Surely you had to observe such a phenomenon: if you bite off (or cut off) a piece from an apple, its flesh will soon darken. Originally white (or with a slightly noticeable pinkish tint), it will take on a brownish color. Moreover, apples of different varieties also darken in different ways: one is faster, the other is slower, and the degree of saturation of the "darkening" is also not the same. Why is this happening?
Pick your recipe
In fact, everything is very simple and natural. The fact is that in an apple (like in all other fruits) it contains quite a lot of various microelements, including such an element useful to the body as iron. As is known from the chemistry course, iron in compounds can be in two main oxidation states: +2 and +3. Iron is present in the pulp of the apple, having an oxidation state of +2. What happens if you bite off, or cut off a piece of fruit?
The exposed pulp comes into contact with oxygen in the air, and under its influence, the iron gradually oxidizes. This oxidation is accelerated by enzymes - oxidases and peroxidases, which are found in apple juice. When bitten off or cut, a lot of juice is released, and the released enzymes "get down to business." As a result, compounds of various compositions (oxides, hydroxides, salts, complex complexes) are formed on the surface of the pulp, in which iron now has an oxidation state of +3. It is these compounds that give the flesh of the apple a brownish tint. The rate of darkening depends on the characteristics of the apple variety, that is, on the content of acids and trace elements.
You can conduct a simple and visual experiment. Cut in half quickly apply lemon juice. After a while, the flesh of the first half darkens, and the flesh of the second remains still light. Why? The reason is that iron +2 ions combined with citrate ions, forming a fairly strong complex and preserving the previous oxidation state. Accordingly, until these complex compounds are destroyed, the iron does not change the oxidation state, and the flesh of the apple does not darken.
In addition, lemon juice contains a lot of ascorbic acid - a powerful natural antioxidant that "binds" oxygen, preventing it from "getting down to business."