School Meal Hot Chain: An Insulated Transport Bound

Cooked school food dispatched at 75.0 °C must, under the design rule adopted here, remain at or above 60.0 °C until serving. A lumped transient cooling model was applied to a full batch of 27.0 kg (60 children at 0.45 kg per child, specific heat capacity 3.9 kJ/kg/K) in five container classes at ambient temperatures of 15, 25 and 35 °C. At 25 °C ambient the full batch stays at or above 60.0 °C for 23.18 h in a vacuum-insulated panel box, 17.39 h in a 50 mm rigid polyurethane cooler, 12.27 h in a locally made haybox with about 100 mm of straw insulation, 10.43 h in a 40 mm expanded polystyrene box, and only 1.74 h in an uninsulated lidded pot. For full insulated loads the thermal bound therefore exceeds any plausible school delivery journey, and distance is not the limiting factor. The bound collapses under three realistic conditions, each quantified here: small batches (a 10-child haybox load holds for only 2.05 h, and a 5-child load for 1.02 h), repeated lid opening (six openings shorten the haybox holding time from 12.27 h to 5.05 h), and cool dispatch (packing at 65 °C instead of 75 °C shortens the haybox holding time to 4.60 h). The haybox is the cheapest insulated option that clears a typical transport-plus-waiting journey with margin, at an annualised $9.61 per container, or $0.16 per child per year. Where holding has already failed, reheating a full batch from 50 °C to 75 °C requires 2,632.5 kJ of useful heat — 0.114 kg of liquefied petroleum gas at a stated 50% stove efficiency, costing $0.14 per batch — cheap in fuel, but reheating does not undo bacterial growth or toxin formation during time spent below 60 °C, and it is treated here as a fallback, not a substitute for holding.