You would like to add some decorative slices of apple to the top of a baking desert. Whenever you try this, unfortunately, the apples brown before you have the chance to serve them to your dinner guests. While watching a cooking show, you learn that the browning of fruits such as apples is a chemical reaction. It is catalyzed by an enzyme called polyphenol oxidase. To prevent browning, the cooking show suggests that you blanch the fruit by placing it in boiling water or steam for a short time. Explain how blanching affects the enzyme, polyphenol oxidase, to stop the browning process.
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Polyphenol oxidase is an enzyme contained in many fruits. It catalyzes the oxidation of phenols into catechols by using oxygen from the air. These catechols themselves then oxidize to quinones, which polymerize to brown colored pigments. This is the reason for the browning of fruits, especially when the skin is pierced, thus letting oxygen contact the inside of the fruit. If you heat the fruit in boiling water or steam it denatures the enzyme. This means that the enzyme loses its shape and therefore its activity. The amino acids are still connected to each other, its just that the tertiary folding structure of the overall enzyme is disrupted. When the polyphenol oxidase loses its structure, it cannot perform its catalytic role in oxidation and the fruit will stay naturally colored for longer. There are other ways to denature an enzyme including the use of surfactants (soaps), but heating is effective for cooking and won't spoil the taste.
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