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How does milk turn into curd?

ScienceLesson on requestFree to readयह पेज हिंदी में पढ़ें

Quick answer

Milk turns into curd when lactic acid bacteria, added through a spoonful of old curd, ferment the milk sugar lactose into lactic acid. As acid builds up in the warm milk over several hours, the casein proteins lose their charge, bond together and form a soft gel that traps water and fat.

Milk turns into curd when lactic acid bacteria ferment its sugar, lactose, into lactic acid, which makes the milk acidic enough for its casein proteins to clump into a soft gel. A spoonful of old curd supplies these bacteria, and warmth lets them multiply until the milk sets.

Step by step

  1. Milk is boiled to kill unwanted microbes, then cooled until it feels comfortably warm to the touch.
  2. A spoonful of curd from the previous batch is stirred in, adding millions of live lactic acid bacteria.
  3. Kept warm, the bacteria multiply and convert lactose into lactic acid, steadily lowering the milk's pH.
  4. Near pH 4.6, casein micelles lose their repelling charge and start linking to one another.
  5. The linked proteins form a gel that holds the liquid in place, giving set curd within several hours.

Key things to understand

  1. Milk contains lactose, a natural sugar, and casein proteins held in tiny clusters called micelles that stay dispersed and keep milk liquid.
  2. Adding a little curd introduces lactic acid bacteria, which feed on lactose and release lactic acid as a waste product.
  3. Warmth around 30 to 40 °C lets the bacteria multiply quickly, so acid builds up over several hours.
  4. As the pH falls toward about 4.6, the casein micelles lose the electric charge that kept them apart and start to bond together.
  5. The linked proteins form a continuous network that traps water and fat, turning the liquid milk into a set, tangy curd.

Labelled diagram

  1. Warm milk

    Lukewarm milk holds lactose sugar and dispersed casein protein micelles.

  2. Starter curd

    A spoonful of old curd adds live lactic acid bacteria.

  3. Lactic acid

    Bacteria ferment lactose into lactic acid, lowering the pH over hours.

  4. Protein network

    Near pH 4.6 casein micelles bond into a continuous gel.

  5. Set curd

    The gel traps water and fat, giving thick, tangy curd.

From warm milk to set curd

Real-world examples

  • Households across India set dahi overnight in steel or clay pots using a spoonful of the previous day's curd.
  • Mishti doi in Bengal is made by fermenting sweetened, thickened milk, often in clay pots that absorb some moisture.
  • Hung curd for shrikhand is made by draining set curd in muslin until most of the whey has dripped away.
  • Greek-style strained yogurt is produced the same way, by removing whey from fermented milk to concentrate the protein.

Common mistake

✗ Myth: Curd sets because the milk goes bad and its proteins rot.

✓ Fact: Curd is controlled fermentation, not rotting. Lactic acid bacteria make the milk acidic, and the acid gently gels the proteins while also discouraging many spoilage microbes.

People also ask

How does curd form from milk scientifically?
Lactic acid bacteria in the starter ferment lactose into lactic acid. As acidity rises, the pH of the milk falls toward about 4.6, the point where casein micelles lose the negative charge that kept them apart. They then bond into a three-dimensional network that traps water and fat, which is the soft gel we call curd.
Why is my curd not setting?
The usual causes are milk that was too cold, so the bacteria worked slowly, or too hot, so they died. A weak or very old starter, too little starter, or milk with antibiotic residues can also stop fermentation. Keeping the bowl in a warm, undisturbed spot usually helps it set.
Can you make curd without a starter?
Some traditional methods rely on bacteria from sources such as green chilli stems or the lining of a used pot, but results are unpredictable because unknown microbes may grow. Boiled milk has had most of its own bacteria killed, so without a reliable starter it may spoil instead of setting into clean-tasting curd.
Why does curd set faster in summer?
Lactic acid bacteria grow fastest in warm conditions. On a hot day the milk stays near their ideal temperature, so they make acid quickly and the curd can set in a few hours, while in winter it may take overnight or need a warm spot.
What is the difference between curd and yogurt?
Both are milk fermented by lactic acid bacteria. Home-made Indian curd uses whatever mixed bacteria are in the starter, while commercial yogurt is made with defined cultures, typically Lactobacillus bulgaricus and Streptococcus thermophilus, at a controlled warm temperature.
Why does curd sometimes release watery liquid?
The clear liquid is whey, the watery part of milk that the protein network had trapped. If curd sets too long, gets too sour or is disturbed, the network tightens and squeezes whey out, a process called syneresis.
Why is milk boiled and cooled before setting curd?
Boiling kills unwanted microbes that could compete with the starter bacteria, and it changes whey proteins so the curd sets thicker. The milk is then cooled to lukewarm, because adding starter to very hot milk would kill the bacteria.

Part of the guide: Food & Kitchen Science →