Mystery Solved: What Tea ‘Foam’ Really Is And How You Can Avoid It

The water we use to prepare the tea is related to the probability of an oily film appearing in the cup.

Tea foam.  ETH Zürich, Department of Health Science and Technology, Institute of Food Nutrition and Health.

Tea is a drink loaded with beneficial properties that ranks second among the preferred infusions in Spain, only behind the omnipresent coffee. The antioxidant and anti-inflammatory compounds that it provides they are related to multiple improvements in health, in particular cardiovascular health, and to greater longevity accompanied by a reduction in the risk of suffering from chronic or neurodegenerative diseases.

Tea lovers are aware that preparing it properly constitutes a true rite -even a ceremony, as in the case of matcha tea- in which each of the elements, from the temperature of the water to the time it is left to rest, influences the the final result. And many observe with displeasure that sometimes a ‘foam’ or oily film appears on the surface.

The truth is researchers at the ETH in Zurich, Switzerland, are also not amused to find this ‘web’ in their freshly brewed cup of black tea, and they considered using their resources to get to the bottom of the matter. Their findings are published in the journal Physics of Fluidsin an article describing how they applied the discipline known as rheology.

The Rheology is the study of the deformation and flow of materials., particularly when they are in a liquid or gaseous state. It is a branch of physics with multiple applications in engineering, geophysics, physiology, biology, pharmaceuticals and materials science, since it is taken into account to manufacture products ranging from cement to chocolate.

In this case, the researchers used it to describe what constitutes an ‘interfacial phenomenon’ on the surface of the teaunderstood as what happens in the transition region between two phases, the first being liquid -the beverage left to cool- and the second being air. The foam is described as “a thin film” that may or may not be visible to the human eye, and that “visibly cracks like ice” on contact.

Using interfacial rheology, specialists were able to determine the mechanical properties of this film, and establish that its formation is affected by factors such as the hardness of the water, its acidity, the fact that sugar or milk is added, the concentration of the herb and the temperature of preparation.

“Interfacial rheology experiments are performed with a piece of metal that is placed on the surface of the tea,” explains one of the authors, Caroline Giacomin. “The rotation of this apparatus is carefully controlled, and the resistance that the film presents will allow us to determine its solidity“.

The residues of the waxy coating that tea leaves can present have traditionally been considered to be the main responsible for the formation of the ‘foam’. But this was ruled out in the nineties when scientists went on to point the finger at calcium carbonate in the waterthe main contributor of compounds that react with the tea to form the film.

“Tap water in many regions comes from limestone aquifers in which calcium carbonate abounds, which is harmless but gives the water a particular flavor”, explains Giacomin. It is the reason why many homes have water softeners to prevent the formation of deposits in the taps.

However, “if we tried to make a tea with totally pure water, we would avoid foaming, but it would taste quite bitter,” he continues. A) Yes, the chances of this film forming increase with the proportion of water hardness.

What if we don’t want to change the water we use regularly despite the fact that it tends to cause foam? Giacomin slips a trick: “Adding an acidic component, such as citrus, to the tea mix will reduce film formation and add flavor“.

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