Browning of the chopped apple can be minimized by :
- (a)using table sugar.
- (b)preserving in a container.
- (c)using lemon juice.
- (d)using milk of magnesia.
Correct — C, using lemon juice. A cut apple browns because the enzyme polyphenol oxidase reacts with oxygen in the air and oxidises the fruit's phenols into brown pigments — enzymatic browning. Lemon juice minimises it two ways: its citric acid lowers the pH so the enzyme works far more slowly, and its ascorbic acid (vitamin C) is an antioxidant that reverses the oxidation. So lemon juice is the effective fix.
- (a)using table sugar. — Sugar can draw out some moisture but does not deactivate the browning enzyme or block oxygen, so it does little to stop the colour change.
- (b)preserving in a container. — A container slows drying, but the trapped and dissolved oxygen still lets the enzyme act, so the cut surface still browns — it is not a reliable fix.
- (d)using milk of magnesia. — Milk of magnesia is a base (alkaline); raising the pH does not switch off polyphenol oxidase the way an acid does, so it will not minimise browning.
Enzymatic browning is caused by polyphenol oxidase, which uses oxygen to convert the phenolic compounds in fruit into brown melanin-like pigments. The reaction needs the enzyme, the phenols and oxygen; remove or slow any one and browning is reduced.
Acid is the key. Lemon juice supplies citric acid (lowers pH, slowing the enzyme) and ascorbic acid (an antioxidant that reverses the oxidation), so it beats sugar, a mere container, or an alkaline substance like milk of magnesia.
- Browning is driven by the enzyme polyphenol oxidase acting on phenols in the presence of oxygen.
- Lemon juice's low pH slows the enzyme; its vitamin C acts as an antioxidant.
- The reaction requires oxygen, so limiting air exposure also helps somewhat.
- Milk of magnesia is alkaline and does not inhibit the enzyme.
- Choosing an alkali (milk of magnesia) when an acid is needed.
- Assuming simply covering the apple fully stops browning.
Asked as an everyday-chemistry item about slowing the browning of cut fruit.
Regular intake of fresh fruits and vegetables is recommended in the diet since they are a good source of antioxidants. How do antioxidants help a person maintain health and promote longevity?
- (a) They activate the enzymes necessary for vitamin synthesis in the body and help prevent vitamin deficiency
- (b) They prevent excessive oxidation of carbohydrates, fats and proteins in the body and help avoid unnecessary wastage of energy
- (c) They neutralize the free radicals produced in the body during metabolism
- (d) They activate certain genes in the cells of the body and help delay the ageing process
Answer(c) They neutralize the free radicals produced in the body during metabolism
Shared idea — antioxidants (like the vitamin C in lemon juice) counter oxidation. UPSC frames it as free-radical neutralisation in the body; this NDA item applies the same anti-oxidation principle to stopping fruit from browning.
- practice — not a real PYQ
The browning of a freshly cut apple is chiefly an example of
- (a)a reduction reaction
- (b)enzymatic oxidation
- (c)fermentation
- (d)caramelisation
Answer(b) enzymatic oxidation — polyphenol oxidase oxidises phenols in the air.
- practice — not a real PYQ
Lemon juice slows the browning of cut fruit mainly because it is
- (a)alkaline
- (b)acidic and rich in vitamin C
- (c)a strong oxidiser
- (d)a source of sugar
Answer(b) acidic and rich in vitamin C — it lowers pH and acts as an antioxidant.