Energy, Exergy and Preservation Function in Isochoric Food Processing: A Comparison Grounded in the Experimental Record

Isochoric preservation holds food below 0 °C without ice by confining it at constant volume: the ice that forms cannot expand, so it pressurises the remaining liquid and depresses the freezing point until the system rests on the ice Iₕ–water liquidus, the product held in the unfrozen fraction. Prior analyses projected large cold-chain energy savings but preceded the experimental literature, now roughly thirty primary studies, and modelled food as saline with constant specific heats. The case is re-examined against the conditions those experiments used, with IAPWS reference properties and every quantity accounted per kilogram of saleable product. Four results follow. The ice fraction requires no correlation: it follows from the constant-volume constraint applied to the equations of state, giving 25.5 % by mass at −5 °C and 52.5 % at −18 °C. Because only the unfrozen fraction is saleable, duty and divisor rise together as temperature falls, so the two-phase advantage over isobaric freezing closes at −10.98 °C: a 59 % saving in electrical work at −5 °C becomes a 48 % penalty at −18 °C. That bound belongs to one regime alone: supercooling forms no ice, and combined freezing-aided supercooling fixes the ice fraction at a halt temperature chosen independently of storage. Dominating the rest, the quality advantage outweighs any energy difference: processing exergy is 0.1 to 2.7 % of the exergy embodied in the food, so the retention delivered repays its cost wherever it reduces product loss by even a few tenths of one per cent.