Question

A cut apple turns brown when kept in air for some time. This is due to presence of
A. Ferrous ions
B. Ferric ions
C. Magnesium ion
D. An enzyme action

Answer 1

Hint: Enzymes are referred to as biocatalysts. These are protein molecules that speed up the rate of reaction of the chemical reactions occurring inside the cells. Enzymes are essential for various reactions in the body such as digestion and metabolism.

Step by step answer:Apples when cut into pieces and kept in the air-exposed for a long time turns brown. This occurs due to the oxidation process that goes on when the inside of the apple is exposed to air having oxygen and water molecules. Apple contains an enzyme known as polyphenol oxidase (PPO). This enzyme when comes in contact with oxygen molecules oxidized polyphenols into o-quinones. These o-quinones thus form reacts with the neighboring amino acids to produce melanins. When melanins are produced, they give a brown coloured appearance to the apple. This process is known as enzymatic browning. There are different methods for the preservation of apples from enzymatic browning. Apples are put in the refrigerator to slow down the process. Putting them in the fridge slows down the metabolic reactions of bacteria thus making the food remain unspoiled for a little longer. Apples can be coated with acidic juices that slow down the reaction of PPO. Oxygen diffusion is also reduced by coating apples with sugar, honey, or caramel. PPO activity is also eliminated by blanching apples into boiling water for 4-5 minutes.
Hence, the correct option is D. An enzyme reaction

Note: Polyphenol oxidase (PPO) is also present in lemons and pineapples. These fruits contain antioxidants and acids in their juices which slows down the reaction. The low pH of the juices reduces the activity of PPO and slows down browning.