Ethylene is the plant’s own ripening signal. Apples, pears, bananas, tomatoes, melons, peaches, apricots, avocados and kiwifruit produce plenty of ethylene as they ripen; ethylene in the surroundings also pushes the neighbouring fruit to ripen, it produces ethylene in turn, and the process accelerates as a chain reaction. This is the chemistry behind the saying "one bad apple spoils the barrel". The effect begins at a very low concentration: 0.1–1 ppm (parts per million) is enough for most products, and in kiwifruit even 0.03 ppm speeds up softening. The gas is colourless; in the store it is noticed only by a sensor or by its consequences.
Its unwanted effects: leaf spotting in lettuce and cabbage, yellowing in broccoli and spinach, yellowing and softening in cucumbers, bitterness in carrots, sprouting in potatoes, early wilting in cut flowers, softening in kiwifruit and persimmons. For this reason ethylene-producing products and ethylene-sensitive products are not kept in the same room. Fruit is not the only source: the exhaust of internal combustion (LPG, diesel) forklifts, decaying product residues and cigarette smoke also carry ethylene; this is why battery forklifts are preferred in fruit and vegetable stores.
Removal methods: regular fresh air exchange (the cheapest method, but it brings a cooling load), filters with granules impregnated with potassium permanganate (which oxidises ethylene; they are replaced when the colour turns from purple to brown), catalytic or ozone ethylene scrubbers, and lowering the oxygen in controlled atmosphere stores. Treatment with 1-MCP (1-methylcyclopropene) switches off the fruit’s perception of ethylene for months; it is common with apples. The most basic measure is still temperature: at 0 °C both ethylene production and sensitivity to ethylene fall to a small fraction of their level at room temperature.
Ethylene is also used deliberately: 100–150 ppm is supplied in banana ripening and ripening rooms, 1–5 ppm in citrus degreening. These rooms need a gas-tight door, a dosing system and ventilation after treatment; because ethylene is explosive in air above 2.7%, a diluted mixture or a generator is used. The door of a ripening room must not open onto a corridor where ethylene-sensitive products are waiting. In stores holding mixed products, when the room plan is drawn up the ethylene relationship of the products should be considered together with the temperature and humidity values in the food storage conditions guide.