Q.The vitamin whose content increases following the conversion of milk into curd by lactic acid bacteria is:
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.
The NCERT emphasises that these processes are not random. They require specific conditions — temperature, pH, moisture, and the right starter culture. If the conditions go wrong, harmful microbes can grow instead. That is why traditional methods (like adding a spoonful of previous curd) are actually precise microbial techniques passed down through generations.
For a commerce or humanities student, the relevance goes beyond the kitchen. Fermented foods are a multi‑billion‑dollar industry. They involve supply chains (milk, grains, fruits), processing technology, quality control, and marketing. Understanding the basic science helps you see why a product like yogurt has a “use by” date, why some cheeses are expensive, and why traditional foods like kimchi or kombucha have become global trends.
In short: microbial fermentation is nature’s way of transforming food using invisible helpers. It is a bridge between biology and everyday life — and it has been feeding humanity long before anyone knew what a microbe was.
Microbial fermentation is a well-established topic in the NCERT Class 12 Biology curriculum, commonly explored through searches like "Microbial Fermentation Foods: definition, examples and applications" and "list microbes used in food production" ahead of CBSE board exams. Students preparing for NEET and other competitive exams often revise this alongside "Microbes in Human Welfare important questions," since it links directly to biotechnology and industrial microbiology.
When milk is converted into curd by Lactobacillus and other lactic acid bacteria (LAB), these bacteria ferment the lactose in milk to produce lactic acid. This process not only coagulates milk proteins but also enriches the curd with certain nutrients. Specifically, LAB are known to synthesise several B-group vitamins during fermentation. Among these, the most significant increase is seen in vitamin B12 (cobalamin), which is otherwise absent or present in very low amounts in raw milk. Vitamins C, D, and E do not increase during this fermentation; in fact, vitamin C content may even decline due to oxidation.
The correct option is (C) vitamin B12, because lactic acid bacteria synthesise and increase the level of this vitamin during the conversion of milk into curd.
The conversion of milk into curd by lactic acid bacteria increases the content of vitamin B12.
Milk is a rich source of many nutrients, but when it is transformed into curd through the action of lactic acid bacteria (LAB), a fascinating biochemical change occurs. These bacteria, primarily Lactobacillus species, ferment the lactose in milk into lactic acid, which coagulates the milk proteins and gives curd its characteristic texture and tangy taste. But fermentation does more than just preserve and flavour the milk — it also alters the nutritional profile.
During the fermentation process, lactic acid bacteria synthesise certain vitamins. While they consume some nutrients for their own growth, they also produce others as metabolic by-products. One of the most significant nutritional gains is in the B-complex group of vitamins. Specifically, the bacteria are capable of producing vitamin B12 (cobalamin), which is naturally present in milk only in very small amounts. The bacterial activity during curd formation enriches the final product with this essential vitamin.
Vitamin B12 is crucial for red blood cell formation and nerve function, and it is almost exclusively found in animal-based foods. Curd, being a fermented dairy product, becomes a better source of this vitamin than fresh milk.
The other vitamins listed — vitamin C, vitamin D, and vitamin E — are not significantly increased by lactic acid fermentation. Vitamin C is actually heat-sensitive and can be lost during processing; vitamin D is fat-soluble and not synthesised by these bacteria; and vitamin E, also fat-soluble, remains largely unchanged. So the only vitamin among the options whose content rises noticeably is vitamin B12.
This is a standard fact from the NCERT Class 12 Biology textbook (Chapter 10: Microbes in Human Welfare). The textbook explicitly states that curd formed by lactic acid bacteria contains higher levels of vitamin B12 compared to milk.
In short, the vitamin whose content increases when milk is converted into curd by lactic acid bacteria is vitamin B12 (option C).
Eliminate by solubility class first -- vitamins C, D and E are not known to be synthesised by lactic acid bacteria during this fermentation, while B12 is the one water-soluble B-vitamin repeatedly linked to bacterial action on milk -- so the correct option survives a class-by-class elimination rather than being recalled cold.
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.
- D — 'wine and beer are produced by distillation of rice' — false on two counts: (i) wine/beer come from fermentation, not distillation (distillation is used for spirits like whisky/brandy); (ii) wine is classically from grapes, beer from malted grains (barley), not specifically rice.
- So the correct pair is A and B → option (A).
Common Mistakes
- Confusing fermentation (used for wine/beer) with distillation (used for spirits).
- Assuming a sludge digester's only or main product is a foul-smelling gas (H₂S) rather than methane.
✓Final answerThe correct option is (A) — A, B.
ANSWER: A
- 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.
- Both true, R not the explanation ⇒ option (B).
Common Mistakes
- Marking one statement false; both are genuine textbook facts.
- Choosing (A) by assuming any two microbiology statements must be cause-and-effect.
✓Final answerThe correct option is (B) — both correct, but R is not the correct explanation of A.
ANSWER: B
- 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.
- (C) "Land contaminated with toxic wastes can be removed [cleaned up] by microbes" — true; this is bioremediation. CORRECT statement.
