Lactic acid bacteria (LAB) are bacteria that grow in milk and change it into curd by making acid. The main example is Lactobacillus. Memory hook: "LAB makes ACID, ACID makes CURD" - and curd made this way has extra vitamin B12.
LAB (Lactobacillus) grow in milk, produce lactic acid that coagulates milk proteins into curd, add vitamin B12, and in the stomach check disease-causing microbes.
Your doubts, answered
What does LAB stand for, and is it one bacterium or a group?
LAB stands for Lactic Acid Bacteria. It is not a single species. It is a group name for bacteria that produce lactic acid. Lactobacillus is the most common example named in NCERT, but the NCERT line says 'Lactobacillus and others commonly called lactic acid bacteria (LAB)'. So LAB is a group, and Lactobacillus is one member of that group.
How exactly do LAB turn milk into curd?
LAB grow inside the milk. While growing, they produce acids. These acids coagulate (thicken) the milk proteins and partially digest them. This is why liquid milk becomes solid curd. NEET often tests the exact words: LAB 'produce acids that coagulate and partially digest the milk proteins'.
Why do we add a little old curd to warm milk to make new curd?
That small amount of old curd is called the inoculum or starter. It already contains millions of LAB. When you add it to fresh warm milk, these LAB multiply fast at a suitable temperature and convert the whole milk into curd. So the starter simply supplies the living LAB needed to begin the process.
Does making curd from milk really add a vitamin? Which one?
Yes. When LAB convert milk to curd, the nutritional quality improves because vitamin B12 increases. This is a favourite NEET fact. The trap options are usually vitamin A, D, or E - but the correct answer is always vitamin B12.
How do LAB help inside our own stomach?
NCERT states that in our stomach, LAB play a beneficial role by checking (controlling) the growth of disease-causing microbes. So LAB are not only useful in food - they also help protect us. This is a one-line fact students often skip, but it can be asked directly.
⚠️ The NEET trap ✗ Curd formation increases vitamin C or vitamin A in the milk. ✓ Curd formation by LAB increases vitamin B12, not vitamin C, A, D, or E. 🧠 Only B12 goes up in curd. Remember: 'B12 in the Bowl of curd.'
Real NEET questions
NEET 2018
Conversion of milk to curd improves its nutritional value by increasing the amount of
A · Vitamin B12 ✓
B · Vitamin A
C · Vitamin D
D · Vitamin E
Solution: When lactic acid bacteria (Lactobacillus) convert milk to curd, they raise the amount of vitamin B12. Vitamins A, D and E are not enriched during this fermentation, so the answer is vitamin B12.
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
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 (a LAB) makes curd, Saccharomyces cerevisiae ferments dough for bread, Aspergillus niger makes citric acid, and Acetobacter aceti makes acetic acid. This gives (a)-(ii), (b)-(iv), (c)-(iii), (d)-(v).
NEET 2021
Match List I with List II. (a) Aspergillus niger (b) Acetobacter aceti (c) Clostridium butylicum (d) Lactobacillus / (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: From the NCERT acid-producer list, Aspergillus niger gives citric acid, Acetobacter aceti gives acetic acid, Clostridium butylicum gives butyric acid, and Lactobacillus (a LAB) gives lactic acid: (a)-(iii); (b)-(i); (c)-(iv); (d)-(ii).
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
