CurdClear

How is curd made?
A spoon of old curd turns a pot of warm milk solid by morning, and nothing was added but bacteria. Zoom into milk, watch them turn milk sugar into acid, see the proteins link up at pH 4.6, then set your own dahi on a winter night and find out why it needs a blanket.

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1. Milk is water with things floating in it. Cow milk is about 87% water. Lactose, minerals and whey proteins are dissolved in it, fat floats as droplets a few thousandths of a millimetre wide, and casein protein hangs in balls called micelles, about 150 nanometres across. Each micelle has a coat of negatively charged hairs, so micelles push each other away and milk stays liquid.
2. A spoon of jaman is a spoon of bacteria. Old curd carries around a hundred million live lactic acid bacteria in every gram. In warm milk they eat lactose and give out lactic acid, and at their best they double about every half hour. Cold slows them almost to a stop and heat above about 50 °C kills them.
3. At pH 4.6 the milk sets. The acid pulls the pH down from about 6.7. Hydrogen ions cancel the charge on the micelles, and near pH 4.6, casein's isoelectric point, the micelles stick together into a network. The network traps all the water and fat, so the liquid becomes a soft solid, a gel. Nothing is removed.
4. Warmth and time decide the pot. Lukewarm milk at about 40 °C sets in roughly five to seven hours. A bare pot on a cold night cools in a couple of hours and may never set, which is why Indian homes wrap it or tuck it into a casserole. Boiling the milk first clears out rival microbes and makes the gel about twenty times stiffer.
5. Whey and sourness have causes. The greenish water on curd is whey, squeezed out as the protein network tightens. Leaving the pot warm for too long, setting it too hot, or shaking it while it sets all make it worse. Sourness is simply more lactic acid, and a fridge nearly stops the bacteria that make it.
6. One trick, many foods. Yoghurt is curd made with two named bacteria at about 42 °C. Paneer uses acid in hot milk and no bacteria. Cheese uses rennet, an enzyme that cuts the hairs off the micelles. From dahi itself come lassi, chaas, shrikhand and mishti doi.

Nearly 9,000 years from milk left in a clay pot to the bacteria in every cup of dahi.

• 6500 BCE · Milk fat in the oldest pots (Neolithic farmers, North-western Anatolia (today Turkey))
• 2500 BCE · Dairy in the Indus valley (People of the Indus Valley Civilisation, Kotada Bhadli, Gujarat)
• 1200 BCE · Dadhi in the Vedas (Composers of the Vedic hymns, North-western Indian subcontinent)
• 1073 · The word yoghurt is written down (Mahmud al-Kashgari, Baghdad)
• 1780 · The acid in sour milk (Carl Wilhelm Scheele, Köping, Sweden)
• 1857 · Souring is the work of living microbes (Louis Pasteur, Lille, France)
• 1905 · The yoghurt bacillus (Stamen Grigorov, Geneva, Switzerland)
• 1919 · Danone begins in Barcelona (Isaac Carasso, Barcelona, Spain)

The full story, with 30 moments, charts, people and 51 sources: glassbox.how/e/curdclear/history. The data lives in history.json.

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