I sliced some apples, along with some other fruit, arranged them neatly on a fruit tray, and set them aside to serve to some guests who were coming over later. Then I sliced a lemon, squeezed out the juice in a bowl, and coated all the sliced apples with this lemon juice. When my guests came over an hour later, my apples were still perfectly fresh.
So whats the mystery behind lemon juice stopping the apples from turning brown? Lets answer this questions first – why do apples start turning brown after a few minutes? When the knife slices through the apple skin, the fruit is exposed to air, and the enzymes and iron present in the apple oxidize. This browning effect is in fact to make the fruit unattractive to other animals.
Lemon juice, when applied to the fruit slows down oxidizing in two ways. First, lemon juice is abundant in vitamin C which is a natural antioxidant and reduces oxidation. Secondly, the citric acid in the juice reduces the pH value of the surface of the apple to a level where oxidation becomes difficult to occur.
Applying lemon juice holds for pears, potatoes, eggplant, and other fruits and vegetables which turn brown just like apples do. If lemon juice is not available, you can also try using salt water which also does a good job in avoiding the browning effect.