Question

Surfaces of fruit and vegetables often become dark because
A. Dirty knife makes it dark
B. Oxidation of tannic acid in the presence of traces of iron from the knife makes it dark
C. Dust of air makes it dark
D. None of the above

Answer 1

Many fruits and vegetables contain tannic acids. Tannic acid is a type of tannin(plant polyphenol). Tannic acid is a weak acid present in vegetables and fruits. Coffee, wine, and chocolate have high amounts of tannic acid.

Complete answer:
The browning of fruit and vegetables is the result of the enzyme browning process. Enzymic browning is a chemical process. Enzymic browning is the process in which the flesh of fruits and vegetables becomes brown due to the oxidation of polyphenol. Oxidation of polyphenol results in the formation of polyphenol oxidase. Tannic acid is a natural polyphenol present in vegetables and fruits. Enzymatic browning can be easily seen in apples and pears. Therefore brown surface is result due to oxidation of tannic acid in the presence of traces of iron from the knife makes it dark.
So the correct option is (B).
Additional Information:
Method to stop enzymatic browning are-
Heating: cooking vegetables and food denature the tannic acid. So cooked food does not undergo an enzymatic browning process. This method can not be applied, when we want to eat uncooked fruit and vegetables.
Acidity: By lowering pH on the surface of vegetables and fruit. Lowering of ph can be attained by applying vinegar or lemon on the surface of a cut or peeled fruits and vegetables.
Water: By putting cut vegetables and fruits in the water. This process helps to cut the supply of oxygen.
Antioxidant: Adding Vitamin C on the surface of fruits and vegetables. Vitamin C slows down the process of oxidation.

Note: Oxidation is a chemical process in which substances get oxidised in the presence of oxygen. enzymatic browning is also a type of oxidation process. Iron also gets oxidised. Oxidation of iron is called the rusting process.