Q.Why is distillation required for producing certain alcoholic drinks?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Microbial Fermentation Foods
Let’s begin with something you already know. Think of a pot of milk left out in warm weather. After a few hours, it turns sour and thickens into curd. That change is not spoilage in the usual sense — it is a controlled transformation caused by tiny living organisms called microbes. This is the heart of microbial fermentation.
Fermentation is a process in which microorganisms — bacteria, yeast, or moulds — break down organic substances (like sugars) in the absence of oxygen, producing energy for themselves and, as a by‑product, substances that change the food. When we deliberately use this process to make food, we call the result microbial fermented foods.
You have eaten many of them without thinking twice: curd (yogurt), idli and dosa batter, bread, cheese, pickles, vinegar, and even the dark chocolate you might enjoy. Each of these relies on specific microbes doing their work.
The NCERT textbook (Class XII, Biology, Chapter 10: Microbes in Human Welfare) introduces fermented foods under the topic “Microbes in Household Products.” It lists curd, cheese, idli, dosa, and bread as common examples. The key point is that these are not modern inventions — they have been part of Indian and global diets for centuries.
Why does this matter? Because fermentation does three things that are valuable for us:
- Preserves food — The acids or alcohol produced by microbes prevent spoilage by harmful bacteria. Pickles stay edible for months because of lactic acid from fermentation.
- Improves digestibility — Microbes break down complex molecules. Lactose in milk becomes easier to digest in curd; the proteins in soy become more digestible in tempeh.
- Enhances flavour and texture — The tang of yogurt, the airy holes in bread, the umami of soy sauce — all come from fermentation.
Let’s look at a few examples from your daily life, as the NCERT would describe them:
- Curd (yogurt): Milk is boiled and cooled, then a small amount of previous curd (containing Lactobacillus bacteria) is added. The bacteria convert lactose into lactic acid, which thickens the milk and gives it a sour taste.
- Idli and dosa batter: Rice and urad dal are soaked, ground, and left to ferment overnight. Naturally occurring Leuconostoc and other bacteria produce carbon dioxide gas, which makes the batter rise and gives idlis their spongy texture.
- Bread: Baker’s yeast (Saccharomyces cerevisiae) ferments the sugars in dough, releasing carbon dioxide that makes the dough rise. The alcohol produced evaporates during baking.
- Cheese: Milk is curdled using rennet (an enzyme) or acid, then specific bacteria and moulds are added to ripen it. Different microbes give different cheeses their distinct flavours — for example, Penicillium roqueforti gives blue cheese its veins and sharp taste. …
Distillation is required for producing certain alcoholic drinks because the natural fermentation process can only yield a limited alcohol concentration — typically around 8–15% — after which the yeast dies or becomes inactive. To obtain drinks with a higher alcohol content, such as whisky, rum, or brandy, the fermented liquid must be heated so that alcohol (which boils at a lower temperature than water) vaporises first. The vapour is then cooled and condensed back into liquid form, giving a much stronger alcoholic product. This process als …
Distillation is required to produce alcoholic drinks with a higher alcohol content than what natural fermentation alone can achieve, because fermentation stops when the alcohol level reaches about 15% — distillation concentrates the alcohol by separating it from water and other components.
Fermentation, the natural process by which sugars are converted into alcohol by yeast, has a built-in limit. Yeast cannot survive in an environment where the alcohol concentration exceeds roughly 15 percent. This means that any drink produced solely by fermentation — beer, wine, cider — will have an alcohol content capped at that level. If you want a drink with a higher alcohol percentage, such as whisky, rum, vodka, or brandy, you need a method to concentrate the alcohol that is already present in the fermented liquid.
That method is distillation. Distillation works on a simple principle: alcohol boils at a lower temperature (about 78°C) than water (100°C). When the fermented liquid — called the "wash" or "wine" — is heated carefully in a still, the alcohol vaporises first. This vapour is then collected and cooled in a condenser, turning it back into liquid. The resulting liquid, called the distillate, has a much higher alcohol content, often between 40% and 60% or even higher.
