Apples when cut with a knife turn brown in color. The oxygen in the air will react with the iron present in the cells of the apple forming iron oxides. This formation of iron oxides is the same as the rust that forms on the iron objects. This reaction is carried out by the enzyme called Polyphenol oxidase which increases the reaction. If the brown apple is cut again, then the inside is still white.  This is because the air has not reacted with the iron present in the apple and the cells are  still intact. Many other fruits also turn brown when they are cut such as pears, bananas, and potatoes.
The chemicals present in the fruits react with the oxygen in the air to make the fruits turn brown. The cut surface of the fruit that is covered with lemon juice will be prevented from the reaction of oxygen and, hence, will not turn brown. When the fruit is cut, many of its cells at the cut end will break, and their inner environment will get exposed to the air. The oxygen mixes with the phenols that are present in the apple and with the help of the Polyphenol oxidase which forms the enzymatic browning. The brown color is formed due to the formation of pigments called melanins. All of this happens due to the process called “oxidation.†If you do not want the brown color to form, then the Polyphenol oxidase enzyme activity has to be stopped by keeping the fruit in water.
The phenolic compounds that are present in the cell chloroplasts are found to be oxidized by the enzyme PPO to O-quinones. The natural phenols present in the apple cells got converted to o-quinones which are colorless precursors. These precursors will later convert to brown-colored products. The o-quinones will produce brown-colored compounds by combining with amino acid and protein. Some fruits will convert to a brown color very rapidly. This might be due to the variation in the growth conditions and maturity of fruits. Browning of fruits can also be prevented by applying sugar.
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