When you slice into an apple, it immediately begins to turn brown, a common occurrence. This rapid discoloration can make cut apples less appealing. Understanding why this happens and how to slow it down can help maintain the fresh appearance of your fruit.
The Browning Reaction
The browning of apples is due to a process called enzymatic browning. This reaction involves specific components within the apple’s cells: polyphenol oxidase (PPO) enzymes, naturally occurring phenolic compounds, and oxygen from the air. When an apple is cut or bruised, its cellular structure is damaged, allowing these three elements to mix.
Once exposed to oxygen, the PPO enzymes act on the phenolic compounds, converting them into o-quinones. These o-quinones then react further to form brown pigments known as melanins. This process is similar to how rust forms on metal. The rate at which an apple browns can vary significantly depending on the apple variety, as some contain higher concentrations of PPO and phenolic compounds than others.
Preventing Browning
Preventing apples from browning involves interfering with one or more parts of this chemical reaction. One effective strategy is to limit the apple’s exposure to oxygen. Submerging cut apple slices in plain water can create a barrier against the air. Wrapping slices tightly in plastic wrap immediately after cutting also works by blocking oxygen contact.
Another common method is to alter the pH level on the apple’s surface, as PPO enzymes are less active in acidic environments. Acidic solutions like lemon juice can effectively slow down the browning process. A solution of salt water can also be highly effective, as the chloride ions can inhibit PPO activity. Brief blanching, or quick heating, can denature the PPO enzymes, permanently stopping the browning reaction, though this also cooks the apple and alters its texture.
Consuming Browned Apples
Despite their less appealing appearance, browned apples are generally safe to eat. The enzymatic browning process is primarily an aesthetic change and does not indicate spoilage or harmful bacteria. While the texture might become slightly softer or grainier, and the flavor subtly altered, the apple remains edible.
However, it is important to distinguish between browning from oxidation and actual spoilage. Apples should be discarded if they show other signs of decay, such as mold growth, an off-smell, or a slimy or mushy texture that extends beyond the browned areas. Unless these additional signs are present, a browned apple is simply a natural result of exposure to air.