Why is a pressure canner needed when canning?

I keep reading the same tip, when it comes to preserves, which essentially is:

Botulism spores can survive 100°C and require hotter than boiling water temperature.

What I don't understand is, why a waterbath in my oven set to 150°C is insufficient to achieve this effect? Or simply a water bath on the stove?

What stops me from heating the content of the jar to above 100°C without a pressure cooker?

• As your source points out, the type of preserve will dictate the process to use when canning. What is it that you want to preserve? – moscafj Sep 3 '19 at 20:41
• In my case it is peppers, which are not acidic. – user1721135 Sep 4 '19 at 7:08

Physics stops you from heating up liquids that consist of mostly water to temperatures above (roughly) 100 C.

The temperature of your heating element can be set higher, but neither the temperature of the water bath nor the liquid in your jars can go higher than the boiling point where water changes from liquid to vapor - which is 100 C at normal pressure 1. To raise the temperature, you have to increase the pressure, so that the new boiling point is at or above the temperature required for safe canning. A pressure cooker may be suitable for the purpose, provided the model is designed for the specific pressure and allows setting/reading the specific pressure or alternatively, temperature. Pressure canners on the other hand are specifically designed for that purpose and marketed as such.

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1 Note that 100 C is correct only at sea level and given a few more constraints. At higher altitudes the boiling point may be significantly lower. For the answer here, 100 C is close enough. (Or far enough from safe canning temperatures for non-acidic items.)

• Also note that pressure canners and pressure cookers are different. Some pressure cookers have no way to read or regulate pressure, which is important for canning. – moscafj Sep 3 '19 at 20:44
• @moscafj, Really? My pressure cooker doesn't let me adjust the pressure. It's fixed (regulated) by the weight of the jiggler on top vent and the diameter of the vent opening. I don't need to read the pressure, because as long as there's steam escaping the weight, I know the pressure is pretty close to the designed pressure of my cooker. – The Photon Sep 4 '19 at 0:28
• @ThePhoton right...that's why you shouldn't use it for canning. – moscafj Sep 4 '19 at 1:01
• @user1721135 the difference is the amount of energy per time you are transferring to your pot. If you want to keep the pot at a certain temperature, a simmer will usually suffice. A rolling boil is better for reducing liquids, as more liquid evaporates in a given time frame. You may have had to add more water to your soup after boiling it hard for a time. Simmer and the reduced movement will also protect delicate items from falling apart, e.g. some dumplings. – Stephie Sep 4 '19 at 8:22
• @user1721135 You're increasing the rate at which the water evaporates, which may be a waste of energy, detriminal to the end product, or desired. But you're not increasing the temperature of the soup above 100 °C. The water is essentially a cheap thermostat. If you could, you would be constantly burning your food. Think about what happens if you just leave a carrot on a pan on full flame for half an hour, and compare what happens with the same carrot if you keep it submerged in water under otherwise the same conditions. Oil works the same way, but has a higher boiling temperature. – Luaan Sep 4 '19 at 9:20

I like to explain the physics this way: heat is a thing, temperature is a place. If you put a hot thing next to a cold thing, heat stuff will flow out of the hot thing into the cold thing. This will cause the cold thing's temperature to rise, and the hot things's temperature to fall, unless other factors interfere. How far and how fast these temperatures move depends on the material: dense things like iron can absorb a lot of heat but temperature rises slowly. Light things like air gain or lose temperature quickly as heat moves.

For a stove burner, you have continuous input of more heat, so the burner's temperature will not fall. The liquid in the pot is mostly water, which can absorb a lot of heat and its temperature will rise--to a point. What interferes is called a phase transition (what you and I call boiling). At this point, further heat energy going into the water is used to turn it into steam, while the remaining water stays in place (this place is about 100C at normal pressure).

Putting the water under pressure changes the place at which this transition happens, and so the water can absorb more heat, going to a higher temperature, before turning into steam.

• Saying that heat is a thing and temperature is a place is neither correct from a technical point of view, nor is it particularly helpful from a lay person's point of view (in my opinion). In what way is temperature like a location? – Kevin Wells Sep 5 '19 at 20:56
• I have found the analogy quite useful in speaking to non-technical people. Temperature is a point along a line, like a mile marker along a road. It helps people understand that "100C" doesn't mean there's 100 units of something anywhere, it's just a label we've given to that point along the line. Calories, on the other hand, are actual things you can count and add up and move around. I find the analogy useful to explain things like why adding two temperatures is meaningless. If you don't find the analogy useful, don't use it. – Lee Daniel Crocker Sep 5 '19 at 22:56

The physics behind why you can't heat liquid water past its boiling point is defined as when vaporization pressure equals atmospheric pressure. So, putting your canning water in an oven, which is at atmospheric pressure, will result in the water getting no hotter than if it were on the stove, which is also at atmospheric pressure, because the energy required for water to change from liquid to gas is the same for both situations.

Why you need to use a pressure cookers is shown in The Ideal Gas Law: PV = nRT. Rearranging the equation for your situation gives: P = (nR/V)T and noting that (nR/V) is a constant, we can rewrite it as P = kT, which indicates that increasing the temperature will increase the internal pressure of the pressure cooker, thus increasing the energy required for a water molecule to become vapor. Therefore, liquid water can become hotter in a pressure cooker, which is what you need to ensure sterilization!

• This is not correct. The fact that water boils is what stops you from heating a water bath hotter than 100C. Its latent heat of vaporization is how much energy it takes to boil a mass of water, but is not important here. The boiling point of water is not linear in temperature: see this figure. The statement that water can become hotter before boiling at higher pressure is correct, which is why pressure canning works. – Ross Millikan Sep 4 '19 at 20:40
• You're correct, I was wrong in attributing it to latent heat of vaporization, I was thinking of the definition of boiling point and used the entirely wrong term. I edited my post accordingly. Thanks for pointing that out! – Jeff Sep 4 '19 at 22:55
• Minor quibble. Because the LHV of water is much higher than the amount of energy needed to raise the temperature 1 degree, you actually tend to raise the temp of the water over 100C before it boils off without needing to raise the pressure. This tends to only be a fraction of a degree though unless done very carefully and is not significant to cooking temps. It is normally less than home equipment can even measure, but in a lab I did see someone force it to about 102 with just the minor pressure increase of putting a lid on a pot. More trivia than anything practical though. – dlb Sep 5 '19 at 13:06