The science behind tea's mysterious film and foam phenomenon
Tea with a film on top might look like a drink from another planet. A mysterious coating appears, shimmering, and then the foam emerges. It may seem insignificant, but it can raise suspicions. Could the tea have gone off? Thankfully not. These phenomena are natural and even beneficial.
If you've ever looked into a cuppa and thought, "Hmm, this looks strange," you're not alone. Many people wonder what's happening to their drink when foam or a characteristic film appears after brewing. While it may look unappetising or even cause concern, science has a reassuring and straightforward explanation for this. So, what does the rainbow film on tea mean?
Tea is a complex chemical mixture composed of hundreds of compounds. Depending on how you brew it, the type of water used, the quality of the leaves, and the method of pouring, various effects can appear on the surface. But there's nothing to fear. Neither the foam nor the film on the tea is harmful.
The film on tea - where does it come from and what does it mean?
Let's start with the phenomenon that raises the most suspicions: a thin, shiny film that floats on the surface of tea, especially one that's been brewed for a long time with strong leaves. It looks a bit like an oily spot, shimmers with colours, and resembles petrol spilt in a puddle. Not pleasant to the eye, but don't worry, it's not a toxin or a by-product of fermentation.
This effect is the result of a reaction between polyphenols in tea, which are valuable antioxidants, and the calcium and magnesium contained in the water. The more mineral-rich the water, the more visible the film on the tea will be. The surface layer forms naturally and does not affect the taste or quality of the drink. However, if you're not a fan of this sight, you can easily prevent it: filter the water before brewing the tea, or add a slice of lemon. Why is this helpful? The acid dissolves the compounds responsible for the film on the tea.
Foam in tea is also desirable
Meanwhile, the foam that appears during brewing is due to the presence of saponins, which are plant compounds that exhibit anti-inflammatory properties and support the immune system. These are completely natural substances that can form a light foam when in contact with hot water. They can be found in green tea, yerba mate, rooibos, and other sources. The appearing foam is a sign that the infusion contains active substances.
The way you pour the tea also affects the formation of foam. The more dynamically you pour the tea, for example, from a greater height, the more air enters the infusion, which further encourages foaming. In some cultures, such as in Morocco, tea is intentionally poured from a height to make the foam visible, treating it as an element of quality.
