf-phs.pdf. , and wine.

There’s also many more wonderful vinegars that the people developing our recipes could try: cane vinegar, coconut vinegar, etc (again, provided they are 5% acid or higher.)

Citric acid with its almost indistinguishable taste is used in a few recipes, but far more tested recipes could be developed using it. (See: Citric Acid and home canning.)

Fermented foods such as sauerkraut and fermented pickles count as acidic because the fermentation process lowers the pH. (Caution: never make fermented foods such as those salt-free.)

In any event, there are many other ways to acidify foods for water-bath canning. Over time, perhaps more tested recipes drawing on them will be added to the canon of safe, tested recipes what we use.

The problem with wine

You will see some more gourmet home canning recipes for water bathing from reputable sources that include wine but no vinegar and wonder where the “acid” is coming from; they’d have drawn on the wine for that in their testing.

Most wine pH’s fall around 3 or 4; about 3.0 to 3.4 is desirable for white wines, while about 3.3 to 3.6 is best for reds.”  ^[8] https://www.winespectator.com/drvinny/show/id/5035 . Accessed July 2015.

An issue with relying on wine is that its pH can vary so widely. By desirable, the writer above from Wine Spectator means what wine makers generally should aim for. That is no guarantee that all will aim for it, or that they will achieve it even if they do, nor is there any legal requirement.

One recipe that relies on wine for acidity is Ball’s Eggplant Pasta Sauce.  After the recipe was published in 2016, there was concern about whether a reliably low pH was always being achieved by everyone, given the wide pH range of wine. In response, Ball admitted that people could optionally add some vinegar to the pasta sauce (we’ve recommended citric acid instead to affect taste less.)

What’s the pH aimed for in food products to be water-bath canned?

The final equilibrium pH must be lower or equal to 4.6. It’s not good enough for just the brine to be below 4.6, but also inside the vegetable and fruit pieces themselves.

The reason the cut off is 4.6 is owing to the risk of botulism above 4.6.  ^[9] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. Accessed January 2015.

In practice, though, tested recipes will generally aim for 4.3 or lower:

…the goal is < pH 4.3 and lower….(that is the) pH target used by commercial canners.” ^[10]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

What is a safe water / vinegar ratio for pickles?

For various reasons, such as acetic acid boiling off, vegetable components being low acid, safety margins, etc, the water / acid level in a pickling mixture should be at least 1 to 1. A higher proportion of vinegar is fine, but not lower.

Home canning author Linda Ziedrich notes,

Fresh pickles generally should contain at least one part 5 percent vinegar for each part water.”  ^[11] Ziedrich, Linda. The Joy of Pickling. Page 11.

There are approved and tested recipes with a lower ratio both in the Ball books and at NCHFP, but should you be assessing a recipe from somewhere else, that 1 to 1 ratio is the lowest ratio considered safe if you are not using a tested, approved recipe (not that you should be, anyway.)

You could also trust a recipe from someplace like Mrs Wages that calls for one of their package mixes, as the mixes will probably have citric acid, etc, mixed in.

You might be interested in reading: Acidity of water and vinegar combinations in home canning.

What fruits have a borderline pH level?

Tomatoes, figs, Asian pears. That’s why some acid must be added to them to can them and nudge them down into the safe zone well below 4.6.

Melon is also low acid, but other than pickling, no safe canning method has been developed for it as a plain fruit, and there are no safe tested melon jam or jelly recipes (recall the outbreak of botulism in Vancouver from watermelon jelly.)

Never do anything to lower the acidity level of a canning recipe to be water bathed

Don’t follow the normal cook’s impulse and add the extra onion or pepper into the relish recipe. “The extra vegetables dilute the acidity and can result in botulism poisoning.”  ^[12] Riggs, Kathleen. Avoiding Common (Major and Minor) Canning Mistakes. Utah State Cooperative Extension. March 2009. FN/Food Preservation/2009-01. 

But, you’re actually free to raise the acidity level anytime you wish!

Kathleen Riggs from Utah State says,

[You may] Add EXTRA vinegar or lemon juice. Bottled acids help obtain required pH (acid levels) in tomatoes and pickles. If a more tart or sour flavor is desired, more vinegar, lemon or lime juice may be added.”  ^[13] Riggs, Kathleen. Avoiding Common (Major and Minor) Canning Mistakes. Utah State Cooperative Extension. March 2009. FN/Food Preservation/2009-01. 

