The browning of a banana is a chemical change, signaling an alteration in the fruit’s chemical composition. This transformation, known as enzymatic browning, is a natural process. It is the same reaction responsible for the discoloration seen in cut apples, avocados, and potatoes. The change from a light fruit color to dark brown involves a series of chemical reactions that create entirely new substances.
Chemical Change Versus Physical Change
Chemical changes are defined by the creation of a new substance with different chemical properties than the original material. This change is typically irreversible without another chemical reaction. For instance, baking a cake transforms flour, eggs, and sugar into a new solid structure and flavor profile.
Physical changes alter only the form or appearance of a substance, not its underlying molecular structure. Melting an ice cube is a physical change because the water molecules remain $\text{H}_2\text{O}$, only changing state. The browning of a banana is a chemical change because the original colorless compounds are converted into dark, polymeric pigments.
The Mechanism of Enzymatic Browning
The specific chemical reaction that causes the browning is an oxidation process catalyzed by an enzyme called Polyphenol Oxidase (PPO). PPO is naturally present in the cells of the banana, but it is physically separated from its reaction partners, the phenolic compounds, within different cell compartments. The phenolic compounds are colorless substances stored in the plant cells.
Browning begins when the cellular structure is damaged, allowing the PPO and the phenolic compounds to mix together. Oxygen from the air then acts as a co-reactant. The PPO catalyzes the oxidation of the colorless phenolic compounds into intermediate molecules called quinones.
These highly reactive quinones then undergo a series of non-enzymatic reactions, polymerizing and combining with amino acids and proteins. This polymerization creates large, complex molecules known as melanins, which are the dark, insoluble brown pigments visible on the banana’s surface. The formation of melanin is the irreversible creation of a new chemical compound, confirming the process as a chemical change. The rate of this reaction is highly dependent on factors like temperature, oxygen availability, and the $\text{pH}$ level of the fruit’s tissue.
The Difference Between Ripening and Damage Browning
The chemical process of enzymatic oxidation is identical in both naturally ripened bananas and those that are bruised or cut, but the trigger for the reaction is different. Natural ripening browning, which appears as small brown spots on the peel, is an internal process. It is driven by the fruit’s hormones, primarily ethylene, which promotes cell wall degradation.
As the banana softens, the compartmentalization between the PPO enzyme and the phenolic compounds gradually breaks down, leading to a slow, internal browning. Bruising or cutting, however, causes external browning by immediately rupturing a large number of cells. This immediate damage instantaneously mixes the PPO and the phenolic substrates, exposing them to atmospheric oxygen and causing a rapid, localized browning reaction.
In both instances, the result is the same melanin pigment, but the speed and location of the reaction differ based on the extent of cellular damage. A fully ripened banana undergoes a slower, more pervasive browning, while a bruised spot is a rapid chemical reaction in a concentrated area. The increase in ethylene during ripening also accelerates the respiration rate, making the fruit more susceptible to the chemical changes that lead to browning.
Practical Methods for Slowing Oxidation
Interfering with the three main components of the browning reaction—the PPO enzyme, oxygen, or $\text{pH}$—can slow the discoloration process. One strategy is to reduce the $\text{pH}$ of the fruit’s surface by applying an acid, such as lemon or pineapple juice. PPO is most active at a $\text{pH}$ range of 5 to 7, and lowering the $\text{pH}$ below 3 can significantly inhibit the enzyme.
Reducing the temperature through refrigeration also slows enzyme activity. While the banana peel may turn dark due to a separate chilling injury process, the internal flesh’s enzymatic browning reaction is delayed by the reduced kinetic energy. Finally, blocking oxygen exposure halts the necessary co-reactant for the oxidation. This can be achieved by vacuum-sealing or tightly wrapping cut banana pieces in plastic film.