The exact alcohol percentage of the final drink depends on how many times the liquid is distilled and whether the distiller keeps only the "heart" of the distillate, discarding the first and last parts (the "heads" and "tails") which contain unwanted compounds.
Distillation does more than just raise the alcohol level. It also allows the producer to control the flavour of the final drink. During distillation, various flavour compounds — called congeners — are separated based on their boiling points. Some of these compounds contribute pleasant aromas and tastes, while others can be harsh or even toxic. By carefully managing the temperature and the cut points, a skilled distiller can create a spirit with a clean, smooth character or one with a rich, complex flavour profile. …
A different angle: reason from the physical chemistry of boiling points, not from yeast biology.
Ethanol boils at roughly 78 degC, well below water's 100 degC. In any mixture, the more volatile component (the one with the lower boiling point) escapes as vapour first when heated. A fermented liquid is mostly water with a modest amount of ethanol dissolved in it — so gently heating it means ethanol vapour rises out preferentially, while most of the water stays behind as liquid. …
Showing the 12 most recent of 16 on this concept.
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Choose the correct statements among the following A) Organisms that enrich the nutrient quality of soil is called biofertilizers B) Certain bacteria groups anaerobically on cellulosic material produce large amount of Methane, CO2 and H2 C) In anaerobic sludge digester bacteria produce hydrogen sulphide only D) Wine and beer are produced by distillation of rice (A) A, B (B) B, A (C) C, D (D) B, C
›Reveal solutionSolution
This checks basic facts about biofertilisers and biogas/microbial fermentation; only A and B hold up, C and D contain clear factual errors.
Concept and Intuition
Biofertilisers are living organisms (bacteria, fungi, cyanobacteria) that enrich soil nutrient quality (nitrogen fixation, phosphate solubilisation, etc.) — a sustainable alternative to chemical fertilisers. Separately, anaerobic microbial digestion of cellulose-rich organic waste (cattle dung, plant residue) in a biogas/sludge digester by a consortium including methanogens produces a mixture of gases dominated by methane, along with CO₂ and hydrogen — this is the basis of biogas plants (gobar gas).
Step-by-Step Solution
- A — 'organisms that enrich the nutrient quality of soil are biofertilisers' — this is exactly the definition used for Rhizobium, Azotobacter, mycorrhiza, cyanobacteria etc. True.
- B — 'certain bacteria acting anaerobically on cellulosic material produce large amounts of methane, CO₂ and H₂' — this describes the microbial consortium in a biogas plant/dung slurry. True.
- C — 'anaerobic sludge digester bacteria produce hydrogen sulphide only' — false; the dominant, useful product is methane (that's the whole point of a biogas/sludge digester), not H₂S. …
- AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQQ.Assertion (A): The puffed up appearance of dough is due to the production of CO2 gas by fermentation of bacteria Reason (R): Lactic acid bacteria improves the nutritional quality of curd by increasing the vitamin B12 (A) Both (A) and (R) are correct and (R) is the correct explanation to (A) (B) Both (A) and (R) are correct but (R) is not correct explanation for (A) (C) (A) is correct (R) is wrong (D) (A) is wrong (R) is correct
›Reveal solutionSolution
Both statements are true but unrelated ⇒ option (B).
Concept and Intuition
Microbes serve many independent roles in food. Fermentation of dough releases CO2 that makes it puff up; separately, lactic acid bacteria in curd both curdle milk and enrich it with vitamin B12. Two true facts about different products do not explain one another.
Step-by-Step Solution
- Assertion: dough's puffed appearance is due to CO2 from fermentation — this matches the NCERT statement, so A is TRUE.
- Reason: LAB improve the nutritional quality of curd by increasing vitamin B12 — also a standard NCERT fact, so R is TRUE.
- Check the link: A is about gas production in dough; R is about vitamin enrichment in curd. R gives no cause for A. …
- AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQQ.Choose the incorrect statement among the following A) By Clostridium acetic acid can be produced commercially B) Saccharomyces is used for commercial production of ethanol C) Land contaminated with toxic wastes can be removed by microbes D) Fleming working on the Aspergillus discovered penicillin antibiotic (A) A, B (B) B, C (C) A, D (D) C, D
›Reveal solutionSolution
Clostridium is linked to butyric acid (not acetic acid, which is Acetobacter's job), and penicillin came from Penicillium (not Aspergillus) — these are the two incorrect statements. Answer: (C) A, D.
