How Long Raw Milk Survives the Bicycle Ride: A Q10 Acidity Bound and an Ice-Jacket Can Sized to Beat It

Across much of the world, raw milk reaches a collection point on the back of a bicycle, hours after milking, with no refrigeration in between. Whether it arrives acceptable or rejected is a race between the transport chain and bacterial acid production, and this paper puts numbers on both sides of that race. A Q10 acidity model (Q10 = 2.5, acid production 0.0045 % lactic acid per hour at 20 °C, rejection at 0.20 % from a fresh value of 0.135 %) gives the time from milking to rejection as 9.14 hours at 25 °C, 5.78 hours at 30 °C, and 3.65 hours at 35 °C. A stated realistic chain — milking and handling, the ride, a queue at the collection point, and a wait before chilling — totals 3.00 hours in its typical form and 6.25 hours in its worst realistic form. The worst chain therefore fails at 30 °C (0.47 hours past the bound) and fails badly at 35 °C (2.60 hours past the bound); at 35 °C the uninsulated failure time is 3.65 hours, barely longer than even the typical chain. An insulated 10-litre can (40 mm polyurethane, heat-leak coefficient 0.168 W/K) with an ice jacket changes the result completely: 3 kg of ice, equilibrating the milk at 7.82 °C, keeps the integrated acidity below the rejection threshold for the full 24-hour model horizon at a 35 °C ambient, and 4 kg of ice equilibrates the milk at 1.41 °C. A static energy check agrees with the simulation: cooling 10.3 kg of milk from 35 °C to 6 °C plus the heat leaked during the worst chain requires 1,283.5 kJ, the latent capacity of 3.84 kg of ice. The equivalent service from a 5 °C phase-change pack (latent heat 180 kJ/kg) requires 7.42 kg of pack for every 4 kg of ice. Ice, where it can be bought at the village in the morning, is the cheapest part of this system; the binding constraint is the clock, and the clock at 35 °C allows 3.65 hours.