Cooling Milk Without Power: The Night-Sky and Evaporation Bound for an Evening Milking

Evening milk on small farms often has to wait until a morning collection, and without refrigeration it spoils during the night. This paper computes what two passive methods, and their combination, can actually do for a stated case: 20 L of milk at 37 C, held for 10 hours overnight. The methods are night-sky radiative cooling from a shallow 1.0 m2 pan, evaporative cooling of a cloth-wrapped can, and a combined design (the pan plus a wetted cloth skirt). Five night climates are considered, from a cool clear highland (night-mean 12 C) to a hot humid overcast night (30 C, 90 % relative humidity). A stated bacterial growth model is integrated along each computed temperature history. Only one combination of method and climate reaches the 4 C refrigeration target: the radiative pan alone on the cool clear highland night, reaching 3.8 C after 10 h and crossing 4 C at 8.7 h. In an arid clear night (22 C, 30 % relative humidity) the combined design brings 20 L to 11.8 C and holds the modelled bacterial count to 8.29e4 CFU/mL, against 5.53e7 CFU/mL for an uncooled can. In hot humid nights passive cooling fails outright: under a hot humid, partly cloudy night (28 C, 85 % relative humidity) every method, including the combination, exceeds a 1e6 CFU/mL comparison threshold within 10 h, so in those climates the milk must be processed the same evening, by fermentation or by heating, rather than stored as liquid milk. SIMULATED, NOT MEASURED: all results are computed from stated assumptions; nothing described has been built or tested. AI disclosure: the computations, drafting, and document preparation were carried out with the assistance of Dave, an artificial intelligence system.