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Grades 6–8 · Everyday Life

How do tea and coffee give their flavors to water?

Tea leaves and roasted coffee particles contain substances that can move into water. Some of these substances contribute color, taste or aroma. Making either drink involves solid–liquid extraction: selected compounds leave a solid material and enter a liquid. Water acts as the solvent, the liquid in which those compounds dissolve.

J75 visual explanationBoth tea and coffee transfer water-soluble substances from plant solids into water. A generic invented sample contains 20 soluble units: 8 remain in the solid and 12 are dissolved in water. Eight separate residue units remain insoluble in this account. The two pictured materials share a mechanism, not a measured extraction rate. Temperature, pressure, grind size, flavor and brewing settings are not calculated.Tea and coffee both use solid–liquid extractionTea leafCoffee particleSolidSolublecompoundsResiduestays solidWaterDissolvedcompoundsWater enters; soluble molecules move out.Some material dissolves; solid residue remains.Plant particles and dissolved material are enlarged.

In the diagramTea and coffee both use solid–liquid extractionTea leafCoffee particleSolidSolublecompoundsResiduestays solidWaterDissolvedWater enters; soluble molecules move out.Some material dissolves; solid residue remains.Plant particles and dissolved material are enlarged.

Both materials release soluble compounds into water, while a solid residue remains.

Water first wets the material and enters spaces within it. Soluble compounds dissolve into that water. Dissolved molecules can then diffuse through water-filled spaces and out into the surrounding liquid. Diffusion is the spreading produced by molecular motion; a concentration difference can give it an overall direction. Mixing can also carry dissolved material away from the surface.

The whole leaf or coffee particle does not have to dissolve. Material that is poorly soluble remains as a solid residue, while dissolved molecules pass into the drink. A filter can hold back larger solid pieces without removing all the dissolved substances that give the liquid its character.

Substances near a particle’s surface need not move at the same rate as those deeper within it. The path through plant material can slow transfer. Tea and coffee also contain many different compounds. Their shared extraction mechanism does not mean the two drinks have identical compositions or extraction rates.

About this explanation

Starting knowledge: Understand dissolved substances, molecules and the difference between a solid and a liquid.

Sources

  • Columbia University Department of Chemistry — Extractions: background
  • Michigan State University Libraries — Extraction and Drying — Cooperative Organic Chemistry Student Laboratory Manual
  • American Chemical Society; Beverly and colleagues — Modeling Mass and Heat Transfer in Multiphase Coffee Aroma Extraction
  • Spiro and Jaganyi; Food Chemistry — Kinetics and equilibria of tea infusion, Part 15: Transport of caffeine across a teabag membrane
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