UKPSC Prelims 2025 Q24: When milk is converted into curd, the sour taste is due

Q: What is the answer to UKPSC Prelims 2025 Q24?

C. Correct — C, Lactic acid. Milk carries a sugar called lactose. When curd is set, the lactic acid bacteria in the starter — chiefly Lactobacillus — ferment that lactose, and the acid they release into the milk is **lactic acid**. That acid is what the tongue registers as sour. The same acid also does the setting. As it accumulates the pH of the milk falls, and near pH 4.6 the casein protein loses its stability and coagulates into the soft solid mass of curd. So carry away one linked idea: **the souring and the setting of curd are a single event — lactose fermented into lactic acid.**

UKPSC Prelims 2025 General Studies Paper-I · Question 24 of 149

Verified against the official question booklet. Answer as per UKPSC's provisional answer key.

Answers from UKPSC's provisional answer key. The Commission publishes a provisional key for UKPSC Prelims 2025 and invites objections before issuing a final key. Every answer shown here is the Commission's own provisional answer (key PDF), not our derivation. If the final key changes an answer, this page is updated to match.

When milk is converted into curd, the sour taste is due to:

(a)Acetic acid
(b)Citric acid
(c)Lactic acid
(d)Tartaric acid

Answer

Why

Correct — C, Lactic acid.

Milk carries a sugar called lactose. When curd is set, the lactic acid bacteria in the starter — chiefly Lactobacillus — ferment that lactose, and the acid they release into the milk is lactic acid. That acid is what the tongue registers as sour.

The same acid also does the setting. As it accumulates the pH of the milk falls, and near pH 4.6 the casein protein loses its stability and coagulates into the soft solid mass of curd.

So carry away one linked idea: the souring and the setting of curd are a single event — lactose fermented into lactic acid.

Why the others are wrong

(a)
Acetic acid — Acetic acid, or ethanoic acid, is the sour principle of vinegar, formed when the ethanol in a fermented liquid is oxidised by acetic-acid bacteria.
That is a different souring from the one the stem describes. Wine or beer left open moves towards acetic acid; milk set with a curd starter moves towards lactic acid, because the starting material is lactose and the organisms are lactic acid bacteria.
(b)
Citric acid — Citric acid is the acid identified with citrus fruits — lemon, lime, orange, sweet lime — and it is widely used as a food acidulant.
The sourness that develops as milk becomes curd tracks the lactic acid built up by fermentation of the milk sugar, so citric acid does not account for the taste change the stem asks about.
(d)
Tartaric acid — Tartaric acid is the acid associated with grapes and tamarind, and its potassium salt, cream of tartar, is used in some baking powders.
Its sourness is comparable, so a stem read loosely as 'which acid tastes sour' can draw a reader here. What the stem asks is which acid milk itself yields on fermenting, and that is the lactic acid built up from lactose.

Concept

Sourness in a food is usually the work of a particular acid, and different foods are identified with different acids. Naming the acid means naming where it comes from.

In milk the change is a fermentation. Lactic acid bacteria convert the milk sugar lactose into lactic acid, which both gives the sour taste and, by lowering the pH, curdles the milk protein casein.

Because a substance exists at the end that was not there at the start, setting curd is treated as a chemical change rather than a mere thickening.

This sits in the school-chemistry unit on acids, bases and salts, in the table that maps naturally occurring acids to the foods they are found in.

It also sits at the join between chemistry and biology. The same lactic acid fermentation preserves curd and many pickles, and lactic acid is what accumulates in muscle when glucose is broken down without enough oxygen.

Key facts

Lactose, the sugar of milk, is fermented by lactic acid bacteria, chiefly Lactobacillus, into lactic acid; this is what turns milk into curd.
Lactic acid is chemically 2-hydroxypropanoic acid, with the formula C3H6O3.
As lactic acid builds up, the pH of the milk falls to about 4.6, where casein coagulates and the curd sets.
Casein is the major milk protein and the one that coagulates; whey proteins stay dissolved in the liquid that separates.
Acetic acid, also called ethanoic acid, is the acid of vinegar.
Citric acid is the characteristic acid of citrus fruits such as lemon, lime and orange.
Tartaric acid occurs in grapes and tamarind; its potassium salt, cream of tartar, is used in some baking powders.
Lactic acid also forms in human muscle when glucose is broken down anaerobically during hard exercise.
Setting curd is classed as a chemical change because a new substance is formed and milk cannot be recovered from curd.

The four acids in the option set and the food each is identified with

Lactic acid→Curd and sour milk

Acetic acid→Vinegar

Citric acid→Lemon, lime, orange

Tartaric acid→Grapes, tamarind

Each option is a real food acid; the stem is decided by which one milk itself produces on fermenting. The keyed row is highlighted.

