Why those an apple become brown after sometime

Enzymes in Apples

The cells of apples, and many other fruits and vegetables, act as balloons, holding enzymes ( a protein that increases the rate of chemical reaction) inside. Portions of the cells contain phenolic compounds, and other parts house enzymes. These enzymes, known as polyphenol oxidase enzymes, are found in the chloroplasts of the cells. When the cells remain intact, as when the apple stays whole, the fruit keeps its normal coloring, but cutting the fruit and exposing it to air changes this.

Oxygen Invasion

Anything that breaks the walls of an apple allows the polyphenol oxidase enzymes and the phenolic compounds to come together. When air enters the mixture, oxidation occurs, producing o-quinones. These mix with amino acids or proteins to create the browning seen on the cut surface of an apple.

Reverse Osmosis

Reverse osmosis is a filtration method removes many types of large molecules and ions from the solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the holes, but should allow smaller components such as the solvent to pass freely.
Reverse osmosis is most commonly known for its use in purify seawater into drinkable water.