UPPSC 2019 Prelims GS Paper-I · Question 25 of 150

Correct — D, 'Pasteurisation — Tea'. The stem asks which pair is NOT correctly matched, and this is the mismatch. Pasteurisation is the mild, controlled heat treatment devised by Louis Pasteur in the 1860s that kills pathogenic and spoilage micro-organisms in a liquid without cooking it — it is applied above all to milk, and also to fruit juice, beer and wine. Tea is not pasteurised at any stage: tea manufacture runs through withering, rolling, oxidation (trade term 'fermentation') and drying, and the beverage is then made by infusing the leaf in near-boiling water, which is not a pasteurisation process. The other three pairs are all correctly matched, so (d) is the answer.

• (a)Desalination — Sea water — Correctly matched, so not the answer. Desalination is precisely the removal of dissolved salts from sea water or brackish water to yield fresh water, whether by distillation (multi-stage flash, multi-effect, low-temperature thermal) or by membranes.
• (b)Reverse osmosis — Potable water — Correctly matched, so not the answer. In reverse osmosis an external pressure greater than the osmotic pressure is applied on the saline side, forcing water through a semi-permeable membrane and leaving dissolved salts behind. It is the standard technology of household water purifiers and of modern desalination plants, and its product is potable water.
• (c)Denaturation — Proteins — Correctly matched, so not the answer. Denaturation is the loss of a protein's native three-dimensional shape (secondary, tertiary and quaternary structure) under heat, acid, alcohol, urea or heavy metals, while the peptide bonds of the primary structure survive. A boiling egg white setting and milk curdling are everyday denaturation.

Four distinct processes, each with one substance it genuinely acts on. Desalination removes dissolved salt from sea or brackish water to produce fresh water. Reverse osmosis is a pressure-driven membrane separation that pushes solvent the 'wrong' way across a semi-permeable membrane — the workhorse of drinking-water purification and one route to desalination. Denaturation is the disruption of a protein's folded structure without breaking its peptide backbone, which is why a denatured enzyme loses its activity. Pasteurisation is the controlled mild heating of a perishable liquid — classically milk — to destroy pathogens and extend shelf life, run either as LTLT (about 63 °C for 30 minutes) or HTST (about 72 °C for roughly 15 seconds). Match process to substance and the odd pair falls out.

Two of the pairs deliberately overlap: reverse osmosis is one of the ways sea water is desalinated, so a candidate may suspect that one of (a) and (b) must be the planted error. Neither is wrong — desalination names the goal and reverse osmosis names one technique for reaching it, and both pairings hold. Work through the options by asking what each process is actually done TO, and only the fourth fails, because pasteurisation belongs with milk and similar perishable beverages, never with tea.

• Desalination = removal of dissolved salts from sea or brackish water; methods include multi-stage flash distillation, multi-effect distillation, low-temperature thermal desalination (LTTD) and reverse osmosis.
• Reverse osmosis = external pressure greater than the osmotic pressure applied on the saline side drives water through a semi-permeable membrane, leaving salts behind.
• Denaturation = loss of a protein's native 3-D structure (by heat, acids, alcohol, urea or heavy metals); the peptide bonds of the primary structure remain intact.
• Pasteurisation (Louis Pasteur, 1860s) = mild heating of milk, juice, beer or wine to kill pathogens — LTLT about 63 °C for 30 minutes, or HTST about 72 °C for around 15 seconds.
• Tea is not pasteurised: the leaf is withered, rolled, oxidised and dried, and the drink is prepared by infusion in near-boiling water.

• Suspecting the desalination / reverse-osmosis overlap — both pairs are correct; RO is simply one route to desalination.
• Believing denaturation breaks peptide bonds — it does not; the primary structure is retained and only the folding is lost.
• Confusing pasteurisation (mild heat, kills most pathogens, product still perishable) with sterilisation (kills all microbes including spores).

The 'which is NOT correctly matched' pairing format is a UPPSC staple. UPSC prefers statement-based versions of the same content — reverse osmosis appeared in 2005 as a question on the direction of osmosis and applied pressure, and desalination in 2008 as a plant-location question.

• practice — not a real PYQ

  In reverse osmosis, the external pressure is applied to:

  • (a)the dilute solution
  • (b)the concentrated (saline) solution
  • (c)the semi-permeable membrane itself
  • (d)neither side

  Answer(b) the concentrated (saline) solution — pressure exceeding the osmotic pressure forces water through the membrane to the fresh-water side.

• practice — not a real PYQ

  Which one of the following is NOT true of the denaturation of a protein?

  • (a)It destroys the secondary and tertiary structure
  • (b)It can be brought about by heat, acids or alcohol
  • (c)It breaks the peptide bonds of the primary structure
  • (d)It usually destroys the protein's biological activity

  Answer(c) — denaturation leaves the peptide bonds of the primary structure intact; only the folded higher-order structure is lost.