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Because the knife strike releases a substance that causes browning. When we cut the fruit, we also shatter some of its cells. “At that time, the enzyme that darkens the apple is released”, says biochemist Maria Inés Genovese, from USP. In addition, contact with oxygen ends up lending a hand to the work of this enzyme – which goes by the name of polyphenol oxidase and is also present in other fruits, such as pears and bananas, and in potatoes. That is why, by the way, there is a custom of leaving the potato slices or sticks in the water, before sending them to the stove. Thus, they are protected from oxygen in the air, and the enzyme is inactive. With apples, another technique is used: dripping lemon drops on exposed parts. “The acidity of the citric fruit prevents the action of the polyphenol”, says Maria Inês. But there is no danger in eating these foods after they have browned. As housewives are well aware, this does not mean that the vegetable is spoiled.