Browning of the cut surface of some fruits and vegetables is due the presence of a group of enzymes called polyphenoloxidases. These enzymes are released by the broken cells and they catalyse the reaction between colourless molecules called polyphenols and molecular oxygen. This reaction creates coloured compounds and these new compounds can spontaenously cross react with one another to form black-brown complexes called melanins.

Food processing and cooking often involve procedures which are intended to inhibit the action of polyphenoloxidases. Why do you think a cook immediately places freshly peeled potatoes into a pan of water? Or why do people squeeze a few drops of lemon juice on to a freshly cut avocado? Mushrooms contain high levels of polyphenoloxidases, so how do you think pre-sliced packaged ones can be prevented from going brown?

In this technique, the change from a colourless solution of catechol to coloured benzoquinone is followed with a colorimeter. A fruit extract is added to a solution of catechol and the rate of formation of coloured benzoquinone is measured. The faster the rate of increase in absorbance of the reaction mixture, the greater the polyphenoloxidase activity of the fruit extract.