Study next

Lactic acid fermentation set against alcoholic fermentationNaturally occurring acids and their sources: oxalic, malic, formic, butyric, ascorbicPhysical change versus chemical change, with the souring of milk as the worked exampleThe pH scale, and why casein coagulates as milk turns acidicAnaerobic respiration in muscle and the build-up of lactic acidMilk composition: lactose, casein, whey protein and milk fatPasteurisation and the role of a starter culture in setting curd

Common traps

Acetic and lactic acid are both fermentation products; the substrate decides which. Ethanol oxidised by acetic-acid bacteria gives acetic acid, lactose fermented by Lactobacillus gives lactic acid.
'Sour' names a taste, not a compound. Several acids taste sour, so the deciding step is which acid this particular fermentation produces, not which acid tastes sourest.
Tartaric acid belongs with grapes and tamarind. Read as a loose 'which acid is sour' question, that option can look as good as the keyed one.
Curd formation is a chemical change, not a thickening — a new substance is formed and the milk cannot be recovered.
Holding only the food half of the lactic acid fact leaves the physiology version unanswerable; the same acid appears in curd and in exercising muscle.

This idea appears as a direct item like the present one, naming the acid responsible for a food's taste, and as a matching set that pairs acids with the foods they occur in.

It also comes from the other side, as an example in physical-versus-chemical change: souring of milk is offered as the chemical change because a new substance, lactic acid, has been formed.

Related PYQs

UPPSC_2025_PRE_GSI_Q772025

The same fact, handed over in the opposite direction. That item supplies 'Lactic acid – Curd' as a pair to be judged rather than asking which acid curd contains, and it puts tartaric acid with grapes, which is one of the options here. It also brings in acetic acid, which belongs to vinegar rather than to potato. What differs is the load: this stem needs one recall, that one needs three pairings weighed together and then converted into a code.

NDA_GAT_2022_I_Q592022

The same event classified instead of named. There, 'Milk turning sour' is the chemical change, set against dissolving sugar, melting ice and crystallisation, which are physical. Here you must name the acid the change produces. Both are answered from one sentence — fermentation of lactose yields lactic acid, a substance not present in the milk before — but one question tests the category and the other tests the compound.

CDS_GK_2022_I_Q82022

Souring again, but examined as oxidation. That item places wine turning sour and butter changing taste on the oxidation side, where oxygen acts on ethanol and on fats. Curd takes a different route: Lactobacillus ferments lactose anaerobically into lactic acid. What is shared is the surface word 'sour'; what differs is the chemistry beneath it, and reading the two together is what keeps the acetic acid route and the lactic acid route apart.

Practice

practice — not a real PYQ
The souring of milk into curd is brought about mainly by which of the following bacteria?
- (a)Rhizobium
- (b)Lactobacillus
- (c)Nitrosomonas
- (d)Clostridium
Answerb — Lactobacillus species ferment the lactose in milk into lactic acid, which sours the milk and coagulates its casein into curd.
Rhizobium fixes atmospheric nitrogen in the root nodules of legumes. Nitrosomonas oxidises ammonia to nitrite in the nitrogen cycle. Clostridium species are anaerobes known for free-living nitrogen fixation and for the toxin of botulism.
practice — not a real PYQ
Which one of the following pairs of an acid and its natural source is correctly matched?
- (a)Tartaric acid — Lemon
- (b)Citric acid — Vinegar
- (c)Acetic acid — Curd
- (d)Lactic acid — Sour milk
Answerd — sour milk and curd owe their taste to lactic acid produced from lactose, so this pairing holds.
The acid of lemon is citric, not tartaric, so (a) fails. The acid of vinegar is acetic, not citric, so (b) fails. Acetic acid belongs to vinegar, while curd is the lactic acid case, so (c) is wrong.
practice — not a real PYQ
The setting of curd from milk is best described as:
- (a)A physical change, because the curd can be turned back into milk
- (b)A chemical change, because a new substance, lactic acid, is formed
- (c)Only a change of state from liquid to solid
- (d)The evaporation of water from milk
Answerb — lactic acid is present at the end and was not present at the start, and the formation of a new substance is the test of a chemical change.
(a) fails because curd cannot be converted back into milk. (c) names only the visible thickening and misses the reaction behind it. (d) fails because curd holds essentially the same water the milk did — the change is a fermentation, not a drying.
practice — not a real PYQ
As curd sets, the acid produced lowers the pH of the milk. The milk protein that coagulates at this stage is:
- (a)Casein
- (b)Keratin
- (c)Collagen
- (d)Myosin
Answera — casein is the major protein of milk, and as lactic acid lowers the pH towards its isoelectric point the casein loses its charge, aggregates and sets as curd.
Keratin is the structural protein of hair, nails and the outer skin. Collagen is the protein of connective tissue, tendon and bone matrix. Myosin is a contractile protein of muscle.