Bananas turn brown after being peeled due to a chemical reaction called oxidation. When the banana flesh comes into contact with oxygen from the air, it causes the formation of phenolic compounds that give the banana its characteristic brown color.
When you peel a banana, you expose its flesh to the air, and that's where we get to the heart of the matter: the encounter between the famous enzyme called polyphenol oxidase (or PPO for those in the know) and certain compounds known as phenols. These phenolic compounds are naturally present in the cells of the banana, but as long as everything remains intact, PPO and phenols stay on their respective sides without much interaction. However, as soon as you remove the skin, it's a party: PPO comes into direct contact with the phenols. This enzyme then plays a catalytic role, meaning it accelerates a chemical reaction between the phenols and oxygen in the air – an oxidation reaction. It is this oxidized mixture that will subsequently lead to the formation of dark brown pigments, responsible for the browning of the banana.
When you peel a banana or cut it, you break cell membranes. Normally, these membranes serve to compartmentalize the different compounds inside the fruit. Once destroyed, they release enzymes, notably polyphenol oxidase, which come into contact with substances called phenolic compounds. Previously, the enzymes and these phenols were separated from each other, but now they mix and react together. Without the breaking of these barriers, there is no reaction: thus, no browning. That's why as long as the banana remains intact, it retains its beautiful yellow appearance.
When the banana is peeled, its interior comes into direct contact with the oxygen present in the air. This is exactly what the enzyme called polyphenol oxidase (or PPO for short) was waiting for. This little ultra-reactive enzyme acts like a kind of accelerator, facilitating the chemical reaction between oxygen and certain compounds called phenols that were comfortably nestled in the cells before peeling. This reaction breaks and modifies these compounds, quickly giving rise to new, darker substances. This process, known as enzymatic oxidation, leads to the rapid browning of the banana's flesh.
The brown color that appears on the surface of peeled bananas is due to a molecule called melanin. This molecule is produced from the chemical reaction between quinones formed previously and the oxygen present in the air. The quinones then combine with each other to create pigmented polymer compounds, thereby forming the famous melanin responsible for the browning we observe. The more this reaction progresses, the darker and more visible the color becomes to the naked eye. This process is completely natural and serves as a defense mechanism for the fruit to protect itself against external aggressions.
Enzymatic browning is the same phenomenon responsible for the brown coloration of apples, avocados, and potatoes after they have been cut or peeled.
Cold temperatures slow down the rate of enzymatic browning; thus, storing a cut banana in the refrigerator can prolong its freshness.
Vacuum packaging or storing in an airtight container reduces exposure to air and significantly prevents the browning of peeled or sliced bananas.
Bananas ripen more quickly in the presence of other fruits such as apples or tomatoes because they naturally release ethylene, a gas that accelerates this process.
Not necessarily. The browning itself does not significantly reduce the nutritional value of the banana. However, as a banana ripens, more of its starch is converted into sugars, primarily altering its taste and carbohydrate content, but without compromising its essential nutritional benefits.
No, this phenomenon also commonly occurs with other fruits and vegetables rich in phenolic compounds, such as apples, pears, or avocados, which brown in the same way when cut or peeled.
The low temperatures of the refrigerator significantly slow down the internal ripening of the fruit. However, the cold weakens the skin and disrupts its cellular structure, leading to rapid external darkening due to the deterioration of the surface cells and the accelerated oxidation of the phenolic compounds present.
Browning can be slowed down by limiting the exposure of the flesh to air. For example, you can brush the surface with lemon juice, which contains citric acid that temporarily inhibits the polyphenol oxidase responsible for browning.
Yes, the brown parts of the banana after peeling pose no health risk. They have simply undergone enzymatic oxidation on the surface, changing their appearance but not their food safety. However, their taste or texture may be slightly altered.
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