- (D) "Fleming working on Aspergillus discovered penicillin" — false; Fleming's mould was Penicillium notatum, not Aspergillus. INCORRECT statement.
- So the incorrect statements are A and D.
Common Mistakes
- Mixing up which acid goes with which bacterium (Clostridium→butyric, Acetobacter→acetic).
- Confusing the two common fungal genera Penicillium and Aspergillus in the penicillin-discovery story.
✓Final answerThe correct option is (C) — A, D.
ANSWER: C
- 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.
- Connect the CO2 release to the signature "holes" of a particular cheese type: Swiss cheese.
Common Mistakes
- Confusing this organism with the lactic acid bacteria (e.g. Lactobacillus) used for curd, or yeast used for bread/toddy fermentation — each product has a distinct characteristic microorganism.
✓Final answerThe correct option is (B) — Cheese.
ANSWER: B
- 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.
- D. Statin → produced by Monascus purpureus → I.
- Assembling: A-IV, B-III, C-II, D-I, which matches option (C).
Common Mistakes
- Swapping Trichoderma and Monascus, since both are fungi/yeast associated with pharmaceutical products — Trichoderma makes cyclosporin-A (immunosuppressant), Monascus makes statins (cholesterol drug).
- Assuming citric acid comes from a bacterium rather than the mould Aspergillus niger.
✓Final answerThe correct option is (C) — A-IV, B-III, C-II, D-I.
ANSWER: C
- 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.
- Link them: since "large holes" is a component of the cheese's characteristic texture, and R states that texture is specific to the microorganism, R directly accounts for why this particular microbe (and no other) produces this particular texture feature (holes) — R explains A.
- Hence, both are true and R is the correct explanation.
Common Mistakes
- Thinking R is "too general" to explain the specific hole-forming mechanism — but the CO₂-driven hole formation is a case of "texture specific to microorganism", so the general reason does logically cover the specific assertion.
- Confusing propionic-acid bacteria's role (hole formation via CO₂) with an unrelated microbe's role (e.g. mould-ripened blue cheeses).
✓Final answerThe correct option is (A) — (A) and (R) are true, and (R) is the correct explanation for (A).
ANSWER: A
- 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.
- Answer: (B) Lactobacillus bacteria.
Common Mistakes
- Confusing this with Saccharomyces (used in bread/alcohol fermentation) or Propionibacterium (used in Swiss cheese, producing propionic acid and CO2 for holes).
✓Final answerThe correct option is (B) — Lactobacillus bacteria.
ANSWER: B
- 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.
- II: clot buster to pectinases - incorrect (should be streptokinase).
- III: Cyclosporin A to immunosuppressive agent - correct.
- IV: biogas to activated sludge - incorrect (activated sludge is a sewage-treatment term).
- Correct pairs: I and III.
Common Mistakes
- Assuming any hydrolytic enzyme (like pectinase) could double as a clot buster simply because both involve breaking down substances.
- Treating activated sludge and biogas digestion as interchangeable anaerobic/microbial waste-processing terms.
✓Final answerThe correct option is (D) — I, III.
ANSWER: D
- 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.
- Clostridium → Butyric acid → B.
- Lactobacillus → Lactic acid → A.
- So I-D, II-C, III-B, IV-A.
Common Mistakes
- Swapping Acetobacter (acetic acid) and Lactobacillus (lactic acid), since both names relate to "acid" fermentation but produce chemically distinct products.
✓Final answerThe correct option is (C) — I-D, II-C, III-B, IV-A.
ANSWER: 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.
- Check III: Penicillium – Fungi – Butyric acid. The organism/category is right, but the product is wrong (should be penicillin). Incorrect.
- Check IV: Acetobacter aceti – Bacterium – Lactic acid. The organism/category is right, but the product is wrong (should be acetic acid). Incorrect.
- Correct combinations: I and II.
Common Mistakes
- Mixing up Acetobacter's acetic acid with Lactobacillus's lactic acid — both are common fermentation acids but from different organisms.
- Assuming Penicillium's category ('fungi', which is correct) means the whole row is correct, without checking the product.
✓Final answerThe correct option is (C) — I, II.
ANSWER: C
- 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.
- Eliminate the other vitamins (D, C, E) — these are not the vitamin conventionally cited as being enhanced by LAB fermentation of milk.
Common Mistakes
- Confusing this fact with Vitamin C enrichment (which is associated with other fermented foods, not curd).
- Assuming curd fermentation affects fat-soluble vitamins (D, E) rather than the water-soluble B12.
✓Final answerThe correct option is (A) — Vitamin B12.
ANSWER: A
- 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.
- This self-toxicity, not lactic acid, pyruvic acid, or insufficient ATP yield per se, is the standard textbook reason yeast "poison themselves to death."
Common Mistakes
- Confusing yeast (alcoholic) fermentation with lactic acid fermentation (as in muscle cells or certain bacteria) — lactic acid accumulation is not the relevant toxin here.
- Assuming low ATP yield of fermentation itself kills the yeast, rather than the toxic accumulation of the specific end-product, ethanol.
✓Final answerThe correct option is (A) — Above 13% of alcohol is hazardous.
ANSWER: A
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