Concept and Intuition
This question checks memorised organism-to-product/discovery pairings in applied microbiology. Different microbes are prized for different fermentation products: Saccharomyces cerevisiae for ethanol (beverages/biofuel), Acetobacter aceti for acetic acid (vinegar), and Clostridium butylicum for butyric acid — so pairing Clostridium with "acetic acid" swaps in the wrong product. Separately, bioremediation — using microbes to detoxify or clean up polluted land/water — is a genuine, widely-used application. Finally, the discovery of penicillin is a landmark microbiology history fact: Alexander Fleming observed that a mould contaminating his bacterial culture plate, later identified as Penicillium notatum, inhibited bacterial growth — Aspergillus was not involved.
Step-by-Step Solution
- (A) "By Clostridium acetic acid can be produced commercially" — Clostridium is associated with butyric acid, not acetic acid (that's Acetobacter aceti's role). INCORRECT statement.
- (B) "Saccharomyces is used for commercial production of ethanol" — true; this is the standard yeast used in alcoholic fermentation. CORRECT statement. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Propionibacterium sharmanii is used for making of (A) Curd (B) Cheese (C) Bread (D) Toddy
›Reveal solutionSolution
Propionibacterium shermanii ferments lactic acid to propionic acid, acetic acid and CO2 — the CO2 bubbles create Swiss cheese's characteristic holes ("eyes"). Answer: cheese.
Concept and Intuition
Traditional Swiss (Emmental-type) cheese-making uses a two-stage fermentation: lactic acid bacteria first ferment milk sugar, and then Propionibacterium shermanii ferments the resulting lactic acid into propionic acid and acetic acid, releasing carbon dioxide gas. This trapped CO2 forms the large characteristic holes ("eyes") seen in Swiss cheese, and the propionic/acetic acids contribute its distinctive tangy flavour.
Step-by-Step Solution
- Identify the organism: Propionibacterium shermanii, a classic dairy fermentation bacterium.
- Recall its metabolic product: propionic acid + CO2 from lactic acid fermentation. …
- AP EAPCET 2026Set ap-2026-05-20-AN1 markMCQQ.Match the following Column-I: A. Butyric acid B. Citric acid C. Cyclosporin-A D. Statin Column-II: I. Monascus II. Trichoderma III. Aspergillus IV. Clostridium (A) A-IV, B-III, C-I, D-II (B) A-III, B-IV, C-II, D-I (C) A-IV, B-III, C-II, D-I (D) A-III, B-IV, C-I, D-II
›Reveal solutionSolution
This tests the standard microbe-product pairings used in industrial
microbiology/biotechnology: Clostridium→butyric acid, Aspergillus→citric acid,
Trichoderma→cyclosporin-A, Monascus→statins.
Concept and Intuition
Several everyday and pharmaceutical products are manufactured industrially using
specific microorganisms selected for their natural metabolic pathways:
- Butyric acid — produced by the anaerobic bacterium Clostridium butylicum.
- Citric acid — produced industrially by fermentation using the mould Aspergillus niger.
- Cyclosporin-A, an immunosuppressant used to prevent organ-transplant rejection, is produced by the fungus Trichoderma polysporum.
- Statins (blood-cholesterol-lowering drugs) are produced by the yeast Monascus purpureus.
Step-by-Step Solution
- A. Butyric acid → match to Column-II: Clostridium is the classic butyric-acid-fermenting anaerobic bacterium → IV.
- B. Citric acid → industrially fermented using Aspergillus niger → III.
- C. Cyclosporin-A → produced by Trichoderma polysporum → II. …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.Assertion (A) : Propionibacterium sharmani is responsible for large holes in swiss cheese. Reason (R) : Characteristic texture, flavour and taste of cheese is specific to microorgansism. Identify the correct option from the following (A) (A) and (R) are true. (R) is correct explanation for (A) (B) (A) and (R) are true. But (R) is not correct explanation for (A) (C) (A) is true, but (R) is false (D) (A) is false, but (R) is true
›Reveal solutionSolution
Both statements about Swiss cheese are true, and the reason (a cheese's specific texture/flavour comes from its specific microorganism) is exactly why Propionibacterium shermanii is responsible for the characteristic large holes.
