Boiling Water Cannot Make Low-Acid Food Safe in a Jar: Spore Inactivation Times at the Boiling Point, and the Altitude at Which Even Acid Processing Times Break

A common belief holds that a low-acid food in a jar can be made safe in a boiling-water bath if only it is boiled for long enough. This paper tests that belief by arithmetic, using stated thermal inactivation parameters for Clostridium botulinum spores: a decimal reduction time of 0.21 minutes at 121.1 °C, a temperature coefficient z of 10.0 °C, and a target of 12 decimal reductions, equivalent to 2.52 minutes at 121.1 °C. At 100 °C the decimal reduction time rises to 27.05 minutes and the required hold is 324.6 minutes (5.41 hours), on the generous assumption that the coldest point in the jar sits at bath temperature throughout. At 95 °C the required hold is 1026.6 minutes (17.11 hours); at 90 °C, 3246.4 minutes (54.11 hours); and at 85 °C, 10266.0 minutes (171.1 hours, 7.129 days). Through a stated boiling-point relation, 95 °C is the boiling point at 1507 m, 90 °C at 3009 m, and 85 °C at 4502 m. Boiling therefore does not offer a practical route to the stated spore target at any altitude, and the shortfall grows by a factor of about 3.16 for every 5 °C the boiling point falls. For acid foods, a second calculation uses a stated target with a decimal reduction time of 2.5 minutes at 100 °C, z of 9.0 °C, and a requirement of 10 decimal reductions (25.0 minutes equivalent at 100 °C), against a stated sea-level schedule of 35.0 minutes. That schedule stops delivering the required inactivation where the boiling point falls to 98.68 °C, at 396 m (1301 ft). At 95 °C the same 35.0 minutes delivers only 9.74 minutes equivalent at 100 °C, 39.0% of the requirement, and the hold that would be needed is 89.8 minutes.