8 min readPathway sheet1-mark MCQ or assertion-reason on curd, bread or cheese, a 2-mark question on lactic acid bacteria; also a NEET favourite

Microbes in food processing

Lactic acid bacteria turn milk into curd, bacteria raise idli and dosa batter, baker's yeast raises bread, and other microbes make toddy and cheeses.

CurdLactobacillus and other lactic acid bacteria (LAB) coagulate and partly digest milk proteins; vitamin B₁₂ increases

BreadDough fermented by baker's yeast, Saccharomyces cerevisiae

Swiss cheeseLarge holes from CO₂ made by Propionibacterium sharmanii

Roquefort cheeseRipened by growing a specific fungus on it, which gives its flavour

How milk becomes curd

1
A spoonful of curd is added to lukewarm milk. It acts as an inoculum or starter, carrying millions of lactic acid bacteria (LAB) such as Lactobacillus.
2
At a suitable warm temperature, the LAB multiply in the milk.
3★
LAB produce acids (mainly lactic acid) that coagulate and partially digest the milk proteins.
4
The milk sets into curd, with better nutritional quality because vitamin B₁₂ increases.
5
In our stomach too, LAB help by checking disease-causing microbes.

★ key step

Batter, dough and traditional foods

The batter for dosa and idli is fermented by bacteria. It puffs up because the bacteria release CO₂ gas during fermentation. Bread dough is fermented by baker's yeast (Saccharomyces cerevisiae), which also releases CO₂ that makes the dough rise. Toddy, a traditional drink of parts of southern India, is made by fermenting the sap of palms. Microbes are also used to ferment fish, soyabean and bamboo shoots into foods.

Household products and their microbes

Product	Microbe	What it does
Curd	Lactobacillus (LAB)	Acid coagulates milk proteins; adds vitamin B₁₂
Idli and dosa batter	Bacteria	CO₂ makes the batter puff up
Bread	Saccharomyces cerevisiae (baker's yeast)	CO₂ makes the dough rise
Toddy	Microbes fermenting palm sap	Traditional fermented drink
Swiss cheese	Propionibacterium sharmanii	Large amount of CO₂ gives large holes
Roquefort cheese	A specific fungus	Ripens the cheese and gives its flavour

Curd vs bread: two kinds of fermentation

Curd (bacterial)Bacteria (LAB) make lactic acid, which sets the milk and gives curd its sour taste and thick texture.

Bread (yeast)A fungus (yeast) ferments sugars and releases CO₂ (and a little alcohol, which evaporates on baking). The gas makes the dough soft and spongy.

In real life

Home-set dahi, idli, dhokla and appam are everyday Indian fermented foods. Curd made at home is a good source of friendly LAB and vitamin B₁₂, which matters for people who eat little meat.

Exam angle

2 marks: “How do lactic acid bacteria convert milk into curd? How does this improve the milk?” LAB in the starter multiply and produce acids that coagulate and partly digest milk proteins. Curd has more vitamin B₁₂, so its nutritional value is higher; LAB also check harmful microbes in the stomach.

2 marks: “Name the microbes responsible for the large holes in Swiss cheese and for the rising of bread dough. What gas is common to both?” Propionibacterium sharmanii (Swiss cheese) and Saccharomyces cerevisiae (bread). Both release CO₂.

Remember it

LAB makes Lactic acid And B₁₂. Swiss cheese holes come from Sharmanii.