Tomatoes: the exception the whole niche gets half-right

The half the niche gets right

Type the question into a search box and you will get five Extension pages, all of them accurate, all of them saying the same thing: acidify tomatoes before you can them. Those pages are good. They are better placed than this one to give you the amounts, they are linked at the foot of this page, and I would rather you read them than take the figures from me.

The reasoning behind that instruction is one sentence on printed page 1-8 of the USDA guide. Tomatoes are usually an acid food, but some now have pH values slightly above 4.6, so they must be acidified to 4.6 or lower with lemon juice or citric acid if they are to be canned as acid foods [F-1.5].

Note the shape of that. The rule is not "tomatoes are dangerous". The rule is "tomatoes are close to the line, some cross it, and you cannot tell which by looking, so treat them all as though they have crossed it."

The half the niche drops

Now the part that is missing almost everywhere, including from pages that are otherwise careful.

The acidification is still required if you pressure can [F-5.2].

This surprises people, because it inverts the mental model most of us build. We learn that acidification is what lets you use a boiling water canner — so it follows, apparently, that owning a pressure canner means you can skip it. The guide closes that door directly. Where a tomato procedure offers both routes, all preparation steps are still required even if you choose the pressure option, and the guide names acidification specifically [F-5.2].

The mechanism is in the same passage. The boiling water and pressure alternatives are equal processes, with different time and temperature combinations, calculated for these products [F-5.2] — meaning for the acidified product the procedure describes. The tomato pressure schedule is short. It is short because it was built for an acid food. Feed it an un-acidified one and the schedule underneath you was developed for something else.

If you take one thing from this page, take that. It is the difference between following a tested procedure and following the part of it you understood.

The disagreement, and why I am not going to resolve it for you

Here is where a page in this niche usually asserts something confident. I am going to do the opposite, because the sources genuinely do not agree, and pretending otherwise would be the same move that produced the bad pages.

Three tested sources, all current, all live, describing why tomatoes drift over the line.

Clemson says the varieties changed. Clemson's Home & Garden Information Center factsheet, written by Dr. Susan Barefoot, PhD, Emeritus Faculty, published 30 April 2018, puts it like this [F-5.4]:

"Tomato varieties have been bred for ease in harvesting; as a result many now have milder flavor and lower acidity than the ancestral tomatoes."

and, on the same page:

"…some current tomato varieties have pH values at or above pH 4.6; a few have values of pH 5 or even higher."

That claim [F-5.4] is a strong and specific one, and it is the version the whole internet has absorbed — usually stripped of its attribution and restated as common knowledge. One caution before you carry it onward: the factsheet names no study behind that upper figure. So this page quotes it as Clemson's position, which it plainly is, and not as a measurement I can vouch for. If you want the underlying survey, Clemson is the place to ask.

NCHFP does not say it at all. The National Center's own tomato introduction — the live page from the body that maintains the USDA guide — makes no variety claim anywhere in its acidity discussion. The words variety, varieties, bred and modern do not appear in it [F-5.4]. Its framing is about the fruit in front of you: select disease-free, preferably vine-ripened, firm fruit; do not can tomatoes from dead or frost-killed vines; and green tomatoes are more acidic than ripened fruit [F-5.4].

And the USDA guide splits the difference. Its sentence is that "some are now known to have pH values slightly above 4.6" [F-1.5] — a statement about some tomatoes, with no explanation attached. No breeding, no varieties, no ripeness. Just the observation and the instruction that follows from it.

So: one source says the plants changed, one source says watch the condition of the fruit, and the primary document says only that some tomatoes are over the line. None of the three is quoting the others. None has been withdrawn. All three were live on the day this page was checked.

I am not going to pick a winner, and you should be wary of a page that does — particularly one that asserts the breeding story without naming Clemson, because that is a citation that went missing rather than a fact that got established.

What all three agree on, which is the part you act on

Acidify every batch, regardless of variety [F-1.5] [F-5.4].

That instruction does not depend on resolving the disagreement, which is the useful thing about it. Whether your tomatoes are over the line because of breeding, because of ripeness, because of the season, or because they always were — the response is identical, and it is cheap, and it does not change the flavour much, and the guide even tells you how to offset the taste if it does [F-5.1].

You do not need to know why the rule exists to follow it. But you do need to know that the people who wrote it have not fully agreed on the why, because that tells you how much weight to put on the next page that explains the why very confidently.

Where to get the actual numbers

Three places, all better sourced than any blog on the subject and all free:

NCHFP's tomato acidification directions page carries the amounts as a table, by jar size, for all three acids. Penn State Extension's acidifying page gives the same figures with the vinegar amount expressed as a fraction of a cup, which is easier to measure. Clemson's factsheet gives the same instruction inside the argument described above.

All three are linked below. This site publishes no processing schedules of its own, and the acidification amounts belong to the same rule: get them from the source that tested them, and if a page gives you a figure without naming where it came from, close it.