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Water bath vs pressure canning

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Canning Fundamentals
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Water bath vs pressure canning

Use a water bath canner only for foods below pH 4.6, because an open pot cannot get hotter than the boiling point of water, about 212°F at sea level, and that is not hot enough to kill botulinum spores. Use a pressure canner for everything else, because steam held under pressure reaches about 240°F, the temperature that destroys the spores. No length of time in a water bath makes up for the missing temperature.

Part of Canning & Pressure Preservation

The two home canning methods differ in the temperature they can reach, and that ceiling, not the duration, decides which foods each method may be used for.

A Pot Of Boiling Water Has A Fixed Ceiling

Water at the surface of an open pot cannot rise above its boiling point. Adding more fire makes it boil harder, not hotter. At sea level that is 212°F (100°C). It is lower at altitude, by about 1.8°F for each 1,000 ft of elevation (recalled from standard atmosphere tables), which is why altitude adds time to every water bath schedule and, as Altitude adjustments explains, makes the pressure correction in a pressure canner non-optional.

The botulinum spore is a different problem from the growing cell. Spores survive boiling for hours. What stops them in a high-acid food is not heat but acidity: below pH 4.6 the spore cannot grow or make toxin (see Acid levels and safety thresholds). That is why a water bath is a safe method for acid foods and an unsafe one for everything else. The heat in a water bath still has a job, because it kills moulds, yeasts and the ordinary bacteria that acid does not stop, and it drives out air to make the seal.

A Pressure Canner Raises The Boiling Point

A pressure canner is a closed pot. Steam cannot escape freely, so pressure builds, and at higher pressure water boils at a higher temperature. The parent article gives about 240°F at 10 PSI gauge pressure at sea level. The physics checks out: 10 PSI above the 14.7 PSI of the surrounding air is about 24.7 PSI absolute, and saturated steam at that pressure is near 240°F (compare the steam table in Pressure Gauge, which lists 250°F at 15 PSI).

That temperature only exists if the canner has been vented of air. A pocket of air in the canner gives a pressure reading that is partly air and partly steam, so the steam, and the food, is cooler than the gauge implies. This is the reason for the 10-minute steady venting step in the parent article.

What You Need

Method: Choosing Between Them

Step 1: Is the food, as packed, below pH 4.6? Fruit, jam, vinegar pickles, and tomatoes with added acid are. Everything else is not.

Step 2: If yes, water bath. If no, or if unsure, pressure canner.

Step 3: If you only have a water bath canner, do not can the low-acid foods. Dry, salt or ferment them.

Step 4: Never substitute a small pressure cooker for a canner. Heat-up and cool-down times in a schedule assume a canner of a certain size and mass, and the schedules for the jar sizes were not tested in a cooker.

Common Mistakes

  • “It boiled for three hours, so it’s safe.” Three hours at 212°F does not substitute for 240°F.
  • Adding salt, sugar or spices and treating them as acid. Sugar and salt lower water activity but do not provide the pH protection that stops botulinum in a canned jar.
  • Treating the water bath as the “easy” method and applying it to green beans or soup. This is the typical route to a canning botulism case.

Safety

A pressure canner is a pressure vessel

A canner that is blocked, overfilled or run with a failed safety valve can rupture. Never lock the lid on a canner whose vent pipe is blocked, never leave it unattended at pressure, and never open it until the pressure has fallen to zero by itself. Steam burns are severe, and opening a canner early can also break jars and spill hot food. For a salvaged canner see Salvaging pressure canners.

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