Why do apples turn brown?

[Inigo_Montoya]

When you peel 'em and send 'em to school in your kid’s lunch. Why is that?

[WhyNot]

They “rust”. Well, they oxidize anyway - when iron compounds on your door panel oxidize, we call it rust. When iron compounds inside an apple oxidize, we call it icky.

Here’s more info.

You can greatly slow it down by tossing the slices in a little lemon juice diluted with water.

[rowrrbazzle]

Natureseal:

http://www.livescience.com/imageoftheday/siod_050429.html
http://www.natureseal.com/about.asp

[Blake]

@WhyNot

… when iron compounds on your door panel oxidize, we call it rust. When iron compounds inside an apple oxidize, we call it icky.

The brown colouration in apples has nothing whatsoever to do with the oxidation of iron.

Then brown colour is in fact melanin, the same pigment that causes the brown colour in our skins. Apples, and most other plants and animals, produce melanin in response to damage because melanin is an effective antibiotic. By dosing a wound site with melanin the risk of infection is reduced.

[WhyNot]

@Blake

The brown colouration in apples has nothing whatsoever to do with the oxidation of iron.

Then brown colour is in fact melanin, the same pigment that causes the brown colour in our skins. Apples, and most other plants and animals, produce melanin in response to damage because melanin is an effective antibiotic. By dosing a wound site with melanin the risk of infection is reduced.

It seems there’s either some debate, or iron compounds are part of the cycle or a secondary reason for browning or iron plays no role in the browning of apples, and it’s all a big myth perpetrated by high school science teachers.

I find no shortage of cites saying that iron plays some role or another in the process. For example:

From here :

Freshly cut apples turn brown when iron-containing chemicals inside apple cells react with oxygen in the air. The chemical reaction is called oxidation, and it is similar to the rusting of iron.

From here:

Apples and other produce (e.g., pears, bananas, peaches, potatoes) contain an enzyme (called polyphenol oxidase or tyrosinase) that reacts with oxygen and iron-containing phenols that are also found in the apple. The oxidation reaction basically forms a sort of rust on the surface of the fruit.

From here :

An apple will soon turn brown when cut, due to the conversion of ferrous iron to ferric iron.

From here :

When body stores of iron are excessive, the production of free radicals accelerates dramatically. To illustrate this, cut an apple in half. Rub lemon juice on the cut surface of one side, and stick a non-galvanized iron nail in the other half. Come back the next day and compare the pieces. The half that was rubbed with lemon juice will look pretty much the same as it did when cut. That’s because the lemon juice contains vitamin C which protects the “flesh” of the apple from free radical oxidation. The cut surface of the other half, however, will be visibly browned by free radical oxidation. Fresh apples (because they contain vitamin C) and green apples in general will be less brown, but you will be able to see a color change. Then remove the nail and cut through the nail hole. There the apple will be dark brown to nearly black from the extraordinary free-radical destruction.

From here :

What happens is that oxygen which makes up about 20% of the air
we breath changes the chemical structure of the apple when its
inner part is exposed. This is the same process that turns iron
into rust or causes the pages of an old book to become brown.

However, this page goes into much more rigorous chemistry than I can remember from college, but I’m not immediately seeing a connection to iron containing compounds.

Blake, perhaps you could lead us to some layman’s explanations of the melanin production cycle.

[Blake]

These endless time-outs are really getting annoying.
WhyNot only two of those articles you quote from suggest that the colouration comes from oxidising iron, and one is a page form some quack remedy business selling, unsurprisingly, iron supplements and antioxidants and the other is an unattributed public submission fluff piece from Wikipedia.

Your first linkis to is a kid’s site that says that “iron-containing chemicals inside apple cells react with oxygen in the air”. That is perfectly true. It never the oxidation of the iron itself is responsible for the colouration.

The second site is also aimed at children and simply refers to an “enzyme that reacts with oxygen and iron-containing phenols” that “* basically* forms a sort of rust”. As a simplified explanation to kids it’s OK. Much like saying that haemoglobin carries oxygen by forming a sort of rust. Quite untrue, but close enough for a 10 year old.

The third site is a Wikipedia affiliated public submission. ‘nuff said.

The fourth site is simply an advertorial for a website selling antioxidants and ferritin tablets using this piece of psuedoscience to warn people why they need to “Keep a watch on ferritin levels” and “Maintain high levels of antioxidants”. You can’t believe everything you read on “natural medicine” web pages. If you did you’d also have to accept the proven benefits of crystals and homeopathy.
The reactions that lead to melanin formation do involve metal ions, sometimes iron but more usually copper. However the metal itself is already bound to an enzyme complex before the fruit is damaged. The colour change has nothing whatsoever to do with the oxidation of iron or any other metal, it is due to the production of melanin.

If there was enough iron in an apple to cause the colour changes we see that would make apples one of the most iron rich plant foods in the world.

As for layman’s explanations, they’re not easy t find. Using Google the entire subject is flooded with Junior High chemistry projects and “Answers for idiots” type pages of the sort we’ve already run across that don’t; actually explain anything much and engage in a lot of handwaving. I think the problem is that the entire process is just a string of reactions using involving uninteresting intermediates. It’s not the type of thing that really interests laypeople. The key point is that the colour change comes from colourless phenols being converted into melanin. The umpteen intermediate reactions are pretty meaningless.

[slaphead]

@WhyNot

However, this page goes into much more rigorous chemistry than I can remember from college

The paper on the end of that link basically answers Inigo’s question perfectly.
From section 2.

Polyphenol oxidase catalyses the initial step in the polymerization of phenolics to produce quinones, which undergo further polymerization to yield dark, insoluble polymers referred to as melanins. These melanins form barriers and have antimicrobial properties which prevent the spread of infection or bruising in plant tissues. Plants, which exhibit comparably high resistance to climatic stress, have been shown to posses relatively higher polyphenol oxidase levels than susceptible varieties. Other enzyme systems in plants, such as chitinase, peroxidase, lipoxygenase, phenylalanine ammonia lyase, b-1,3-glucanase, etc., also show increased activity when subjected to stress.

From section 3

Enzymatic browning does not occur in intact plant cells since phenolic compounds in cell vacuoles are separated from the polyphenol oxidase which is present in the cytoplasm. Once tissue is damaged by slicing, cutting or pulping, however, the formation of brown pigments occurs. Both the organoleptic and biochemical characteristics of fruits and vegetables are altered by pigment formation. The rate of enzymatic browning in fruit and vegetables is governed by the active polyphenol oxidase content of the tissues, the phenolic content of the tissue, pH, temperature and oxygen availability within the tissue.

As a bonus, it goes into considerable detail on how to control it. As well as the old lemon juice tricky, you can apparently try carbon monoxide, 4-Hexylresorcinol, honey, Aromatic carboxylic acids, Ring-cleaving oxygenases and other goodies. Good luck with the lunches.

[Colophon]

Why do apples and potatoes turn brown when you slice them?

It appears that not only is the brown colour due to melanin, but the enzyme in apples that catalyses the reaction is apparently the same one that produces melanin in your skin when you tan.

More about that enzyme from Wikipedia.