Concept and Intuition
In "Microbes in Human Welfare", specific bacteria/fungi are noted for giving specific cheeses their signature identity:
- Propionibacterium shermanii ferments sugars/lactate producing propionic acid, acetic acid, and carbon dioxide gas. The CO₂ produced during the slow ripening of Swiss-type cheese gets trapped as bubbles within the semi-solid cheese matrix, forming the large characteristic "eyes" (holes).
- More generally, the flavour, texture, and taste unique to each cheese variety (Roquefort's blue veins from a Penicillium mould, Swiss cheese's holes from P. shermanii, etc.) arise because each is fermented/ripened by a specific microorganism whose particular metabolism (specific enzymes, specific byproducts) shapes that cheese's identity.
Step-by-Step Solution
- Confirm A: yes, P. shermanii's CO₂ production during ripening is the documented cause of Swiss cheese's large holes — true.
- Confirm R: yes, cheese identity (texture/flavour/taste) is specific to the microorganism used — a general and true statement. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Microorganism that improves nutritional quality of curd by increasing vitamin B12 (A) Saccharomyces cerevisiae (B) Lactobacillus bacteria (C) Propionibacterium sharmanii (D) Pencillium notatum
›Reveal solutionSolution
Lactic acid bacteria (LAB), including Lactobacillus, enrich curd nutritionally, including boosting vitamin B12.
Concept and Intuition
During curd (yoghurt) fermentation, lactic acid bacteria such as Lactobacillus multiply in milk, converting it to curd while also increasing the vitamin B12 content, improving its nutritional quality in addition to lowering pH (which checks pathogenic growth).
Step-by-Step Solution
- Curd fermentation is driven by lactic acid bacteria (LAB), notably Lactobacillus.
- Besides acidifying the milk, LAB growth increases vitamin B12 content in curd. …
- AP EAPCET 2024Set ap-2024-05-16-AN1 markMCQQ.Choose the correct pair related to Bio active compounds I. Aspergillus Niger - Citric acid II. Clot bluster - Pectinases III. Cyclosporin A - Immunosuppressive agent IV. Biogas - Activated sludge (A) II, III (B) I, II (C) I, IV (D) I, III
›Reveal solutionSolution
As with the standard bioactive-compound pairings, Aspergillus niger to citric acid and Cyclosporin A to immunosuppressant are correct (I, III); the clot-dissolving enzyme is streptokinase (not pectinase) and biogas isn't specifically activated sludge (II and IV are wrong).
Concept and Intuition
This question repeats the same set of bioactive-compound statements tested elsewhere in this paper. Aspergillus niger's industrial-scale citric acid production and Cyclosporin A's role as an immunosuppressive drug (used in organ transplantation) are two of the most frequently cited facts in biotechnology/microbiology chapters, and both are factually accurate as stated. By contrast, the enzyme used clinically to dissolve blood clots (clot buster) is streptokinase, produced by Streptococcus; pectinases are unrelated enzymes used in fruit-juice clarification. Likewise, activated sludge specifically refers to the biological floc used in aerobic sewage treatment, distinct from the anaerobic microbial digestion process that produces biogas in a biogas plant.
Step-by-Step Solution
- I: Aspergillus niger to citric acid - correct. …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.Match the following Table I / Table II I Aspergillus | A Lactic acid II Acetobacter | B Butyric acid III Clostridium | C Acetic acid IV Lactobacillus | D Citric acid (A) I-D II-A III-B IV-C (B) I-C II-D III-B IV-A (C) I-D II-C III-B IV-A (D) I-D II-C III-A IV-B
›Reveal solutionSolution
Matching each microbe to its industrially significant fermentation product: Aspergillus–citric acid, Acetobacter–acetic acid, Clostridium–butyric acid, Lactobacillus–lactic acid.
Concept and Intuition
Several microorganisms are industrially exploited to produce specific organic acids by fermentation — a classic NCERT biotechnology-application table:
- Aspergillus niger (a fungus) → citric acid.
