Lemon juice may help prevent sliced apples from oxidizing in pie filling. Its acidity slows the enzymatic browning that begins when cut apple flesh meets oxygen.
The reaction involves polyphenol oxidase, an enzyme in apples that converts phenolic compounds into brown-colored quinones. Lemon juice lowers the mixture’s pH and also adds antioxidant vitamin C, helping preserve a lighter appearance while the apples are prepared. It is usually tossed with the slices rather than used as a heavy flavoring ingredient.
This is a preparation step, not a guarantee that the apples will never brown. Other acidic juices can work too, and quick cooking also limits browning. The lemon juice may contribute a small amount of tartness, which is often welcome alongside sugar, butter, cinnamon, or nutmeg. The technique is especially associated with cooking apples such as Bramley, whose slices are commonly covered with lemon juice before being baked or stewed. Browning itself is not spoilage; it is primarily a visual and flavor-quality change caused by oxidation.