What a pressure canner does that a boiling pot cannot

One machine reaches a temperature. The other one cannot.

Put the two canners side by side and almost everything is the same. Both hold jars on a rack. Both apply heat for a timed period. Both are covered.

The difference is a ceiling. Water boils at 212°F at sea level [F-6.3] and cooler than that above it, and a boiling water canner cannot go past whatever that day's boiling point is, no matter how hard the burner is working. A pressure canner seals, builds pressure, and raises the boiling point of the water inside it, so the steam in the vessel gets hotter than 212°F. The tested figure is 240° to 250°F [F-1.2].

Twenty-eight degrees does not sound like a lot. For Clostridium botulinum spores it is the difference between a process measured in minutes and a process measured in hours — the guide's own figures are 20 to 100 minutes at 240° to 250°F, against 7 to 11 hours in boiling water [F-1.3].

That is the whole argument for owning the more expensive machine. Not sturdiness. Not speed. A temperature.

Pressure is the means. Temperature is the mechanism.

The single most useful sentence in the guide on this subject is the one that takes the pressure out of the starring role: pressure does not destroy microorganisms, but high temperatures applied for an adequate period of time do [F-1.8].

Keep that separation and several confusing things become simple.

Why the gauge is only a proxy. The relationship between pressure and temperature holds for pure steam with the air driven out [F-1.8]. Trapped air lowers the temperature you get at a given pressure reading [F-3.3]. The gauge cannot see the difference, which is why venting is not optional and not a warm-up.

Why "but it reached the right pressure" is not the end of the argument. It is the start of one. A vessel can show a correct pressure and sit below the temperature the process was written for. That is the crux of the electric appliance question, and it is why the people who maintain the guidance state flatly that what matters is temperature, not pressure [F-7.2].

Why altitude changes the pressure and not the time. In a boiling water canner you cannot raise the temperature, so tested tables raise the time. In a pressure canner you can raise the temperature, so tested tables raise the pressure and leave the time alone [F-4.1]. Same correction, applied where the machine allows it.

What is actually in the jar

The organism is Clostridium botulinum, and it is worth naming rather than gesturing at.

It exists in two forms. Vegetative cells, which are ordinary bacteria and are dealt with by ordinary cooking temperatures. And spores — dormant, tough, and the reason the whole method exists. The guide compares them to plant seeds and notes they can survive harmlessly in soil and water for many years [F-1.4].

Spores are on most fresh food surfaces already, and washing reduces their numbers only slightly [F-1.4]. There is no version of this where careful sourcing or thorough scrubbing removes the problem. The spores go into the jar. The question is whether the process destroys them.

They are harmless on your kitchen counter because they need the absence of air to grow [F-1.4]. A sealed jar of low-acid food supplies that, along with the moisture and the temperature range in the list on this page. Toxin can be produced within 3 to 4 days of growth beginning [F-1.4].

None of this is a reason to be frightened of canning. It is the reason the process is specific.

Acid does the same job by a different route

Two ways exist to make a sealed jar of food safe at room temperature, and the pressure canner is only one of them.

Above a pH of 4.6, the food is low-acid and needs the temperature [F-1.1]. At or below 4.6, the acid itself prevents the growth, and a boiling-water process is enough [F-1.1]. Pickling is that manoeuvre done deliberately: adding enough vinegar or lemon juice to a low-acid food to lower its pH to 4.6 or lower [F-1.6].

So the sentence "vegetables need a pressure canner" is not quite right, and the imprecision matters. Plain green beans need a pressure canner. Green beans in a tested pickling brine do not, because they are no longer a low-acid food. What decides is the pH, and the person who decided it was the one who tested the recipe.

If you want the full version of that decision, it is on water bath or pressure.

What you are signing up for

The guide's own timeline for a pressure load: about 12 to 15 minutes of heating before it begins to vent [F-3.2], another 10 minutes to vent, another 5 minutes to pressurise, the process itself, and then another 20 to 60 minutes to cool before jars come out [F-3.2].

Two things follow from seeing it laid out.

The first is scheduling. A "twenty-minute" process is not a twenty-minute evening, and knowing that in advance is the difference between a calm batch and a rushed one. Rushing a canner load is how the cool-down gets forced.

The second is more important. Every stage in that list is part of the treatment or a precondition for it. The come-up and cool-down periods contribute heat to the food; the venting period is what makes the gauge reading mean what it is supposed to mean [F-3.3]. None of it is padding, which is why none of it can be trimmed. That is the subject of the next page but one.

Before you buy

Three things, and only one of them is a preference.

Size is a safety spec, not a convenience. The minimum is a canner that holds four quart jars standing upright on the rack, and pressure saucepans below that are not recommended for canning [F-3.1]. Small vessels cool quickly, and quick cooling shortens the heat treatment.

Gauge type is a real choice. A dial gauge has to be checked for accuracy every year and a weighted gauge does not [F-3.5]. That is a recurring obligation attached to one of them, and it is worth deciding on deliberately rather than discovering later: dial gauge or weighted gauge.

An electric multi-cooker with a canning button is not this machine. That question has more in it than a yes or a no, and it gets its own page: Instant Pot and electric canners.