Question

The pungency of onion is due to

• A. Allyl propyl disulphide
• B. Quercetin
• C. Sinigrin
• D. Khudioxidase

Hint:

Onion's sharp flavor and tear-inducing property come from volatile sulfur compounds. These are formed when the onion is cut, releasing the enzyme alliinase, which breaks down sulfur precursors. Allyl propyl disulphide is a well-known example of these pungent compounds.

Answer 1

The Correct Option is A

The characteristic sharp taste and smell (pungency) of onions, as well as their ability to cause tears (lachrymatory effect), are primarily due to a group of volatile sulfur-containing organic compounds. When an onion is cut or crushed, an enzyme called alliinase is released. Alliinase acts on sulfur-containing precursors (amino acid sulfoxides like S-alk(en)yl cysteine sulfoxides) present in the onion cells. This enzymatic reaction produces unstable thiosulfinates, which then further break down into various volatile sulfur compounds, including disulfides.
(1) Allyl propyl disulphide is one of the key sulfur compounds responsible for the characteristic pungency and flavor of onions. Other related compounds like dipropyl disulfide and methyl propyl disulfide also contribute.
(2) Quercetin is a flavonoid, an antioxidant found in onions, but it contributes to color and health benefits, not pungency.
(3) Sinigrin is a glucosinolate found in plants of the Brassicaceae family (like mustard, horseradish), responsible for their pungent flavor, but not in onions.
(4) "Khudioxidase" is not a recognized standard enzyme name associated with onion pungency. The primary enzyme is alliinase. Therefore, allyl propyl disulphide is a major contributor to onion pungency. Allyl propyl disulphide

Answer 2

Biochemical-pathway approach:
When onion tissue is damaged, the enzyme alliinase is released from cell vacuoles and comes into contact with sulfur-containing amino acid precursors stored in the cytoplasm. This reaction generates unstable sulfenic acids that rapidly rearrange into volatile organosulfur compounds, chiefly various propyl and allyl disulfides, of which allyl propyl disulphide is a principal contributor to the characteristic pungent smell and taste of onion. Quercetin is a flavonoid pigment linked to colour and antioxidant activity, sinigrin is a glucosinolate found in Brassica crops rather than onion, and "Khudioxidase" is not a recognised enzyme in this pathway at all.