When bananas ripen, they turn brown in colour. This is due to the formation of a brown product called melanin. The enzyme catechol oxidase acts on its substrate, catechol, leading to the formation of a brown product, melanin, as shown in Fig. 1.1. catechol oxidase catechol (pale pink substrate) melanin (brown product) [Figure 1.1] As melanin is produced, the colour of the reaction mixture changes to brown. The intensity of the brown colour produced is proportional to the concentration of melanin. The more active the enzyme, the more intense the brown colour. A colorimeter is used to measure the absorbance of the reaction mixture. Absorbance is a measure of the light absorbed by a coloured solution. With the reaction shown in Fig. 1.1, the more intense the brown colour, the higher the absorbance. Catechol oxidase can be extracted from bananas.