The answer to whether an apple turning brown is a chemical change is a definitive yes. This common observation is a result of enzymatic browning, a complex biological process that fundamentally alters the composition of the apple’s flesh. The color change from white or green to brown indicates that a new substance has formed within the fruit.
Defining Chemical and Physical Changes
To understand the browning process, it helps to distinguish between the two primary ways matter changes. A physical change alters the form or state of a substance, but its molecular composition remains the same. For instance, slicing an apple or melting an ice cube are physical changes, as the original material is still present and the change is often easily reversible.
A chemical change, in contrast, results in the formation of one or more entirely new substances with different properties than the starting materials. These changes are typically irreversible, such as when wood burns and turns into ash and smoke. The color change seen in a sliced apple is a transformation involving molecules reacting to create a completely new compound.
The Enzymatic Reaction Causing Apple Browning
The browning is triggered when the apple tissue is cut, bruised, or damaged, which breaks open individual plant cells. Inside these cells, two separate components are normally kept apart: the enzyme Polyphenol Oxidase (PPO) and phenolic compounds. When the cell walls rupture, the PPO and the phenolic compounds mix together and are exposed to atmospheric oxygen.
Polyphenol Oxidase acts as a catalyst, meaning it speeds up the chemical reaction between oxygen and the phenolic compounds, which are naturally occurring substances within the apple. This reaction is an oxidation process where the PPO uses oxygen to convert the colorless phenolic compounds into intermediate molecules called quinones.
These quinones are highly reactive and quickly undergo a second, non-enzymatic reaction, polymerizing with other quinones and amino acids. This polymerization results in the formation of large, dark-colored pigments known as melanins. The melanins are completely new molecules, providing the irreversible brown color and confirming this sequence of events as a chemical change.
Practical Ways to Prevent Apple Oxidation
Preventing brown discoloration centers on disrupting the PPO-catalyzed oxidation reaction. One effective method involves using acidic solutions, like lemon or lime juice, which have a low pH. The low pH environment causes the PPO enzyme to deform (denaturation), which significantly slows or halts its ability to catalyze the browning reaction.
Another approach uses heat, such as quickly blanching the apple slices in boiling water. High temperatures permanently destroy the structure of the PPO enzyme, effectively deactivating it so it can no longer react with oxygen. Once the enzyme is destroyed, the chemical change that causes browning cannot occur.
Creating a physical barrier is a simpler way to stop the reaction by preventing oxygen from reaching the exposed tissue. Submerging apple slices in water or a sugar syrup, or tightly wrapping them in plastic, physically isolates the fruit from the air. Without oxygen, the PPO enzyme cannot perform the oxidation step, preventing the chemical change and the formation of melanin pigments.