When an apple is sliced or bruised, its crisp, pale flesh often turns an unappealing brown. This natural process, observed shortly after exposure to air, makes the fruit less desirable, though it remains safe to consume.
The Science Behind Apple Browning
The discoloration in cut apples results from enzymatic browning. This biochemical process involves an enzyme called polyphenol oxidase (PPO), naturally occurring chemical compounds known as phenolic compounds, and oxygen from the air. In an intact apple, PPO and phenolic compounds are stored in separate compartments. When the apple’s cellular structure is damaged by slicing, bruising, or biting, these compartments break open, allowing the PPO and phenolic compounds to mix and contact oxygen.
Upon oxygen exposure, PPO acts as a catalyst, accelerating the oxidation of phenolic compounds into o-quinones. These o-quinones are highly reactive and quickly undergo further reactions to form dark-colored pigments called melanins. The formation of these melanin pigments gives the apple’s surface its characteristic brown appearance.
Factors That Influence Browning
Several factors influence an apple’s browning speed and intensity. Oxygen exposure is significant; more oxygen generally leads to faster browning because it is a necessary reactant in the PPO-catalyzed reaction. The degree of cellular damage also plays a role, as more extensive damage exposes more PPO and phenolic compounds to oxygen, accelerating the reaction.
Temperature also affects enzymatic browning. Higher temperatures typically increase PPO activity, leading to faster browning, while colder temperatures slow down enzyme activity. Different apple varieties possess varying amounts of PPO and phenolic compounds, causing some types to brown more quickly than others. For instance, highly acidic apple varieties, like Granny Smith, tend to brown slower due to PPO being less effective in more acidic conditions.
Preventing Apple Browning
Preventing apple browning involves strategies that either limit oxygen exposure, alter the acidity (pH) to inhibit PPO, or deactivate the enzyme itself. One common method is to create a barrier against oxygen. Submerging cut apple slices in cold water can reduce their exposure to air, thereby slowing the browning. Storing sliced apples in an airtight container or a sealed bag, with as much air removed as possible, also helps by minimizing oxygen contact.
Applying acidic solutions, such as lemon juice, is another effective technique. The acid lowers the pH of the apple’s surface. A lower pH can inactivate the enzyme, preventing it from catalyzing the browning reaction, as polyphenol oxidase functions optimally within a specific pH range. Ascorbic acid also acts as an antioxidant, reacting with oxygen before PPO can, further delaying browning. A salt water solution can also inhibit PPO activity. Briefly blanching apple slices in boiling water can effectively stop browning by denaturing the PPO enzyme due to high heat.