Why does cooked vegetables turn brown?

Cooked vegetables turn brown due to a chemical reaction called enzymatic browning. This reaction is triggered by the release of enzymes called polyphenol oxidases when plant tissues are damaged or cooked. These enzymes react with polyphenols, which are naturally occurring compounds in vegetables, to produce brown pigments called melanin. The rate of enzymatic browning depends on several factors, including the type of vegetable, the cooking temperature, and the presence of oxygen. Some vegetables, such as potatoes and apples, are more prone to browning than others, such as carrots and peas. Higher cooking temperatures and longer cooking times can also cause more browning. The presence of oxygen is also essential for enzymatic browning, which is why vegetables often turn brown when they are exposed to air after being cooked. To prevent or reduce browning, vegetables can be cooked quickly at high temperatures, kept in an acidic environment (such as lemon juice or vinegar), or stored in an airtight container to minimize exposure to oxygen.