A freshly sliced apple turning brown is a common occurrence in kitchens worldwide. This transformation is not a sign of spoilage but rather a natural chemical process known as enzymatic oxidation, which begins immediately after the apple’s protective structure is broken. When an apple is cut, bitten, or bruised, the damage triggers an internal defensive reaction that changes the exposed surface color. Understanding the specific components involved in this reaction provides clarity on why the apple changes from white to brown.
The Role of Enzymes and Oxygen
Apple browning is a protective mechanism initiated by the fruit itself. This reaction requires three main components: an enzyme called Polyphenol Oxidase (PPO), phenolic compounds, and oxygen from the surrounding air. Inside the apple’s cells, PPO and the phenolic compounds are usually stored in separate compartments, preventing them from interacting.
When the apple’s cell walls are damaged by slicing, the compartmentalization breaks down, allowing the PPO and the phenolic compounds to mix freely. This mixture is then exposed to atmospheric oxygen. PPO acts as a catalyst, using the oxygen to chemically transform the colorless phenolic compounds into new, colored molecules called quinones.
The quinones are highly reactive and rapidly undergo further reactions, leading to the formation of larger, brown-colored pigments known as melanins. These melanins provide the distinct brown coloration on the apple’s surface. The formation of melanin serves to seal the damaged area, potentially helping to prevent further water loss or the entry of pathogens.
Why Apples Brown at Different Rates
The speed at which an apple surface darkens is not uniform across all varieties and is influenced by several factors. A primary determinant is the inherent concentration of the PPO enzyme present within the fruit’s tissue. Certain apple cultivars, like Granny Smith or Cortland, naturally contain lower levels of active PPO, causing them to brown much slower than varieties like Red Delicious or Golden Delicious.
Genetic differences also affect the total amount of phenolic compounds available for the reaction. Varieties with a lower concentration of these compounds will have less material to convert into brown pigments, slowing the overall rate of darkening. Furthermore, the apple’s stage of maturity plays a role, with fruits picked slightly earlier often exhibiting a slower browning rate than those that are fully ripe.
External conditions also impact the speed of the reaction, particularly temperature. Warmer temperatures accelerate the activity of the PPO enzyme, causing the transformation from colorless phenols to brown melanins to happen much more quickly. Refrigerated apple slices show a significantly reduced rate of browning because the lower temperature slows the enzyme’s catalytic function.
Keeping Apples Fresh: Stopping Oxidation
Preventing or slowing the browning reaction involves targeting one or more of the three necessary components: the enzyme, the phenolic compounds, or the oxygen. One of the most effective and common methods is to inhibit the PPO enzyme by lowering the pH of the apple’s surface. Applying an acidic solution, such as lemon juice or a mixture of water and citric acid, lowers the pH below the optimal range for PPO activity, effectively pausing the reaction.
Submerging apple slices in cold water or wrapping them tightly in plastic film works by physically limiting the fruit’s exposure to atmospheric oxygen. While this method does not stop the PPO enzyme from working, it removes the necessary third component for the reaction to proceed, thereby delaying the onset of browning. Commercial anti-browning solutions often contain combinations of calcium and ascorbate, which is a form of Vitamin C, to achieve similar results.
Another method involves using heat to permanently alter the structure of the PPO enzyme. Brief exposure to high temperatures, such as blanching the slices in boiling water for a short period, denatures the enzyme. Once the enzyme’s structure is changed by heat, it can no longer function as a catalyst, and the browning reaction is permanently halted.