Polyphenol oxidase is a chemical enzyme found in fruits and vegetables that catalyzes the oxidation of phenols to quinones when exposed to air. These quinones can then form polymers that are dark in color and that is why fruit will brown over time, especially if the skin is pierced thus exposing the inside to oxygen. Blanching is a process that deactiviates the enzyme to slow down the browning process so the fruit or vegetable will remain fresher for longer. Basically is it heating the food to 75-100 degrees Celsius for a short period of time to deactivate the polyphenol oxidase. Enzymes are very temperature dependent to work properly and heating them even for a short period of time will cause then to distort their shape and therefore deactivate them from their chemical process.