Sour taste of vinegar is due to

Question

UnlockIAS · Accepted Answer

Correct — D, None of these. Vinegar is a dilute aqueous solution of acetic acid, and acetic acid is not on the list. That is the whole question. Vinegar typically contains between 4 and 18 per cent acetic acid by volume, the rest being water plus trace flavour compounds, and the acid is produced by a double fermentation: yeast converts simple sugars to ethanol, and acetic acid bacteria then oxidise the ethanol to acetic acid. The very word records the chemistry — vinegar comes from the Old French vyn egre, sour wine. Acetic acid, CH₃COOH, is the second simplest carboxylic acid after formic acid, and its water-free form is called glacial acetic acid because it sets into ice-like crystals a little below room temperature, at 16.6 °C. Every one of the three named acids is a real food acid, but each belongs to a different foodstuff, and none of them belongs to vinegar. Because acetic acid is absent from the options, the escape option is the only truthful answer — and this is the disciplined use of 'None of these': you may take it here because you can name the substance that ought to have been there and show that it is not.

Why the other options are wrong — (a) The acid of citrus fruit — lemon, lime, orange — and the commonest acidulant in soft drinks, where it supplies the tartness. It also gives its name to the citric acid cycle of cellular respiration. It is the most tempting wrong answer here simply because it is the food acid most people can name, but a bottle of vinegar contains essentially none of it.  (b) The acid of grapes and tamarind. In the kitchen it is met as cream of tartar, the potassium salt used in baking powder, and in winemaking it is the acid that precipitates as crystals in the cask. It is genuinely a souring agent, but of imli and of wine — and note that when wine turns to vinegar it is not the tartaric acid that has changed, it is the alcohol that has been oxidised to acetic acid.  (c) The acid of curd and sour milk, formed when lactic acid bacteria ferment lactose, and the compound that accumulates in muscle during prolonged anaerobic exertion and is associated with the sensation of fatigue. It sours dahi, not sirka. Both vinegar and curd are products of microbial fermentation, which is exactly why this option is placed here — the process is shared, the acid is not.

Concept: Sourness is the taste of hydrogen ions, so every sour food owes its taste to some acid, and school chemistry expects you to hold a short table pairing the common weak organic acids with the foods that carry them. Acetic acid is vinegar. Citric acid is citrus fruit. Tartaric acid is tamarind and grapes. Lactic acid is curd and sour milk. Formic acid is the ant's sting — the name comes from formica, Latin for ant. Oxalic acid is tomato and spinach, malic acid is the apple, butyric acid is rancid butter, and ascorbic acid is vitamin C in amla and citrus. All of these are weak acids: they only partly dissociate in water, which is why they are safe to eat while mineral acids of similar strength on paper are not. Acetic acid's dissociation constant corresponds to a pKa of about 4.76, making it a slightly weaker acid than formic acid at about 3.75 — a comparison UPSC has asked directly.

State PCS 2025 Q61 — Science & Technology PYQ with Answer

State PCS 2025 Q61 — Science & Technology PYQ with Answer