A bit of acidity won’t help you a bit in stuff that needs pressure canning

Sneaking a bit of vinegar or acid into food products that must be pressure canned doesn’t give you any kind of free pass away from having to pressure can them for the full, completely recommended time and procedure: “Is it safe to can green beans in a boiling water bath if vinegar is used? No. Recommended processing methods must be used to assure safety. Recommended processing times can not be shortened if vinegar is used in canning fresh vegetables (this does not refer to pickled vegetables).  ^[14] Missisippi State University Extension Service. Canning. Accessed January 2015. You’d have to add so much vinegar to your “plain” green beans that you’d end up with “pickled” green beans. Which is fine, it’s just a different food product all together.

Acidity alone is not enough, a canning process is still needed

Bernardin says,

However, high acidity alone is not sufficient to preserve foods.  Heat processing in a boiling water canner is still required to inactivate enzymes, yeasts and other microoganisms that can thrive in unprocessed High Acid Foods.”  ^[15] Bernardin. The Science Behind Heat Processing. Accessed March 2015.

Bacteria such as campylobacter, listeria, salmonella, etc, can survive acidity and thrive in a sealed jar, but, they can’t survive a proper hot water bath processing.

Heating a product — either through cooking a relish mixture, or processing jars of pickles in vinegar, also helps to make the product safer by driving the acidity inside the food particles:

“The techniques for reducing pH are….. (3)…..The product is heated and the high temperature improves the rate of penetration into solid particles.”  ^[16]Ontario Agency for Health Protection and Promotion (Public Health Ontario). Home canning: literature review. Toronto, ON: Queen’s Printer for Ontario; 2014. ISBN 978-1-4606-4166-8 [PDF] Page 8.

Acidity also helps with taste

Putting Food By says,

We add acid to enhance flavor, making it brighter or more tangy, in condiments like ketchup or chili sauce or chutney. We add it to help create the balance that makes a gel in combination with pectin and sugar, in cooked jellies and jams. And we add it more more lavishly to aid preservation of a number of pickles served as garnishes.”  ^[17] Janet Greene et al. Putting Food By. Page 39.

University of Georgia video on acidity

Appropriate heat processing

Controlling for acidity does not negate the need for adequate heat processing.  Many harmful microorganisms can either survive or even thrive in acidity.

“…high acid environments inhibit the growth of bacteria and other spoilage organisms. However, high acidity alone is not sufficient to preserve foods. Heat processing in a boiling water canner is still required to inactivate enzymes, yeasts and other microorganisms that can thrive unprocessed in High Acid Foods.” ^[18] Bernardin. The science behind heat processing. Accessed March 2016.

E-coli, listeria and salmonella are a few examples of pathogens than can survive in a jar that was not adequately heat treated by processing.

“While these pathogens [Escherichia coli O157:H7, Salmonella enterica, Listeria monocytogenes and other pathogenic bacteria] do not grow in acidified vegetables, they may survive long enough to cause disease. The infectious dose for E. coli O157:H7 may be as low as one to ten cells. For this reason, acidified vegetables must be processed to assure a five log reduction in acid resistant pathogenic bacteria….E. coli O157:H7 has been found to be the most acid resistant pathogen of concern for these products….The research done here documents how innocuous items such as pickled onions could hold E. coli that aren’t immediately killed by the vinegar, and what must be done to ensure its destruction.” ^[19] 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.

To know what is required to adequately process a jar requires lab testing. They will test on variables such as size of jar, what the item packed into the jar is and how the heat currents move through that jar in order to reach everything in that jar for long enough to kill it off, whether it’s “nooks and crannies” as in a jar of sliced pickles or high density as in a jar of jam. They will recommend the right time and temperature of heat processing — just enough to ensure safety, without any more time which could damage the quality of the product you are making.

But we don’t have to worry about what an adequate heat processing time is, because we have access to literally thousands of lab-tested recipes where that has already been done for us! (If you see a company giving out “one processing time fits all” recommendations for a variety of home-canned products, it could be a sign they have in fact not invested in doing testing for each product.)

So just remember the combination: correct acidity PLUS correct heat processing.