A pressure canning schedule cannot be calculated: each pressure and time was measured for one food, pack and jar size by recording the temperature at the slowest-heating point of the jar, because the laboratory minimum for spores, 12 decimal reductions at 250°F, is about 2.52 minutes of exposure at that point and says nothing about how long the clock must run before the centre of a jar gets there. Take every pressure and time from a tested source such as the USDA Complete Guide to Home Canning or the National Center for Home Food Preservation, and if no tested schedule exists for a food, do not can it.
Part of Canning & Pressure Preservation
Pressure and time requirements are the tested pairs of canner pressure and clock time that deliver enough lethal heat to the coldest point of a jar of low-acid food, and the reasons they cannot be reasoned to.
What The Laboratory Number Actually Measures
Bacterial spores die along a predictable curve. At a fixed temperature, each fixed interval of time kills the same fraction of those that remain. The D-value is the time at a given temperature to kill 90% of the spores, a tenfold reduction. For the spores of Clostridium botulinum, the figure usually quoted is a D-value of about 0.21 minutes at 250°F (121.1°C) (Esty and Meyer, 1922; also tabulated in Stumbo, Thermobacteriology in Food Processing; recalled, not re-measured here).
The commercial canning standard, the “botulinum cook”, requires 12 decimal reductions, so the minimum lethal exposure at that temperature is
12 × 0.21 min = 2.52 min
of time at 250°F, at the coldest point in the container (US FDA, 21 CFR part 113 governs commercial low-acid canning; recalled). That number is a spore-kill requirement, not a canning time. It is also written for 250°F, hotter than the roughly 240°F a home canner reaches at 10 PSI (the parent article’s figure), and the z-value, about 18°F (10°C) for these spores (same sources; recalled), says what a cooler canner costs: the required exposure rises tenfold for each z of temperature lost. The tested schedule does that bookkeeping for you. Doing it by hand is the point at which people go wrong.
Why The Clock Time Is So Much Longer Than 2.52 Minutes
The jar is not at canner temperature. The centre of a jar of meat or beans is the last place to heat. Heat must travel from the steam, through the glass, through the outer food, to the middle, and the time that takes depends on:
- Jar size and shape. In an idealised conducting solid the time to heat the centre grows with the square of the distance, so a bigger radius is disproportionately slower. Jars of different sizes are not scaled copies of each other, so even this rule cannot be applied directly.
- How the food heats. Liquid circulates; a thick mass conducts only. Packing density, particle size, and the viscosity of the liquid all change the answer.
- Starting temperature. A hot pack starts ahead of a raw pack.
- Headspace and trapped air. Both change the heat transfer.
- Canner come-up and cool-down time. Lethal heat keeps accumulating while the canner is still hot after the clock stops, which is why the tested schedules assume a particular cool-down and why forcing a canner to cool changes the dose.
The result is measured with thermocouples placed in the cold spot of real jars in a real canner, then converted to lethality with a safety margin. That is what a tested schedule is.
The Schedule Ratios Show Why You Cannot Interpolate
If a rule could scale times from one jar size to another, the ratio of quart time to pint time would be roughly constant. Test that against the pint and quart lines in the parent article’s pressure table, dividing each quart time by its pint time (the minutes are left in the parent table, where you can check them):
| Food | Pint to quart time ratio |
|---|---|
| Chicken, bone-in | 1.15 |
| Carrots | 1.2 |
| Beef or venison chunks | 1.2 |
| Dry beans | 1.2 |
| Green beans | 1.25 |
| Soup or stew | 1.25 |
| Corn, whole kernel | 1.55 |
(The parent table has not been checked against the published guide; the point is the spread, not the values.) The ratio runs from about 1.15 to about 1.55, so no single factor converts one size to another, and a size or a food that is not in the table has no honest conversion at all.
What Goes Wrong When People Reason Their Way To A Time
- Interpolation. “A pint-and-a-half is halfway between” assumes a straight line that the ratios above refute.
- Scaling down. Half-pint jars sometimes have their own tested lines and sometimes none; shortening the pint time is a guess.
- Trading pressure for time. Because the temperature response is exponential, a lower pressure for a longer time is not a simple swap, and a lower-pressure schedule exists only where someone tested it.
- Borrowing from a similar food. Peas and beans, or chicken and rabbit, can differ in density and fat.
- Mixed jars. A mixture heats like its slowest component and may have no tested schedule.
- Skipping the venting step. Air left in the canner gives a pressure reading but a cooler canner, and the schedule’s temperature is never reached.
- Stopping the clock early because “it looks done”. Cooked texture happens far below lethal temperature.
What You Need
- A verified copy of a tested schedule collection, printed, with its edition and year noted. Where a community has only a handwritten or remembered table, that table is unverified.
- Two independent copies, compared line by line, so a transcription error does not become a standing rule.
- A canner with a gauge or weight confirmed accurate (Gauges, weights, and safety valves).
Method: Treat The Schedule As A Controlled Document
Step 1: Keep the source schedule in one protected place and copy it by checking, not by recall.
Step 2: Before first use of the parent article’s tables, compare each line you will use with the published guide. Two discrepancies to check for are the altitude rules: the parent states one table of pressure corrections where, as I recall, the published guidance distinguishes dial gauges from weighted gauges, and one water bath rule of 5 extra minutes per 3,000 feet where the published guidance gives altitude bands. These are recollections, not verified findings; the verification is the task.
Step 3: Record every batch: food, jar size, pack, pressure, time, gauge reading, altitude, date.
Step 4: If a food has no tested line, preserve it another way (Food Preservation).
Common Mistakes
- Treating a remembered number as a schedule. A family’s “always did 90 minutes” may have been right for one food and wrong for another.
- Believing past luck is evidence. Under-processing usually leaves the jar safe, which is why it persists, until one jar is not.
Safety
This is the one place where a plausible number is worse than none
A schedule that is off by a few minutes for one jar size will usually produce food that looks and tastes normal. When it fails the toxin is undetectable, the illness appears a day or more later, and the cause is rarely traced to the jar. If a time or pressure cannot be taken from a tested source for the exact food and jar size, do not can that food.
What’s Next
- Learn next: Altitude adjustments — why the same pressure reading at altitude is a cooler canner
- Combine with: Processing times and headspace — the same logic applied to the water bath