Among the following, which is not the correct method for keeping the curd?
- (a)Keeping in stainless steel vessel
- (b)Keeping in copper vessel
- (c)Keeping in plastic vessel
- (d)Keeping in glass vessel
Correct — B, keeping curd in a copper vessel. Curd is acidic (fermentation produces lactic acid, taking its pH down to roughly 4 to 4.6). Copper is a reactive metal that is attacked by acids, so acidic curd slowly dissolves the metal to form soluble copper salts, which are toxic and spoil the food. Copper storage is therefore the one method that is not correct.
- (a)Keeping in stainless steel vessel — Stainless steel is passivated by a chromium-oxide layer and does not react with the mild acid of curd, so it is a safe, standard storage vessel — a correct method, not the exception asked for.
- (c)Keeping in plastic vessel — Food-grade plastic is chemically inert towards lactic acid and does not release metal ions, so it stores curd safely — a correct method.
- (d)Keeping in glass vessel — Glass is chemically inert and unreactive to acids, making it an ideal container for curd — a correct method, not the wrong one.
Curd (dahi) is a fermented, acidic food; lactobacillus bacteria convert lactose to lactic acid, giving it a sour taste and a low pH. Storage vessels must be non-reactive so that the acid does not corrode the container or leach harmful ions into the food. Reactive metals such as copper (and brass) are corroded by dietary acids, whereas stainless steel, glass and food-grade plastic are inert.
The trick is that copper is widely praised for storing drinking water, so a candidate may wrongly assume it is good for everything. But plain water is near-neutral, while curd is distinctly acidic — the same copper that is fine for water reacts with the sour curd. Spot the one reactive metal among four options.
- Curd's sourness comes from lactic acid produced during fermentation; its pH is roughly 4 to 4.6.
- Copper reacts with acids to form soluble copper salts; ingesting these can cause copper toxicity (nausea, vomiting).
- Stainless steel, glass and food-grade plastic are non-reactive with dietary acids and are recommended for acidic foods.
- The same rule bars storing pickles, lemon, tamarind and sambar in copper or brass vessels.
Only the copper vessel reacts with the acid in curd, so option (b) is the incorrect method.
- Assuming copper is good for storing everything because it is recommended for drinking water — water is neutral, curd is acidic.
- Confusing 'non-reactive' with 'expensive' — a cheap food-grade plastic tub is chemically safer for curd than a costly copper pot.
Framed as a 'which is NOT correct' everyday-chemistry item testing metal-acid reactivity.
No directly related past PYQ was found.
- practice — not a real PYQ
Why are pickles and lemon juice traditionally not stored in brass or copper vessels?
- (a)They are too heavy
- (b)The acid in them reacts with the metal
- (c)The metal changes the colour to green permanently without harm
- (d)They evaporate faster
Answer(b) The acid in them reacts with the metal — forming toxic salts.
- practice — not a real PYQ
Which of the following materials is chemically inert and hence suitable for storing acidic foods?
- (a)Copper
- (b)Iron
- (c)Glass
- (d)Zinc
Answer(c) Glass — it does not react with dietary acids.