It is a puzzle that has stared almost every scientist in the face at one time or other, especially during the laboratory tea break. But judging how long the tea bag should be left in the cup to make the perfect brew is no longer an art. A chemical engineer has successfully modelled the kinetics of tea and coffee infusion, writes Steve Farrar.
Andrew Stapley, lecturer in chemical engineering at Loughborough University, has applied Fick's second law, an established equation that predicts the diffusion of solids to liquids, to the humble cuppa.
The approach matches experimental data far better than the standard equation found in scientific papers.
"It adds to the theoretical understanding of the problem and tells you a bit more about how tea infusion varies with time," Dr Stapley said. He found it predicted the rapid burst of infusion immediately after the tea leaves or coffee granules come into contact with hot water.
Diffusion from the surface layers of each leaf or particle is at first much faster than from deeper inside.
The model will make it possible to calculate how long it will take a cuppa to reach a desired concentration working from first principles.
But given the complexity of mathematics involved, it might be easier to rely on trial and error.
Dr Stapley devised the model while preparing undergraduate lectures and is sending it to tea and coffee manufacturers, where it could be useful in commercial product analysis.
The findings are published in the Journal of the Science of Food and Agriculture .