Making Milk Safe Without a Thermometer or a Boil: Hold Times at Temperatures Below Boiling, Computed from Lethality Accumulation

Households without a thermometer often boil milk vigorously because boiling is the only visible sign that enough heat has been applied. This paper computes whether that sign is necessary. Thermal inactivation is modelled with a stated decimal reduction time at 63 °C of 2.5 minutes and a stated temperature sensitivity of 5.0 °C for Coxiella burnetii, a stated second organism Mycobacterium tuberculosis at 0.8 minutes and 5.5 °C, and a target of five decimal reductions of the first organism, equivalent to 12.5 minutes at 63 °C. For 5.0 litres of milk heated at a stated 3.0 °C per minute from 20 °C, lethality accumulated during the ramp is counted as well as during the hold. The net hold required is 49.04 minutes at 60 °C, 11.78 minutes at 63 °C, and 2.42 minutes at 66 °C. At 70 °C and at 75 °C the ramp alone accumulates 18.182 and 181.816 minutes of 63 °C equivalent, respectively, so no additional hold is required under the stated ramp. Heating to 63 °C and holding uses 0.247 kg of dry wood at a stated stove efficiency of 0.25, compared with 0.445 kg for heating to 100 °C and holding a rolling boil for two minutes, a saving of 44.4 percent. A rolling boil is therefore not required for the stated safety target; it is a costly visual substitute for time at a lower temperature. SIMULATED, NOT MEASURED.