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Food Canning (Low-Acid Foods)

5 min read

Low-acid foods, meaning those above pH 4.6 such as meat, vegetables, beans and soups, can be kept safely in sealed jars only if every jar is processed under pressure to a tested schedule for that exact food, jar size and altitude, because botulinum spores survive boiling and the toxin they make gives no sign in the jar. A larger autoclave does not change that rule: it must reproduce the conditions the schedule was tested under, and it must never be run on a time or pressure worked out by guesswork.

Part of Applications

Using a pressure vessel to can meat, vegetables and other low-acid foods: why a schedule cannot be improvised, and what changes when the vessel is larger than a home canner.

The Schedule Belongs To A Food, A Jar, A Canner And An Altitude

The corpus already covers the food side in detail, and this article does not restate it. The method, the pressure and the altitude tables are in Canning & Pressure Preservation, which is the one place to take processing pressure from. That article records that dial-gauge and weighted-gauge canners use different pressure tables, and that the altitude correction is part of the schedule. Do not substitute a pressure from anywhere else, including this tree.

The reasons are covered in Acid levels and safety thresholds, which gives the pH 4.6 limit below which botulinum spores cannot make toxin, and in Pressure and time requirements, which explains why a schedule cannot be calculated: it was measured at the coldest point of real jars, and it cannot be interpolated from one jar size to another.

The Carrier’s Processing Table Is Not A Tested Schedule

The carrier article lists processing times for 500 mL jars at 121 °C (15 psi): meat, poultry, dense vegetables, soups. This article does not reproduce that table. It is not attributed to a tested source, it uses a pressure basis that the canning article’s tables do not give for the same job, and it names 500 mL jars where tested schedules name pint and quart jars. Where the carrier’s table and the canning article disagree, take the canning article. The warning the carrier attaches is right: under-processing is invisible, because the food looks and smells fine and can be lethal.

What Changes In A Large Autoclave

A schedule is not only a pressure and a time. The measured time includes the canner’s own come-up and cool-down, as the Pressure and time requirements article says: lethal heat keeps accumulating while the canner is still hot after the clock stops, so the schedules assume a particular cool-down. A big autoclave heats and cools at a different rate from a home canner, so the same clock time may deliver more or less heat to the jar centre. The honest position:

  • Use the vessel at the conditions the schedule assumes, which means a small load and a cool-down like the tested canner’s, or
  • Have the schedule re-established for your vessel by someone able to measure the temperature at the centre of a jar, and
  • Do not scale up by loading more jars and keeping the time.

What You Need

  • A sound pressure vessel with all the safety devices, and a gauge whose type (dial or weighted) matches the schedule used.
  • A tested schedule for the exact food, jar size and altitude, copied and checked line by line.
  • Jars and lids in good order (Pressure Canning (Low-Acid)).
  • A thermometer in the chamber, to confirm the steam is pure.
  • A batch record: food, jar size, pressure, time, altitude, date.

Method: Running Jars In A Vessel

Step 1: Arrange jars on a rack, upright, with room for steam, never stacked solid.

Step 2: Vent the air. The carrier gives five minutes of venting at atmospheric pressure before closing; the canning article says ten minutes of steady steam. Use the longer. Air trapped in the vessel makes the gauge read high while the steam is cooler than the schedule assumed (Medical Instrument Sterilization works the numbers).

Step 3: Process to the schedule, starting the clock when the pressure the schedule names has been reached, and holding it steady.

Step 4: Let the vessel cool and depressurise by itself. Jars full of hot liquid lose contents and can break if the pressure falls fast, for the same reason a vessel of hot water flashes (Failure Modes). Do not run cold water over the vessel or open the vent to hurry it.

Step 5: Open at zero pressure, tilting the lid away, and leave the jars undisturbed to seal.

Common Mistakes

  • Copying a time from the carrier table. It is not a tested schedule.
  • Scaling up the load without re-establishing the schedule.
  • Trading pressure for time. The relationship is not a simple swap.
  • Judging a jar by taste or smell. A bulging lid, spurting liquid, cloudiness or off odour means discard it unopened and untasted, and not to animals.

Safety

Botulism toxin has no taste, no smell and no visible sign

A low-acid jar processed short of its schedule can look perfect. If any pressure, time, jar size or altitude cannot be taken from a tested source, do not can that food; preserve it another way (Food Preservation). The vessel is a pressure vessel: never open it while it holds pressure.

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