Pears, like many fruits, can oxidize and turn brown when exposed to air after being cut. To prevent browning and keep your sliced pears looking fresh, you can use lemon juice. Coat the cut surfaces of the pears with lemon juice. Lemon juice contains citric acid, which helps to slow down the oxidation process and prevent browning. You can either squeeze fresh lemon juice directly over the pears or dip the slices in a bowl of water mixed with lemon juice.