That Shiny Film on Your Cuppa? It Reflects the Complex Chemistry of Making Tea

(Inside Science) -- Many tea drinkers will have noticed the oily-looking sheen that sometimes appears atop their brew. “Depending on your perspective, it can be beautiful or annoying,” said Caroline Giacomin, a doctoral student at the Institute of Food, Nutrition and Health at ETH Zurich who has been studying tea films for the past couple of years.

Giacomin presented new research on how factors such as tea type or proteins from milk affect the mechanical properties of tea films earlier this month at a meeting of the Society of Rheology in Bangor, Maine.

Scientists didn’t actually know what caused tea films until the mid-1990s. They had thought the waxy coating of tea leaves might be melting in hot water and floating to the surface, but when chemists from Imperial College in London actually analyzed the film , they found it was made of calcium carbonate and other salts (which came from the water), and a mix of organic compounds from the tea leaves.

Giacomin’s work builds on that original research. In July of this year, she and her advisor, Peter Fischer, published a paper describing how the mechanical properties of films in black tea changed depending on the amount of calcium carbonate in the water and the presence or absence of traditional tea additives such as sugar, milk, or citric acid as found in lemon juice. They found, for example, that the acid weakened the films.

In new research not yet published, Giacomin and her colleagues branched out from black tea to include green tea, which is made from the unfermented leaves of the same plant as black tea (Camellia sinensis). They also studied red tea, which is also called rooibos tea and is made from the leaves of the shrub Aspalathus linearis. Green tea films were less brittle than black tea films, and red tea produced no film. The team also began testing how the addition of certain dairy proteins affected the film properties, finding that two common proteins found in milk made the films harder to break.

Giacomin’s research is motivated in part by the idea that makers of bottled or dried tea mixes may wish to control the formation of tea films without using preservatives. But for the scientifically curious average tea drinker who simply wishes to see the film, Giacomin recommends using hard tap water (which has high mineral content) and pure black tea as the best way to observe the phenomenon in the kitchen. And if you don’t have hard water from your tap, the researchers say, neglecting to wash your teacup can let the necessary minerals build up.