Answer :
Final answer:
The darkening of fruits when exposed to oxygen, known as browning, is due to the enzymatic reaction of oxidation, where polyphenol oxidase in the fruit reacts with oxygen to create the brown pigment melanin. Antioxidants like lemon juice can prevent this browning. Some dark spots on fruits, however, may be caused by diseases or pests rather than oxidation.
Explanation:
The darkening of fruits exposed to oxygen is primarily caused by a chemical reaction known as oxidation. When fruits are cut or damaged, enzymes present in the fruit tissue interact with oxygen in the air. An enzyme in particular, known as polyphenol oxidase, facilitates the reaction between oxygen and certain compounds in the fruit, leading to the production of melanin, the brown pigment responsible for the darkened areas on the fruit's surface. This process is quite similar to how iron rusts when exposed to oxygen, resulting in a brown color.
To prevent this browning, one can apply antioxidants such as lemon juice. Lemon juice contains ascorbic acid (vitamin C), which neutralizes the polyphenol oxidase enzyme, thus protecting the fruit flesh from discoloration. Additionally, storing fruits in environments with reduced oxygen levels can also slow the oxidation process. When packaging fresh fruits and vegetables, an atmosphere with about 8% oxygen is typically used to balance the need for plant respiration with the prevention of oxidation.
Not all dark spots on fruits are due to oxidation, however. Some may be caused by disease or pests, leading to symptoms such as sunken, dark spots or the presence of brown, frass-filled tunnels in the fruit, indicating insect activity rather than a simple reaction with air.
