This post explains the role of acid in canning and how to can safely therein the bounds of the pH scale.
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Canning is a mysterious and amazing thing. This post will attempt to clarify one aspect of canning and hopefully my Dear Readers will feel more confident taking their first water bath plunge. As an English teacher, I hope to make a really science-y thing understandable to any eager water bath canner, regardless of the last time you saw a piece of litmus paper or read the introductory chapter of nearly any good canning cookbook. Here we go…
Foods have a pH value, denoted by a number. Low numbers mean higher acid (limes are a 1.8) and higher numbers mean less acid (spinach is a 6.5). An acidic environment (inside your canning jar) prohibits the growth of spoilers, which can make us sick and can cause our hard work to be wasted. To safely water bath can, one element that must be present is a safe acid level.
We preserve foods that are naturally high in acid and/or we add ingredients to our recipes to bring the acid level up. For example, strawberries are a great choice for any canner because they naturally are acidic at a 3. Strawberry jam recipes often are very, very simple because you just need sugar and the berries to reach a safe acid level.
The important number that recipe creators and curious canners need to keep in mind is 4.5. Any number higher than that and the recipe will call for something to bring the acid level up (and the number down). Green beans are a 4.6 – not acidic enough all by themselves to safely water bath can, but, what about the old fashioned favorite Dilly Bean?! They are canned safely because of the addition of vinegar (quite acidic at about 2.0-3.4 depending on variety). This is why vegetables (generally lower in acid) are often pickled in vinegar, and fruit (generally higher acid) is practically foolproof and a great starting point for a new preserver.
Tomatoes are the perfect example of this pH scale and its importance. We tend to think of tomatoes as being really acidic but they are actually at about a 4.2- 4.9, depending on all kinds of things like where they were grown and their variety. Most recipes for canning tomatoes call for either lemon juice or citric acid to be certain that the acid level is safe. Unless you are going to measure the pH level of the tomatoes you haul home from the farmer’s market, you must follow the recipe even if the tiny bit of citric acid seems unnecessary or the whole lemon you have on hand seems just as good as a jug of store bought lemon juice. Store bought lemon juice is verified to be a specific acid level. You would have no way of knowing if your whole lemon happened to be a bit less acidic than need be. Thus, it is critical to follow the recipe to maintain safety in canning.
This science lesson is applicable to a canner who might feel inclined to step out of the recipe safe-zone and into lower acid territory. One should ALWAYS follow a recipe from a trusted source when preserving food in jars. However, it is important to understand the chemistry so you make wise canning decisions, Wildflowers. I confess; I am a corner cutter, rule breaker, and shortcut taker 9 times out of 10 but NOT with canning.
The pH values of various foods list is available on the FDA website in a very long and not so easy to read format, so I created a FREE downloadable PDF for my readers for your reference. The PDF is condensed and easier to read than the list on the site, and you can print it or read it on your phone while you’re canning away in the kitchen:)
The idea with this list is you can refer to it to check if it is safe to replace lemon juice with lime (probably safe because their acid levels are very similar) or replace apricots with figs (definitely NOT because apricots are in the safe zone of 3.3 and figs are a 4.6). This tool is meant to educate and inform. I mentioned apricots and figs purposely. You might think, hey, they are both sweet, about the same size, it won’t matter if I swap out some of the apricots for figs…but it matters a lot. Canning is not like regular cooking where you can make substitutions willy-nilly. You might also notice the wide range of acid values for some produce. That should also indicate to you the necessity to check the list, or better yet, find a trusted recipe.
Download the free Acid & Canning Chart here!