Scientists say 'crisp' water makes hot tea taste better

If you love to kick off the morning with a hot cup of black tea, a new study details how to make it taste better. As it turns out, the 'pure' water systems found in many homes are reducing the flavor of your tea, making it more bitter than it would be if brewed using 'impure' water that forms a film on the beverage.

If you've ever made a cup of hot tea and noticed a thin film form over the surface as it cools, that's a sign of a good brew, according to new research from ETH Zurich. That waxy film forms due to a combination of milk, sugar, acidity, how concentrated the tea is, the temperature at which it was brewed, and how hard the water is.

That latter part may be key to improve one's tea brewing capabilities, depending on where one lives. Though many places have naturally 'hard' water that lends a more crisp taste, homes often have water purifiers installed that produce soft water. While that helps prevent the build-up of minerals in water pipes, it also leads to bitter-tasting tea.

In describing how they studied the film, the study's co-author Caroline Giacomin said:

In interfacial rheology, experiments performed involve a metal device placed at the surface of the tea. The rotation of that device is carefully controlled, and the resistance to rotation that the film applies is what allows us to determine its strength.

The researchers found that the film on tea is mostly the result of calcium carbonate reacting with tea compounds. Adding something acidic to the tea when using a dried blend will, the researchers noted, boosting the flavor while reducing how visible the film is. The acidic compound could be, for example, citrus.