The browning of fruit over time is a chemical process. Polyphenols are chemicals naturally found inside the fleshy bodies of all fruits (and other foods too). Enzymes called polyphenol oxidases can oxidize these polyphenols into quinone polymers which have a distinctive dark color. Oxygen is also required for this reaction. Inside the fruit, the chemicals are protected from oxygen. But when the skin is pierced, oxygen enters the fruit and the oxidation reaction occurs. Blanching is a cooking technique where the fruit is exposed temporarily to high heat. This causes the enzyme to denature, or fall apart. When the enzyme unfolds and comes apart, it loses its activity and the browning oxidation reaction is halted.