Respiration Rate

Once a commodity is detached from the plant, it is cut off from the water, minerals, and photosynthetic sugars that once sustained it, but its cells remain alive and metabolically active. Respiration continues to consume the sugars, starches, and organic acids already stored in the tissue, releasing carbon dioxide, water, and heat as by-products. Because those reserves cannot be replenished after harvest, respiration rate is effectively a countdown: the faster a commodity respires, the sooner its stored reserves, and with them its eating quality, are used up.

Respiration rate is highly sensitive to temperature. As a general biological pattern, the rate commonly rises two- to three-fold for every ten-degree increase in temperature, a relationship often described in biology as the Q-ten relationship. This is a general pattern, not a fixed factor that applies identically to every commodity across every temperature band, but it explains why cooling is the single most effective intervention available in the entire post-harvest chain — see Precooling, Cold Chain, and Cold Storage. The heat that respiration releases, sometimes called vital heat, does not disappear; it must be removed by the refrigeration system, and the amount of heat a commodity is expected to generate is itself a factor the system is sized around.

Reducing the oxygen available to a commodity and raising the carbon dioxide around it both suppress respiration further, beyond what temperature alone achieves. This is the physiological basis of controlled-atmosphere storage and modified-atmosphere packaging, which deliberately manage the surrounding gas mixture to extend storage life — see Controlled-Atmosphere Storage and Modified-Atmosphere Packaging.

Different commodities respire at very different paces even under identical conditions. Leafy greens, broccoli, and sweetcorn are among the fastest-respiring fresh commodities and correspondingly have short potential storage lives. Onions, potatoes, and tree nuts respire slowly and can be held for months under suitable conditions. This inherent variation is a major reason why blanket storage-life expectations do not transfer between commodities, and why storage guidance is written commodity by commodity rather than as a single rule.

A measured respiration rate reflects the sampled tissue’s recent history as much as its species. Wounding, exposure to ethylene, and disease all raise the measured rate, so two lots of the same commodity can show different rates for reasons that have nothing to do with genetics or ripeness.

• Not ripeness — young, growing tissue and wounded tissue both respire fast, independent of ripeness.
• Not a climacteric verdict on its own — a rise after harvest identifies climacteric behaviour; a high but steady rate does not.
• Not a measure of remaining shelf life in absolute terms — it shows the pace of use, not the size of the reserve.
• Not an ethylene measurement — the two are related in climacteric fruit but are distinct processes, measured separately.