Curd is made when Lactobacillus and other lactic acid bacteria (LAB) grow in milk and produce acids. These acids coagulate (set) and partially digest the milk proteins, so liquid milk becomes solid curd. Memory hook: L-A-B = "Lactobacillus Acidifies to Bind" milk proteins.
Milk to curd pathway: a small amount of curd (inoculum/starter) adds millions of LAB; at warm temperature Lactobacillus produces acids that coagulate and partially digest milk proteins, forming curd with extra vitamin B12.
Your doubts, answered
Which microbe converts milk to curd?
Lactobacillus and other bacteria commonly called lactic acid bacteria (LAB). NCERT names Lactobacillus as the standard example. In match-type NEET questions, always pair Lactobacillus with curd.
How exactly does milk become solid curd?
During growth the LAB produce acids. These acids coagulate (set) and partially digest the milk proteins. The proteins clump together, so the liquid milk changes into semi-solid curd. No high heat is needed, only a suitable warm temperature.
What is LAB and how is it different from Lactobacillus?
LAB means lactic acid bacteria, a whole group of bacteria that make lactic acid. Lactobacillus is one member (the most famous example) of this group. So every Lactobacillus is a LAB, but LAB also includes others.
Do you need to add anything to fresh milk to make curd?
Yes. A small amount of curd is added to fresh milk as an inoculum or starter. This starter contains millions of LAB. At a suitable temperature they multiply and convert the whole milk into curd.
Does curd have more nutrition than milk?
Yes. NCERT states that converting milk to curd also improves its nutritional quality by increasing vitamin B12. This exact point was asked in NEET 2018, so remember: curd to B12.
⚠️ The NEET trap ✗ Curd forms because Lactobacillus produces vitamin B12 which thickens the milk. ✓ Curd forms because LAB produce ACIDS that coagulate and partially digest milk proteins. The vitamin B12 increase is a separate nutritional benefit, not the setting mechanism. 🧠 NTA mixes the cause (acids set proteins) with the benefit (more B12). Setting = acids; nutrition = B12. Keep the two facts apart.
Real NEET questions
NEET 2019
Match the following organisms with the products they produce: (a) Lactobacillus (b) Saccharomyces cerevisiae (c) Aspergillus niger (d) Acetobacter aceti; (i) Cheese (ii) Curd (iii) Citric Acid (iv) Bread (v) Acetic Acid. Choose the correct option.
A · (a)-(ii), (b)-(iv), (c)-(v), (d)-(iii)
B · (a)-(ii), (b)-(iv), (c)-(iii), (d)-(v) ✓
C · (a)-(iii), (b)-(iv), (c)-(v), (d)-(i)
D · (a)-(ii), (b)-(i), (c)-(iii), (d)-(v)
Solution: Lactobacillus converts milk to curd (ii), Saccharomyces cerevisiae ferments dough for bread (iv), Aspergillus niger produces citric acid (iii), and Acetobacter aceti produces acetic acid (v). So a-(ii), b-(iv), c-(iii), d-(v).
NEET 2021
Match List I with List II. List I (Microorganism): (a) Aspergillus niger (b) Acetobacter aceti (c) Clostridium butylicum (d) Lactobacillus; List II (Product): (i) Acetic Acid (ii) Lactic Acid (iii) Citric Acid (iv) Butyric Acid.
A · (a)-(ii); (b)-(iii); (c)-(i); (d)-(iv)
B · (a)-(iv); (b)-(ii); (c)-(i); (d)-(iii)
C · (a)-(iii); (b)-(i); (c)-(iv); (d)-(ii) ✓
D · (a)-(i); (b)-(ii); (c)-(iii); (d)-(iv)
Solution: Per the NCERT list of organic-acid producers: Aspergillus niger yields citric acid (iii), Acetobacter aceti yields acetic acid (i), Clostridium butylicum yields butyric acid (iv), and Lactobacillus yields lactic acid (ii). This is why Lactobacillus (a LAB) acidifies milk and sets it into curd.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. LAB grow in milk and ferment milk sugar into acids. This microbial fermentation is why curd making is included in Microbes in Human Welfare.
Why is a warm temperature needed for curd?
The LAB in the starter multiply best at a suitable warm temperature. That is why milk sets faster into curd in warm weather and slower when it is cold.
Can NEET ask curd along with cheese and bread microbes?
Yes. It is a favourite match-the-column format: Lactobacillus to curd, Saccharomyces cerevisiae to bread, Aspergillus niger to citric acid. Learn the full set together.
What is the single most tested fact about curd for NEET?
Two facts: (1) Lactobacillus/LAB acids coagulate milk proteins to form curd, and (2) curd has increased vitamin B12 compared with milk (asked in NEET 2018).
