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Why do apples turn brown when cut?


What happens to an apple that has been cut for a long time? It looks like it has changed its "face" and turned into an ugly brown color, which makes people lose their appetite. Then why does the apple cut brown? Some people think that the apple in the divalent ferrous ions by the air O2 oxidation into trivalent iron ions, so the apple is yellow after peeling, but in fact it is not. According to the experiment, the "yellow-green" color in the light yellow-green juice is caused by cytochrome, not Fe2+ plasma. The brown substance on the peeled side of the apple is the color reaction of phenolic compounds. Phenolic compounds (especially polyphenols) are easily oxidized to quinone, quinone compounds have color, such as o-benzoquinone for red, p-benzoquinone for yellow, tannin also contains phenolic hydroxyl, oxidation for black. With the participation of enzymes, the rate of phenol oxidation to quinone is faster, for example, catechol (catechol) is quickly oxidized to o-benzoquinone by oxygen under the catalytic effect of polyphenol oxidase.


Apple cut surface brown, is because the apple cells of a variety of phenolic compounds are oxidized to generate quinones, quinone slightly soluble in water so that the apple cut surface brownish yellow. Not only apples, pears and some vegetables (such as potatoes, eggplant, etc.) peeled or injured brown, is also the result of phenolic oxidase action.

So how to make the cut apple does not change color? These methods can be used.


Adding antioxidant substances such as vitamin C. However, this method has the disadvantage that a person can be poisoned by excessive intake of the vitamin. By isolating oxygen and preventing chemical reactions from occurring, the apples will not change color. For example, if you fill the bowl containing the apples with some lightly salted water, diffuse it over the apples and soak them for two or three minutes, the enzymes in the apples will not be able to combine with oxygen, and the apples will not change color and taste bad, and the salted water will also improve the nutrition of the apples. Start with the apples themselves. After research, it was found that there is one - polyphenol oxidase plays a key role in the apple discoloration process. If this enzyme is inactive, then the discoloration reaction will not happen. Scientists thought of turning off the gene that synthesizes this enzyme directly, thus working on an apple that does not discolor.

The organic tannins in apple flesh also react chemically with the iron in the knife blade to produce a black iron tannate that covers the surface of the apple flesh, making the cut surface of the apple discolored. The discolored apple flesh does not affect consumption, and the small amount of iron tannate is not harmful to human health. However, the tannic acid in the flesh is difficult to dissolve in water, so it is difficult to wash off if it is stained on a towel or handkerchief.