The Water Bath Side
What genuinely belongs in a boiling water canner, why tomatoes are the exception everyone gets half-right, and what the vinegar in a pickle is actually doing.
5 pages here.
A boiling water canner is the simpler machine and the more misunderstood one. It has one job: hold filled jars under briskly boiling water for a timed process, with at least an inch of water over the lids [F-2.1]. Whether that is enough is never a question about the canner. It is a question about the food.
Food at or below a pH of 4.6 can be processed in boiling water [F-1.1]. Food above it cannot, at any length of time, because the thing that has to be dealt with is dealt with by temperature and boiling water has a ceiling [F-1.2]. Everything on this side of the site is downstream of that one line.
Two of the five pages below are about the same manoeuvre: moving a food across the line deliberately. Some tomatoes sit slightly above 4.6 and are acidified so that they do not [F-1.5]. Cucumbers sit well above it and are pickled so that they do not [F-1.6]. In both cases the added acid is doing the work that a pressure canner would otherwise have to do with heat, and in both cases the quantity is a laboratory finding rather than a matter of taste.
The other three: what belongs in the canner in the first place, why the tested directions specify bottled lemon juice rather than the lemon on your counter, and the steam canner — a boiling-water tool with a live disagreement attached to it that is worth reading before you buy one.
What is not here. No processing times, no headspaces, no altitude tables of ours. When you need one, these pages name the tested document that holds it and link you to it.
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What genuinely belongs in a boiling water canner
The USDA guide names the foods that can be processed in boiling water, in one sentence, on one page. Here is that sentence, what sits at its edges, what counts as a canner, and the one time empty jars have to be sterilised.
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Tomatoes: the exception the whole niche gets half-right
Every page tells you to acidify tomatoes for a water bath. The guide says the acidification is still required if you pressure can them, and three tested sources disagree in public about why tomatoes drift over the line. Both halves are here, with the page numbers.
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Why bottled lemon juice, and not the lemon
One question with a one-sentence answer: bottled lemon juice has a constant acidity and one lemon's acidity varies from another's. Here is the sentence, from Penn State Extension, and what it does and does not rule out.
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Pickles: what the vinegar is actually doing
A pickle is not a vegetable stored in vinegar. It is a low-acid food that a tested quantity of acid has moved across the pH line, which is the only reason boiling water is enough. Here is the mechanism, from the guide's own glossary.
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Steam canning: what changed, and when
The USDA guide says steam canners are not recommended. The research centre that maintains that guide approved them the same year. Both texts are live. Here is what each one says and what it means for you.