Produce goes on living after it is picked: it respires, loses water and softens. The rate of these processes multiplies with temperature; a strawberry at 30 °C respires nearly ten times as fast as one at 0 °C. Pre-cooling stops this loss by bringing the product down to storage temperature in the first hours after harvest. A common field rule is this: for delicate products such as strawberries, cherries and apricots, every hour of delay between harvest and cooling takes about one day off the shelf life. Pre-cooling is not storage; it is a separate job and needs separate capacity.
The main methods: room cooling (the product cools by itself in a cold room, taking 1–3 days), forced-air cooling (cold air is drawn through the crates by a fan, cutting the time to 2–8 hours), hydrocooling (a shower of water at 0–2 °C; 15–40 minutes for cherries, carrots and sweetcorn), vacuum cooling (20–30 minutes for leafy produce such as lettuce and spinach; about 1% water loss for every 6 °C drop) and icing (broccoli, fish). The time is usually given as the "7/8 cooling time": the time in which seven eighths of the difference between the product and the cooling medium is removed.
The capacity calculation is very different from that of a storage room, because all the product heat is removed within a few hours. Bringing 10 tonnes of cherries from 28 °C to 2 °C in 6 hours takes about 45–50 kW of cooling capacity; a few kilowatts are enough to store the same cherries once cooled. In forced-air systems 1–2 litres of air per second are passed per kilogram of product; the evaporator is selected with a large surface and a low temperature difference (TD, the difference between the room air and the evaporating temperature) so that the product does not dry out. The cooling load calculator gives this capacity from the daily intake and the cooling time.
Points to watch: crates and cartons must be perforated so that air passes through them (at least 5% of the side surface open), pallets must be stacked against the fan without gaps, and the air must pass through the crates, not around them. When the product approaches the target temperature the fan speed is reduced or the product is moved to the storage room; air flow that goes on longer than necessary means weight loss. For products sensitive to chilling injury, such as bananas, tomatoes and cucumbers, the water and air temperature is not taken below the product’s lower limit. Target temperatures by product are given in the food storage conditions guide.