Packaging That Does Not Pay: Sealed Film, Respiration Heat, and the Shortened Saleable Life of Market Produce

A common belief at small produce markets is that sealing produce in plastic film keeps it fresher. This paper tests that belief with a simple thermal and humidity model for 1 kg packs of tomatoes, lettuce, and carrots held at a market stall. Sealing reduces the rate at which respiration heat can leave the pack, so the produce warms itself, respires faster, and consumes its own saleable life more quickly. In shade at an ambient temperature of 22 °C, the computed temperature rise from sealing is 0.655 K for tomatoes, 1.761 K for lettuce, and 0.407 K for carrots, with corresponding reductions in saleable life of 5.3%, 13.6%, and 3.3%. In intermittent sun, represented by an effective ambient temperature of 28 °C, a sealed pack of lettuce reaches 32.556 °C and its saleable life falls to 1.74 days, a 56.6% reduction relative to an open pack in shade. The air inside a sealed bag reaches saturation in about 2.0 minutes for tomatoes, so the humidity benefit claimed for film is exhausted almost immediately, after which the bag holds saturated air against the surface for the rest of the trading period. The belief fails as a general rule. It holds in one narrow case identified here: for leafy produce sold before wilting becomes binding, sealing prevents water loss that would otherwise end saleability in 1.79 days. Outside that case, and in particular for fruiting and root produce held for more than a day, open or ventilated packaging preserves more saleable life at no packaging cost.