Henry Shaw, UC Master Food Preserver Online Program Volunteer
If you've done any pressure canning, you've probably noticed something curious: the jars often keep boiling long after you remove them from the canner and place them on your counter (Fig.1). This bubbling can go on for minutes — or even hours. Believe it or not, that's a good sign! It usually means your jars have sealed properly. But why does this boiling continue, especially when the jars are no longer hot enough to reach water’s normal boiling point? Let’s explore the science behind this fascinating process.
Figure 1. Bubbling jar left to cool on counter (Image credit: Henry Shaw, 2025).
Boiling occurs when the “vapor pressure” of a liquid equals the pressure of the surrounding environment. Vapor pressure is essentially a measure of how easily a liquid turns into gas. The warmer a substance gets, the higher its vapor pressure becomes — this is why water evaporates faster when heated. Even solid materials have vapor pressure (yes, even iron!), but at everyday temperatures, it’s usually negligible. For water, however, vapor pressure plays a crucial role in the pressure-canning process.
A quick word on pressure: psia vs. psig
Before diving into the canning process, I need to briefly discuss pressure measurements. Pressure is commonly measured in pounds per square inch (psi), but there are two different ways this is reported:
- psia (absolute pressure): Measured relative to a perfect vacuum. At sea level, atmospheric pressure is about 14.7 psia and a perfect vacuum is 0 psia.
- psig (gauge pressure): Measured relative to the ambient atmospheric pressure; at whatever elevation you are, the ambient pressure is defined as 0 psig.
When we say we’re pressure canning at “10 pounds pressure,” we mean 10 psig, which means the pressure inside the canner is 10 psi above the surrounding atmospheric pressure. That translates to about 24.7 psia at sea level but less at higher elevations.
Because water’s boiling point depends on absolute pressure, canning at higher elevations — where atmospheric pressure is lower — requires increasing the gauge pressure to reach safe temperatures for destroying pathogens like Clostridium botulinum.
Figure 2. The pressure (P)-temperature (T) path of a canning jar that's processed at 10psig at sea level. The black curve is the "boiling curve" for water, at pressures and temperatures below this curve (shaded blue), water is as a liquid; above the curve (shaded pink), water is a gas. Gray arrows show the P-T path of the conditions inside the jar during the entire process (Image credit: Henry Shaw, 2025).
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Let’s follow a jar of food through a typical pressure canning cycle at sea level. We’ll ignore the small effects of water activity for simplicity.
- Starting out: At room temperature and normal atmospheric pressure, the jar sits cool and calm on the counter (blue dot in Fig. 2).
- Heating up: The canner is vented and heated until it reaches 212°F (the boiling point at sea level). Steam begins escaping to purge the system of air (purple dot). We vent the canner for 10 minutes to purge the canner interior (including the headspace of the jars) of as much air as possible and replace it with water vapor.
- Sealing the vent: After 10 minutes of venting, the vent is closed. The canner, now a sealed environment, begins building pressure as it continues to heat.
- Approaching processing conditions: As both temperature and pressure increase, the contents of the jars follow a path along the boiling curve of water until the target processing conditions (240°F and 24.7 psig this case) are reached (red dot).
- Holding steady: The jars are held at this pressure-temperature point for the required processing time to ensure safety.
Cooling Down: After processing, the heat is turned off. The canner cools and depressurizes slowly, retracing the same pressure-temperature path in reverse. When it reaches atmospheric pressure again, it’s safe to open the lid and remove the jars.
Why the Jars Keep Boiling
Now comes the part you’ve probably been waiting for: why do the jars keep boiling!
Here’s why: Canning lids act like one-way valves. While the internal pressure is higher than outside, gas can escape. But as the contents cool and pressure drops inside the jar, the lid seals shut. What we often call a "vacuum seal" is really the result of external air pressure pressing the lid down once the internal pressure drops. If there is “gunk” on the rim of the jar, perhaps due to siphoning of product during processing, this will interfere with the lid’s ability to form a good one-way seal with the jar’s rim, resulting with a poor seal or no seal at all. Following the recommendations for appropriate headspace helps avoid this problem, as does cleaning the rim of the jar after filling it and before placing the lid.
As the jar contents cool, water vapor inside the headspace condenses into liquid. Because water vapor takes up far more space than liquid — over 1,200 times the volume! — this condensation drastically lowers the pressure inside the jar, and with the lid sealed, no air can enter to balance it out. The drop in internal pressure allows the remaining liquid to boil — even though the temperature is well below the normal boiling point. On a “phase diagram” (like Fig. 2), the jar follows a path along which boiling occurs at temperatures lower than the normal boiling point due to the reduced pressure (the path from the purple dot to the maroon dot in Fig. 2). The jars stop boiling when they reach the temperature at which the vapor pressure of the contents is lower than the internal pressure of the jar. The pressure in the jar continues to decrease (from maroon dot to the green dot in Fig. 2) as the headspace gas cools because the pressure of a gas at constant volume is proportional to its temperature.
The Final Seal
By the time the jars reach room temperature, the internal pressure will have dropped to between 9 and 13 psia 1 — lower than surrounding air pressure. That difference in pressure (roughly 1.7 to 5.7 psi) is enough to hold the lid tightly in place, keeping your food safe and shelf stable.
So, the next time you hear gentle bubbling from a cooling jar, take it as a quiet sign of success — thermodynamics in action right on your countertop.
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1 Sivanandam, G., 2014, Evaluation and Comparison of the Sealing Performance of Three Major Types of Jar Lids Available For Home Canning. Univ. GA Master of Science thesis. https://openscholar.uga.edu/record/12666?ln=en&v=pd. Accessed 7/24/25.