How is coffee decoction prepared?
Two steel cups, a pierced disc and three centimetres of boiling water: that is all it takes to make the strongest coffee in an Indian kitchen. Slow the drip with Darcy's law, watch the first drops come out strongest, find out what the chicory is doing, and see why decoction left warm on the stove turns sour.
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- Two cups and a disc. A South Indian coffee filter is two steel cups. The upper one has a floor of pinholes and holds about two heaped tablespoons of medium-fine powder, usually blended with chicory. A pierced pressing disc sits on the powder to keep it level, boiling water goes on top, and a strong decoction drips into the lower cup.
- Why a quarter of an hour. Only about 3 cm of water stands on the bed, pushing with about 400 pascals. Darcy's law says the flow goes with that push and the bed's permeability, which shrinks with the square of the grain size. So a medium-fine powder drips in about 15 minutes, a fine one chokes for most of an hour, and a coarse one runs through weak. The drip slows as the head of water falls.
- Four times as strong. The filter uses about 7 or 8 grams of water per gram of coffee, against 15 to 17 for drip coffee. The decoction comes out near 5% dissolved solids, about four times a drip coffee's 1.2 to 1.4%. The first drops are the strongest, the chamber cools from about 90 °C as it drains, and a second decoction from the same grounds is weaker than drip coffee.
- Chicory. Roasted chicory root has no caffeine but dissolves far more readily than coffee: roasting turns its inulin into browned, caramel-like compounds. It darkens and thickens the decoction and adds bitterness. FSSAI says a coffee-chicory mixture must be at least 51% coffee, with both percentages printed on the pack.
- Fresh, stale, sour. Decoction loses its aroma and slowly turns sour as lactones from roasting split back into acids. Both speed up with heat: warm on the stove it fades in minutes, in the fridge over days. Reheat it gently, never boil it. A tablespoon holds about 20 to 25 mg of caffeine; ICMR-NIN advises staying within 300 mg a day.
- A root, a pot and a law. Chicory came into coffee during a Prussian coffee ban in 1766. A French drip pot of 1795 dripped boiling water through a pierced metal plate. Henry Darcy found the law of flow through sand in Dijon in 1856. Coffee came into Tamil homes from the late 19th century, helped by coffee hotels, the India Coffee House and the Coffee Board.
| Term | Meaning |
|---|---|
| Decoction | The strong coffee concentrate that drips through a South Indian filter, later mixed with hot milk. |
| Pressing disc | The pierced dome on a stem that rests on the powder, keeping the bed level and spreading the water. |
| Darcy's law | Flow through a porous bed is proportional to the pressure pushing it and the bed's permeability, and inversely to its thickness and the liquid's viscosity. |
| Permeability | How easily a packed bed lets liquid through. It falls with the square of the grain size. |
| Head | The height of liquid above a point; its weight is the push that drives the drip. |
| Extraction yield | The share of the coffee powder that ends up dissolved in the drink. |
| Strength (TDS) | How much of a drink is dissolved coffee: about 1.15 to 1.45% for drip coffee, about 5% for decoction. |
| Chicory | A roasted, caffeine-free root blended into coffee powder. FSSAI caps it at 49% of a coffee-chicory mixture. |
| Lactone | A ring-shaped molecule made in roasting that slowly turns back into acid, souring stored coffee. |
From a Prussian coffee ban and a Rouen drip pot to Darcy's sand filters and India's 51% rule.
- 1766 · Chicory steps in for coffee (Christian Gottlieb Förster, Brunswick, Germany)
- 1795 · A pot that drips through holes (François-Antoine-Henri Descroizilles, Rouen, France)
- 1856 · Darcy's law (Henry Darcy, Dijon, France)
- 1890 · Coffee comes into Tamil homes (households of colonial Madras Presidency, Tamil Nadu)
- 1936 · The first India Coffee House (Coffee Cess Committee, Churchgate, Bombay (Mumbai))
- 1942 · The Coffee Board (Government of India, Bangalore (Bengaluru))
- 2011 · At least 51% coffee (Food Safety and Standards Authority of India, New Delhi)
- 2021 · FSSAI reminds coffee makers of the label rules (Food Safety and Standards Authority of India, New Delhi)
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