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Canning Fundamentals

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Canning Fundamentals
Sub-Topics
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Water bath vs pressure canning
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Acid levels and safety thresholds
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Jar types, lids, and seal mechanics

Canning preserves food by sealing it in a jar and heating the jar enough to destroy the organisms that spoil it, and the organism that decides every rule is Clostridium botulinum, whose spores survive boiling at 212°F and make a deadly toxin in low-acid food. Acidic foods below pH 4.6 can be processed in boiling water, while every low-acid food needs a pressure canner and a tested schedule. A badly processed jar looks and smells normal, so no inspection by eye or taste can tell you it is safe.

Part of Canning & Pressure Preservation

Canning is the heat-and-seal method of storing food in jars at room temperature for years, safe only when the acidity of the food, the heat and the time of processing are matched to a tested schedule.

A Sealed Jar Is Only As Safe As The Heat It Received

Canning does three jobs at once. Heat kills the moulds, yeasts and bacteria already in the food. The same heat drives air out of the jar, so that as the jar cools a partial vacuum pulls the lid down and keeps new organisms out. The sealed, airless jar then stores at room temperature, which is why the parent article calls it the most reliable long-term storage you can make without refrigeration or continuous smoking.

The trouble is the third job nobody sees. A sealed, airless, room-temperature jar of low-acid food is exactly the environment Clostridium botulinum prefers: no oxygen, plenty of moisture, no competition. Its spores are in ordinary soil, so they ride in on every vegetable and every piece of meat. The spores survive 212°F, the temperature of boiling water, which is why boiling a jar for hours does not make low-acid food safe. The limit is temperature, not time.

Two Classes Of Food, Two Methods

The dividing line is acidity, measured as pH. Spores will not grow and make toxin below pH 4.6 (the figure used in food-safety regulation; the acid article in this series explains where it comes from).

  • High-acid foods (below pH 4.6): most fruit, jams, vinegar pickles, and tomatoes only when acid is added. These go in a boiling-water bath. See Water Bath Canning (High-Acid).
  • Low-acid foods (above pH 4.6): vegetables, meat, poultry, fish, soups, stews and beans. These need a pressure canner, which reaches about 240°F at the pressures in the parent article. See Pressure Canning (Low-Acid).

There is no third class. Open-kettle canning (cooking the food, pouring it into jars and relying on the lid) is not a method, because nothing in it heats the food inside the jar.

What You Need

Method: Decide Before You Pack

Step 1: Classify the food. Is it high-acid or low-acid? If you are unsure, it is low-acid until a tested source says otherwise.

Step 2: Choose the method from the class. Never from what equipment you happen to own.

Step 3: Find the tested schedule. The time, pressure, jar size, headspace and pack style all come from a source that tested them, such as the USDA Complete Guide to Home Canning or the National Center for Home Food Preservation. Schedules cannot be worked out by reasoning; Pressure and time requirements explains why.

Step 4: Pack, seal, process and cool. Follow the schedule exactly. Leave jars undisturbed while they cool; the seal forms as they cool.

Step 5: Check, label and store. A sealed lid is pulled down and does not flex. Label with the date and contents, remove the screw bands, and store cool and dark.

Common Mistakes

  • Trusting the senses. Botulism toxin does not change the look, smell or taste of food. Illness typically appears a day or more after eating, so a death is never traced back to the jar.
  • Shortening a time because the jar is small or the food looks done. Cooked-looking food is not sterilised food.
  • Using a pot lid, a plate or a mayonnaise-jar lid as a “seal”. Only a purpose-made seal gives a reliable vacuum.
  • Canning something no source has tested. If no tested schedule exists, do not can it. Salt, dry or ferment it instead (Food Preservation).

Safety

Botulism cannot be detected and cannot be cooked out of a bad jar after the fact

C. botulinum toxin causes descending paralysis: double or blurred vision, drooping eyelids, slurred speech and difficulty swallowing, followed by failure of the breathing muscles. Symptoms usually appear within a day or two of eating, sometimes within hours and occasionally after more than a week (CDC; recalled). Without antitoxin and breathing support it is often fatal. Bulging lids, spurting liquid, cloudiness or an off smell mean discard the jar unopened and do not taste it. The absence of those signs proves nothing.

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