Venting, come-up and cool-down: the parts people skip

The five ways to spoil low-acid food, according to the guide

There is a single sentence in the 2015 guide that lists them, and three of the five are the steps this page is about. Food may spoil if you fail to select the proper process times for specific altitudes, fail to exhaust canners properly, process at lower pressure than specified, process for fewer minutes than specified, or cool the canner with water [F-3.9].

Read down that list and notice the shape of it. Two of the five are about the timed process, and they are the two everybody already worries about. The other three are the altitude adjustment, the venting, and the cooling — the parts that look like preliminaries and tidying up.

They are not preliminaries. They are three of the five named failure modes.

Venting: the gauge is a promise about pure steam

A pressure figure is a stand-in for a temperature, and the substitution only works in pure steam free of air [F-1.8]. Air in the canner breaks it: trapped air lowers the temperature obtained at 5, 10, or 15 pounds of pressure and results in under-processing [F-3.3].

The gauge cannot tell. It is measuring pressure, and a canner full of hot air and steam can be at exactly the right pressure and the wrong temperature.

So the venting period is not a warm-up and not a stage of getting the canner going. It is the step that makes the number on the dial mean the thing the process was written about. All types of pressure canner must be vented for ten minutes before they are pressurised, timed from when steam first escapes [F-3.3].

Two practical notes.

The worst case for trapped air is a raw pack in a dial-gauge canner [F-3.3]. Raw-packed jars carry more air into the canner, and dial-gauge canners do not vent during processing [F-3.3]. If that is how you can, this step is carrying more weight in your kitchen than in most.

And the ten minutes are ten minutes of real steam. A canner that has been vented for "about that long" while you did something else has been vented for an unknown period, which is the same as an uncontrolled one.

Come-up and cool-down: the heat you are not counting

Here is the idea that makes the rest of the pressure section coherent.

A tested process time is not a stopwatch measurement of how long a jar sits at temperature. It is the output of laboratory work on a whole thermal history — the heat the food takes on while the canner comes up to pressure, the heat during the timed process, and the heat it keeps receiving during the early stages of cooling. NCHFP states this plainly: bacteria are not killed in the food only during the process time [F-7.3].

Two things follow.

The stages either side of the timer are inside the number. They are not overhead. That is why the guide is willing to say that forced cooling may result in unsafe food [F-3.8], which otherwise sounds like an overreaction to a bit of cold water on a pan.

And you cannot substitute one for another. You cannot vent less and process longer, or cool faster and add minutes, because the exchange rate is not published — and, per NCHFP, is not known [F-14.9].

That last point is the honest one and it is worth being clear-eyed about. There is a hole in the tested guidance here. The people who write the processes have said publicly that they cannot state what fraction of the treatment the cooling supplies [F-14.9]. What that gap actually forbids is improvising around it. A number you cannot decompose is a number you have to reproduce whole.

Losing pressure

If at any time the pressure goes below the recommended amount, bring the canner back to pressure and begin timing over from the beginning, using the total original process time [F-3.7].

That is the rule, stated in the guide under a heading in capital letters, with the reason attached: this is important for the safety of the food [F-3.7].

Three situations produce it in ordinary kitchens.

The heat gets turned down too far. The most common one, and it usually happens a few minutes after pressure is reached, when the canner is settling and the temptation is to back the burner off to stop it climbing.

The stove cycles. Some electric and induction hobs modulate their output on a cycle, and a setting that holds pressure at the top of the cycle can lose it at the bottom.

Someone moves the canner. Sliding it across a hob to a cooler ring is a pressure event.

The habit that prevents all three is dull and effective: stay in the room for the first five minutes after pressure is reached, make one small heat adjustment at a time, and wait to see the result before making another.

Cooling, and the ten minutes after the ten minutes

When the process finishes, the heat goes off and the canner is moved off the burner if that is possible, and then it depressurises on its own [F-3.8]. About 30 minutes for standard heavy-walled canners loaded with pints, about 45 with quarts [F-3.8].

Do not force it. Cold water over the canner, or opening the vent port before it is fully depressurised, causes loss of liquid from jars and seal failures — and forced cooling may result in unsafe food or spoilage [F-3.8]. Both reasons are given, and only the second one is visible.

Then the step that gets skipped. Once the canner is depressurised the weight comes off the vent port or the petcock opens, and there is a further ten-minute wait before the lid is unfastened [F-3.8].

The reason is pressure inside the jars rather than inside the canner. Jar contents are still above boiling point and still under some internal pressure of their own; opening the canner into cool room air pulls liquid out through the seal that is in the middle of forming. Ten minutes of standing still is the whole fix, and it is free.

What this all costs you

Time, and only time.

The guide's own worked example: about 12 to 15 minutes of heating before the canner begins to vent [F-3.2], another 10 minutes to vent, another 5 minutes to pressurise, then the process, then another 20 to 60 minutes to cool before jars come out [F-3.2].

Plan around that sequence rather than around the process time in the recipe, and none of these steps ever becomes the thing you are cutting because the evening got late. Almost every skipped vent and every forced cool-down in home canning is a scheduling failure wearing a technique costume.

If you have not yet chosen a canner, the gauge decision changes what you have to watch during all of this: dial gauge or weighted gauge. And if you are at elevation, the altitude adjustment is the first of the guide's five failure modes for a reason — find your altitude first.