Bananas are harvested green, transported at 13–14 °C and ripened in the city where they will be sold. The ripening room does this to a programme: the pulp temperature is brought to 16–18 °C, 100–150 ppm (parts per million) of ethylene is supplied to the room for 24 hours, then the room is ventilated and the temperature is lowered in steps to 14–15 °C. The programme is set between 4 and 8 days according to the desired day of sale. Ripeness is followed by peel colour on a colour scale from 1 (dark green) to 7 (yellow with brown flecks); fruit is usually dispatched to the shop at colour 3–4.
Here the refrigeration system has to deal with a load that behaves the opposite of what is usual: the respiration of a banana that has begun to ripen increases several times over and the product heats itself. The system is designed to remove this heat, to change the fruit temperature by no more than 1 K per hour and to heat when necessary; the evaporator has heating elements or a hot gas coil. Control is based not on the air temperature but on the temperature of a probe inserted into the fruit. Humidity is held at 90–95%; in dry air the peel turns dull and darkens quickly, so an ultrasonic humidifier is standard in most rooms.
Even ripening depends on air distribution. In pressurised rooms the pallets are stacked in two rows, the aisle between them is closed with a tarpaulin and the fans force the air through the holes in the cartons; in this way the carton at the front of the room and the one at the back come out the same colour. After the ethylene has been supplied, the carbon dioxide that builds up through respiration must not exceed 1%, otherwise ripening stops; the rooms are ventilated with fresh air for 15–20 minutes once or twice a day. Room capacity is stated as a number of pallets; one pallet holds about 48–54 cartons of 18 kilograms.
Warnings: below 13 °C bananas suffer chilling injury, the peel turns grey and never ripens properly again; above 20 °C the pulp softens while the peel stays green ("cooked banana"). Ethylene is explosive in air above 2.7%; that is why a mixture diluted with nitrogen, or a generator producing ethylene from alcohol, is used instead of a pure cylinder, and the dose is limited automatically. By changing the programme the same room can also be used to ripen avocados, mangoes, tomatoes and kiwifruit. You can see the preliminary capacity with the cooling load calculator and the storage values of bananas in the food storage conditions guide.