Forty-Eight Hours in the Freezer: Where the Pantry Moth Rule Holds for Single Bags and Fails for Stacked Flour

A common household rule says that 48 hours in a freezer clears pantry moths from flour and grain. This study tests the rule as a conduction problem: a bag of packed flour with thermal diffusivity 1.1e-7 m2/s enters a minus 22 C freezer at 24 C, and the centre plane must reach minus 15 C. A single 6 cm bag reaches the criterion in 1.96 hours, and even a 20 cm bag needs only 21.74 hours, or 27.18 hours with a 25 percent margin, so the 48-hour rule is generous for any single bag. The same arithmetic applied to a tight stack of bags acting as a 40 cm block gives 86.96 hours, or 108.71 hours (4.53 days) with margin, so the centre of a stack is still far above the criterion when the rule says to take it out. The originated rule replaces a single duration with a geometry condition: freeze bags singly or in a single layer for 48 hours; if bags must be stacked, allow about four and a half days, or restack and turn the pile halfway through. This study is simulated, not measured. SIMULATED, NOT MEASURED.