Surely you had to observe this phenomenon: if you bite off (or cut off) a piece from an apple, its flesh will darken pretty soon. Initially white (or with a subtle pinkish tinge), it will take on a brownish color. Moreover, apples of different varieties 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?
In fact, everything is very simple and natural. The fact is that an apple (like all other fruits) contains quite a lot of various microelements, including such a useful element for the body as iron. As you know from the course of chemistry, iron in compounds can be in two main oxidation states: +2 and +3. The apple pulp contains iron, which has an oxidation state of +2. What happens if you bite off or cut off a piece of the fruit?
The exposed pulp comes into contact with atmospheric oxygen, and under its influence the iron is gradually oxidized. This oxidation is accelerated by enzymes - oxidases and peroxidases, which are found in apple juice. When biting off or cutting, 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 apple pulp a brownish tint. The speed of darkening depends on the characteristics of the apple variety, that is, on the content of acids and trace elements.
A simple and intuitive experiment can be carried out. Cut in half and quickly apply lemon juice. After a while, the flesh of the first half will darken, while the flesh of the second half will remain light. Why? The reason is that iron +2 ions combined with citrate ions, forming a fairly strong complex and retaining the same oxidation state. Accordingly, until these complex compounds are destroyed, the iron will not change the oxidation state, and the apple pulp will 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."
