Handle With Care: Why Produce Bruises
The biology behind types of bruising // Can you still eat it? // 4 ways to damage a fruit // do's and don'ts // sensitivity index // sources
Apples like eggs
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Nobody throws a box of eggs into a shopping cart.
The damage would be immediate, visible, and all over your hands. Yet fruits and vegetables like apples, oranges and potatoes often land hard on the checkout belt, get dropped into a bag and get a ride home in a bicycle bag over cobblestones. Seemingly, your fruits and veg is fine, but underneath, the bruise is already there, it just takes a day or two to show. Between the field and the shop, mechanical damage is estimated to account for around thirty to forty percent of all postharvest losses, and bruising is the largest part of that (source - source). |
What happens after the shop has barely been studied at all.
Nobody counts the apples that get bruised on the way home, which is probably why nobody thinks about it either. |
One bruise is not like the other
Apple (and many others): the brown stain
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Most produce bruises the way people picture it: it turns brown.
Fruit and vegetables are built from living cells, each with a thin wall and a membrane holding its contents in place. When a force pushes those cells past what they can take, walls and membranes break. Enzymes that normally sit in one part of the cell come into contact with phenolic compounds stored in another, and with oxygen. In an intact cell these are kept apart; once the compartments are broken, the enzyme polyphenol oxidase turns the phenolics into brown pigments. (source and source) That is the brown you see under the skin of a bruised apple. Under intact skin it can take a day or more to become visible, which is why bruises so often turn up long after the moment that caused them. Bruising like an apple: pear, quince, medlar, peach, nectarine, apricot, plum, cherry, grape, fig, avocado, mango, guava, papaya, persimmon, kiwi, lychee, longan, rambutan, mangosteen, soursop, breadfruit, jackfruit, aubergine, artichoke, and mushrooms. |
Lettuce makes its own 'brown'.
Leafy greens look like they belong in the first group, but they arrive at the brown by a different route. There are few phenolics in an undamaged lettuce leaf to react with. Instead, the wound sets off a signal that travels through the leaf, and the plant starts manufacturing phenolics where none were before. The enzyme that does this appears about four hours after the damage and peaks around a day later, which is why cut and bruised lettuce browns slowly, and why the brown spreads outwards from the injury rather than staying where you touched it. (source) Berries do it backwards. The red and purple in berries comes from anthocyanins, and these are poor substrates for the browning enzyme: it barely touches them. (source) What happens instead is a detour. The enzyme oxidises other phenolics in the cell into quinones, and those quinones attack the pigment. (source) So the colour does not turn brown so much as fade away, leaving a dull, washed-out patch behind. Which is why a bruised strawberry looks tired rather than marked. |
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Tomato: clear & watery instead of brown
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Tomatoes go clear, not brown.
When the cells break here, the fluid runs into the air spaces between them. The tissue turns glassy and translucent rather than brown, which is what makes a bruised tomato look dark and wet in patches. Cut one open and there is usually more than you expected: water-soaked flesh, and the gel around the seeds shrunken, disordered and discoloured to a whitish green. (source) At the same time the impact sets off the breakdown of the cell wall itself, the same process that normally happens during ripening, only faster and only there. So a tomato bruise is a two-step affair: the physical damage, and then the fruit's own softening machinery running unchecked at that spot. The bruise is genuinely soft, not just discoloured. (source) |
Peppers do much the same, and badly. That thin skin covers crisp, fragile flesh, and about one in ten peppers bruises from a fall of just eight centimetres onto a hard surface. Cold, wet conditions make it worse: a chilled pepper bruises more easily than a warm one. In the worst bruises the centre goes water-soaked, exactly as in a tomato. (source)
A cucumber looks the same but isn't. Its bigger enemy is the fridge. Below about ten degrees it develops water-soaked patches and pitting that look exactly like bruising, and often only appear once it warms up again. That is cold damage, not impact: the cell membranes weaken and the fluid seeps out on its own, with nothing having hit the cucumber at all. (source) NB All of it is perfectly edible. But do so fast, as it goes off faster than it should. |
Citrus: burns itself with toxic oils
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Citrus skips the browning route almost entirely.
The rind is full of oil glands sitting just under the surface, and a knock or a squeeze ruptures them. The released oil is toxic to the cells around it, so the fruit effectively burns itself with its own essential oil. (source) Under the microscope the sequence is clear: the impact tears the skin directly above a gland, the escaping oil seeps back into the rind through that same tear, and the cells beneath break down rapidly while the outer skin itself stays intact. (source) So the surface sinks around each gland while the glands stand out from it. Fresh oil spotting is barely visible; over the following days the damaged areas darken and sink, and that is the pitting you can feel with a fingertip. An orange that looked fine in the bowl on Monday can be marked by Thursday. (source) |
It can happen to any citrus, but the ones with many prominent oil glands are the most vulnerable: navel oranges, grapefruit, mandarins, lemons and limes.
Small fruit is more susceptible than large. And the fuller of water the fruit is, the less it takes: in a really turgid fruit, the slight pressure of a bump or a rub is enough to release oil. That is why fruit picked in wet or foggy weather suffers most, and why lemon growers are advised to let the fruit sit for a day before handling it. (source) The mango does it with sap The mango has its own version: When the stem breaks at harvest, sap under pressure sprays across the skin. Its damaging component is terpinolene, a volatile oil that enters through the pores in the skin and injures the tissue underneath, and those injured cells then brown in the ordinary enzymatic way, with dark streaks appearing within hours. Applying pure terpinolene to a mango produces marks indistinguishable from the real thing. (source) |
The potato: internal blackening
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Here nothing collapses. The impact damages the membranes inside the cells rather than the walls around them, so the tissue keeps its shape and the skin stays smooth. What breaks is the compartmentation that keeps enzyme and substrate apart.
The substrate is different too. In the potato it is mainly tyrosine, an amino acid, and the end product is melanin, the same pigment that darkens human skin. Which is why the mark is grey to black rather than brown, and why it sits deep, close to the vascular ring and usually at the stem end. (source, source) You have seen this reaction before, on a banana. Its peel runs the same route, tyrosine to L-DOPA to melanin, and the black speckles on a ripe banana are exactly that, happening where you can watch it. A bruised peel goes black rather than brown for the same reason. (source) |
The potato runs the identical chemistry out of sight, and slowly.
The enzyme does not increase after an impact; it simply moves, and that redistribution takes around twelve hours. By sixteen hours the cells in the damaged zone are dying, with almost none of the repair response a cut would get. The discolouration itself follows a day or two after that. (source, source) A potato can look perfect right up to the cutting board. Do you see black spots? Don't worry. Black and firm is fine to eat. Cut it out for the taste, not for safety. Green is the one to watch. The colour itself is just chlorophyll, but it signals that the potato has been making glycoalkaloids, its own defence compounds, and those act on the nervous system: too much brings nausea, cramps and headache. Cooking does not destroy them. A firm potato with a shallow green patch or a few sprouts is fine once you cut them away generously; one that is green deep into the flesh, heavily sprouted, shrivelled or bitter should go. (source) |
