Why Some Fruit and Vegetables Should Not Be Stored Together: Ethylene Works at Parts per Billion
A ripe avocado releases over 100 µL of ethylene per kilogram per hour, and just 2 µL per litre of air halves broccoli's shelf life. Who in the fridge produces ethylene, who suffers and how to arrange the drawers.
Ethylene is a gas that works at concentrations of parts per billion
Ethylene (C₂H₄) is a colourless gas that plants make themselves from the amino acid methionine, and it is biologically active at concentrations measured in parts per million and parts per billion, according to the "Ethylene Effects" chapter of the USDA's Agriculture Handbook 66 (2016). In so-called climacteric fruit, which includes apples, avocados, bananas, melons, pears and tomatoes, ethylene stimulates its own synthesis, so during ripening its production rises up to a thousandfold. Once such a fruit has started ripening, lowering the ethylene in the surrounding air no longer stops it.
The same source lists what ethylene does to other produce: it accelerates senescence, excessive softening, chlorophyll loss (yellowing), browning and leaf drop. Besides ripe fruit, wounded and diseased plant tissue is a significant source of ethylene, and outside the kitchen so are engine exhaust and smoke.
Who in the fridge gives off ethylene
Handbook 66 gives, in its introductory table, ethylene production rates by commodity, in microlitres per kilogram per hour. The differences are enormous: from practically zero to several hundred.
| Produce | Ethylene production (µL/kg/h, USDA HB 66) | Chilling sensitivity |
|---|---|---|
| Passion fruit | 280 (20 °C / 68 °F) | below 10 °C (50 °F) |
| Avocado, ripe | more than 100 (20 °C / 68 °F) | below 4.5–13 °C (40–55 °F) |
| Kiwifruit, ripe | 75 | not sensitive |
| Tomato | 10 (20 °C / 68 °F) | ripe below 7–10 °C (45–50 °F) |
| Banana, ripe | 5 (15 °C / 59 °F) | below 11.5–13 °C (53–56 °F) |
| Peach, nectarine, ripe | 5 (0 °C / 32 °F) | not sensitive |
| Apple | "varies greatly" | most varieties not sensitive |
| Cherry, strawberry, broccoli, carrot | less than 0.1 | not sensitive |
The third column explains why half of these producers do not go in the fridge at all: according to the USDA's chilling-injury table, bananas are damaged below 11.5 to 13 °C (53 to 56 °F), ripe tomatoes below 7 to 10 °C (45 to 50 °F) and avocados, depending on variety, below 4.5 to 13 °C (40 to 55 °F). They stay on the counter, which also solves part of the ethylene problem in the fridge, because at 4 °C (40 °F) mostly apples and pears remain.
Who suffers from ethylene: broccoli, lettuce, carrots
Broccoli itself produces less than 0.1 µL of ethylene per kilogram per hour but is, per Handbook 66, "extremely sensitive" to it: exposure to 2 µL per litre of air at 10 °C (50 °F) halves its shelf life and yellows the florets. Lettuce responds to ethylene with rusty brown lesions on the leaf midribs (russet spotting), so the USDA explicitly recommends not storing it with apples, cantaloupes, pears and peaches. Carrots form the bitter compound isocoumarin at just 0.2 µL per litre, mostly in the peel, so whole and cut carrots must not sit next to ethylene. Cherries produce very little ethylene but respond to others' with faster respiration and loss of quality; strawberries are the exception that does not respond to ethylene at all.
What this looks like in a home fridge
The "Ethylene Effects" chapter gives a list of measures for reducing ethylene's effect that translates directly to the kitchen: keep the air free of ethylene, hold produce at the lowest temperature it tolerates, segregate producers from sensitive items and shorten the time between exposure and use. In practice:
- One drawer for apples and pears, the other for lettuce, broccoli, carrots and other vegetables; vegetables in a perforated bag, which also slows water loss.
- Bananas, tomatoes and avocados outside the fridge, in a spot away from the bowl with the rest of the fruit.
- Bruised, split and mouldy fruit out immediately, because wounded and diseased tissue gives off more ethylene than healthy tissue.
- The fridge at 40 °F (4 °C) or below (FDA) with room for air to circulate; for lettuce the USDA notes that russet spotting is easily prevented if the temperature is below 2 °C (36 °F) and the air is ethylene-free.
Temperature matters more than anything else: the figure the USDA gives for broccoli applies at 10 °C (50 °F), and a lower temperature slows both respiration and the response to ethylene.
When ethylene helps
The same mechanism serves for ripening. The USDA states that external ethylene promotes the ripening of avocado, banana, melon and tomato, and that the gas released by a ripe fruit accumulates inside closed packaging. Commercially, bananas are ripened with 10 to 50 µL of ethylene per litre for 24 to 48 hours at 14 to 18 °C (58 to 64 °F). At home a hard avocado or green banana in a closed paper bag with one ripe apple gets the same gas, just without the measuring. For longer storage the reverse applies: producers on one side, sensitive items on the other. How to apply this to individual foods, from apples to cherries, is covered in the kitchen guides at jelo.hr/blog; seasonal recipes are at jelo.hr/recepti.
Izvori
- USDA ARS, Agriculture Handbook 66: The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks (2016), chapters "Ethylene Effects", "Respiration and Ethylene Production Rates", "Chilling and Freezing Injury", "Broccoli", "Lettuce" and "Carrot"
- FDA, Selecting and Serving Produce Safely
- FDA, Refrigerator Thermometers – Cold Facts about Food Safety