- Acetobacter aceti (a bacterium) → acetic acid (vinegar).
- Clostridium butylicum (an anaerobic bacterium) → butyric acid.
- Lactobacillus (bacteria, used in curd formation) → lactic acid.
Step-by-Step Solution
- Aspergillus → Citric acid → D.
- Acetobacter → Acetic acid → C. …
- AP EAPCET 2024Set ap-2024-05-17-FN1 markMCQQ.Select the correct combinations(A) II, III (B) III, IV (C) I, II (D) I, IV
I Monascus purpureus Statins Yeast II Trichoderma Cyclosporin-A Immunosuppresive agent III Pencillium Fungi Butyric acid IV Acetobacter aceti Bacterium Lactic acid ›Reveal solutionSolution
Only I (Monascus purpureus/statins) and II (Trichoderma/cyclosporin-A) are correctly matched; III and IV name the wrong products for their organisms. Answer: (C).
Concept and Intuition
This is a straightforward organism-product recall from the 'Microbes in Human Welfare' chapter:
- Monascus purpureus (classified in the textbook alongside yeasts) is the source of statins, cholesterol-lowering drugs.
- Trichoderma polysporum, a fungus, yields cyclosporin-A, an immunosuppressive drug used to prevent rejection in organ transplantation.
- Penicillium, though correctly identified as a fungus, is famous for producing the antibiotic penicillin — not butyric acid, which is actually produced by the bacterium Clostridium butylicum.
- Acetobacter aceti, correctly a bacterium, produces acetic acid (vinegar) — not lactic acid, which is the signature product of Lactobacillus.
Step-by-Step Solution
- Check I: Monascus purpureus – Statins – Yeast. Matches the standard textbook fact exactly. Correct.
- Check II: Trichoderma – Cyclosporin-A – Immunosuppressive agent. Matches exactly. Correct. …
- AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQQ.Conversion of milk into curd by lactic acid bacteria increases (A) Vitamin B12 (B) Vitamin D (C) Vitamin C (D) Vitamin E
›Reveal solutionSolution
LAB-driven curdling of milk is nutritionally beneficial because, alongside converting milk to curd, these bacteria increase the curd's Vitamin B12 content.
Concept and Intuition
Lactic acid bacteria (e.g., Lactobacillus) multiply in milk, fermenting lactose to lactic acid, which coagulates milk proteins into curd. Beyond this textural/preservation change, LAB metabolism also improves the nutritional profile of the product — specifically increasing the amount of Vitamin B12 available in the curd compared to the starting milk.
Step-by-Step Solution
- Identify the microbial group: lactic acid bacteria performing milk fermentation to curd.
- Recall the specific nutritional benefit attributed to this fermentation in the standard microbiology-in-human-welfare curriculum: an increase in Vitamin B12. …
- AP EAPCET 2023Set ap-2023-05-23-AN1 markMCQQ.Yeast poison themselves to death, because (A) Above 13% of alcohol is hazardous. (B) Above 13% of lactic acid is hazardous. (C) Above 13% of pyruvic acid is hazardous. (D) Low energy produced cannot support their growth.
›Reveal solutionSolution
This tests understanding of why yeast fermentation is self-limiting — the ethanol produced becomes toxic to yeast once its concentration crosses roughly 13%.
Concept and Intuition
During anaerobic fermentation, yeast converts sugars into ethanol and CO2 to regenerate NAD+ for continued glycolysis. However, ethanol is not merely a waste product for the yeast — at sufficiently high concentrations it is toxic to the yeast cell's own membranes and enzymes. This is why fermented beverages produced solely by yeast fermentation (without distillation) rarely exceed roughly 13–14% alcohol content — the yeast effectively poisons itself once that threshold is crossed, stopping further fermentation.
Step-by-Step Solution
- Yeast ferments sugars anaerobically to produce ethanol as the main by-product.
- As fermentation proceeds, ethanol accumulates in the surrounding medium.
- Above roughly 13% ethanol concentration, the alcohol becomes toxic to the yeast cells, inhibiting their metabolism and eventually killing them. …
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