How is filter coffee prepared?
A spoon of strong decoction, a lot of boiling milk, some sugar, and a long pour between two steel cups. See why milk climbs out of the pan, why sugar left alone sulks at the bottom, and how eight pours from 45 cm bring the coffee under 65 °C in half a minute.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Decoction, milk, sugar, two steel cups. Filter kaapi is a little strong decoction from a two-chamber steel filter, a lot of hot milk and sugar, served in a steel tumbler standing in a wide davara. A common tumbler is 35 mL of decoction to 105 mL of milk, about 1 : 3, with about 60 mg of caffeine from arabica beans.
- Why milk boils over. From about 65 °C milk's whey proteins unfold and gather with fat into a skin. At the boil, steam bubbles are trapped under it as foam. A full LPG flame makes about half a litre of steam a second, so the foam can lift over the rim in seconds; turning the flame down cuts the steam and the foam with it.
- Strength and the stubborn sugar. The decoction is about 6% dissolved coffee, so mixed 1 : 3 the cup is about as strong as a mug of drip coffee. Sugar dissolves in seconds if it moves, but left on the floor of the tumbler it makes a heavy syrup that stays put and spreads only by slow diffusion.
- The metre pour. Poured from 45 cm, the stream hits at √(2gh), about 3 m/s, and drags air in, which milk proteins hold as froth. Each pour also mixes the drink with cooler steel and lets water evaporate from the stream and the bubbles: seven or eight pours bring 82 °C coffee under IARC's 65 °C line in about half a minute.
- Why a davara. The davara spreads the coffee over more than twice the surface of the tumbler, so it cools almost twice as fast. The thin steel wall above the coffee sheds heat to the air, so the rim you hold is far cooler than the coffee. 'Degree' coffee may be named after the lactometer that checked the milk, but that story is not documented.
- From coffee clubs to coffee houses. A 1923 flood sent Udupi's cooks to Mysuru, Bengaluru and Madras, where coffee hotels flourished. The first India Coffee House opened in Bombay in 1936, the Coffee Board followed in 1942, and since 1957 workers' co-operatives have run the Indian Coffee House.
| Term | Meaning |
|---|---|
| Decoction | The strong coffee that drips through a South Indian filter, about 6% dissolved coffee, mixed with hot milk. |
| Davara | A wide steel bowl with a flared lip. The tumbler stands in it, and the coffee is poured into it to cool and froth. |
| Milk skin | A film of unfolded whey protein and fat on hot milk. It traps steam as foam, which is why milk boils over. |
| Toned milk | Milk with its fat adjusted down using skim milk; FSSAI requires at least 3.0% fat and 8.5% solids-not-fat. |
| Diffusion | Molecules spreading by their own random motion: quick across a hair's width, very slow across a cup. |
| Free fall | A stream dropped from height h reaches about √(2gh): 3 m/s from 45 cm. |
| Air entrainment | A plunging jet dragging air under the surface, which comes back as bubbles and, in milk, as froth. |
| Evaporative cooling | Water leaving as vapour carries about 2,260 joules per gram, so the liquid it leaves cools. |
| Lactometer | A float that reads milk's density. It sinks deeper in watered milk. |
About 350 years from seven seeds in the Karnataka hills to a GI application for a tumbler of Kumbakonam degree coffee.
- 1670 · Seven seeds in the Baba Budan hills (Baba Budan, Chandragiri hills, Chikkamagaluru, Karnataka)
- 1890 · Coffee enters Tamil homes (Households across the Madras Presidency, Tamil Nadu)
- 1923 · A flood sends Udupi's cooks to the cities (People of Udupi, Udupi, Karnataka)
- 1924 · A coffee club on Lalbagh Road (Parampalli Yagnanarayana Maiya and his brothers, Lalbagh Road, Bangalore (Bengaluru))
- 1936 · The first India Coffee House (Coffee Cess Committee, Churchgate, Bombay (Mumbai))
- 1942 · The Coffee Board of India (Government of India, Bangalore (Bengaluru))
- 1946 · Toned milk is born in Bombay (Dara Nusserwanji Khurody, Bombay (Mumbai))
- 1957 · Workers take over the coffee houses (A. K. Gopalan and the coffee-house workers, Bangalore and Delhi)
The full story, with 24 moments, charts, people and 32 sources: glassbox.how/e/filtercoffeeclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
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|---|---|---|
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glassbox/video.mp4 |
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Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
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js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
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js/kaapi.js |
The drink's physics (boiling milk and its foam, FSSAI milk classes, mixing, sugar dissolving, the metre pour, the hot rim, the lactometer), the generic tumbler, davara, pan and burner, and the chart boards; the extraction model is CoffeeClear's |
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