The words: process, headspace, siphoning, vent, seal

Process

A process is a timed exposure to a specified temperature, in a specified vessel, for a specified food in a specified jar. All four parts are the process. Change one and you have left the schedule behind.

The reason instructions are so insistent about this is that heat travels slowly through food in glass, and the number that matters is the temperature at the coldest point in the middle of the jar, which nobody at home can see. The published time is how long the outside has to be held at the canner's temperature for the inside to have got where it needs to be.

That is also why "it was bubbling for ages" is not a process and a maintained rolling boil for the published time is.

Headspace

Headspace is the unfilled space between the surface of the food and the underside of the lid, and the tested procedures specify it precisely: 1/4 inch for jams and jellies, 1/2 inch for fruits and tomatoes going into boiling water, and 1 to 1-1/4 inches for low-acid foods going into a pressure canner [F-2.7].

It is doing two jobs at once. It gives the contents room to expand while heating without pushing out past the sealing surface, and it gives the air somewhere to be driven out from, so that the space left behind can become a vacuum as the jar cools.

Both failure directions are real. Too little headspace and food is forced over the rim during processing, which fouls the sealing surface. Too much and there is more air in the jar than the process was designed to clear, which can leave you with a weak vacuum and discoloured food at the top.

The figure comes from the recipe. The three figures above are the common pattern, not a substitute for reading the procedure in front of you.

Siphoning

Siphoning is liquid being drawn or forced out of a jar during processing or cooling. You see it as a drop in the level after the batch is done, and sometimes as food stuck to the rim under the band.

It is caused by pressure differences that are too abrupt: a violent boil, a canner opened or lifted too soon, jars moved while still very hot, or in pressure canning a canner that was force-cooled or opened before it had fully depressurised — which the guide says will cause loss of liquid from jars and seal failures [F-3.8].

The rule that matters is what you do afterwards, and it is short: if a jar loses excessive liquid during processing, do not open it to add more liquid [F-11.2]. Opening the jar undoes the only thing that has been achieved. A jar that has siphoned is either sealed, in which case store it and use it sooner for quality's sake, or unsealed, in which case it belongs in the refrigerator.

Vent

Venting belongs to pressure canning only. It is the period at the start when steam is allowed to escape freely from the open vent port, before the port is closed and pressure is allowed to build [F-3.3].

There is no equivalent step in a boiling water canner, and no reason for one: a boiling water canner is open to the atmosphere throughout, so the air is never trapped in the first place.

If somebody tells you that venting is just letting the canner warm up, the slab above is the answer. Air left in the canner makes it cooler at the same gauge reading, and the guide's glossary connects skipping the step directly to a risk of botulism in low-acid canned foods [F-3.3]. Venting, come-up and cool-down get a page to themselves: the parts people skip.

Seal

A seal is a vacuum. The lid is held down by the difference between the air pressure outside the jar and the lower pressure inside it, and that difference exists because the contents cooled and contracted after the air was driven out.

Everything that follows from that definition is worth being exact about.

A seal proves that a vacuum formed. It does not prove what temperature the food reached, how long it was held there, or whether the process was the right one for that food. Jars filled by methods with no heat processing at all seal routinely [F-1.7].

So a formed seal is necessary and not sufficient. An unsealed jar is definitely not shelf-stable. A sealed jar is one that has passed one of several tests, and the others were passed or failed in the canner, before you ever heard the lid go down. How to run the tests, and what each one can and cannot tell you, is on sealed, or just lidded.

Two more, because they tell you about the page you are reading

Pickled has a technical meaning: acid added in a quantity sufficient to bring the pH to 4.6 or below, which moves a food across the line to the boiling-water side [F-1.6]. A page that uses "pickled" to mean "has vinegar in it" is using the word loosely, and the looseness is exactly where unsafe recipes come from. The mechanism is worked through on pickles: what the vinegar is actually doing.

Open kettle describes filling jars with hot food and closing them with no processing afterwards. It is worth knowing the term because it still turns up in inherited recipe cards and in comment sections, usually phrased as though it were a shortcut rather than a different, discredited method [F-1.7]. When you meet it, you are reading something written before, or in ignorance of, the tested guidance.