Image source: Pixabay
Michelle Francl "confessed": "If the tea is cold, I will heat it in the microwave before drinking it." "Franzer, a chemist at Brynmore College in the United States, wrote in an article titled "A chemist's cup of tea" that a membrane floats on the surface of herbal tea, which makes it difficult for her to take a mouthful—to reheat the tea or squeeze a little lemon juice before making the tea.
Written by | Wang Yibo
Review | Twenty-seven
This membrane did not initially attract the special attention of Caroline Giacomin. Until a classmate from Taiwan complained to Jia Keming that the membrane on the tea was really unbearable for him, so he did not plan to drink tea again.
Jacobin is a PhD student in the Department of Health Sciences and Technology at the Swiss Federal Institute of Technology in Zurich, where she joins a research group that focuses on scientific questions about interfaces. When she discussed the doctoral research topic with her supervisor, the supervisor provided her with some topics worth studying. To Jia Keming's surprise, the "interface of tea" was listed in it, and with the previous conversation with her classmates, she decided to study this membrane herself. Eventually, her findings were published in the journal Fluid Physics.
In Jacobmin's teacup, she sometimes sees a glossy tea film, sometimes not. Moreover, if you wait for a while to drink the brewed tea, you will find that the tea film cracks like an ice layer. But if it is invisible to the naked eye, does this membrane really not exist? What factors are involved in the fragmentation of the membrane? Is it possible to keep the membrane from rupturing?
Chemists "pave" the way
In the 1990s, there were two tea lovers: Michael Spiro and Degoratius Jaganyi. They wrote a total of 14 papers on tea, of which 7 explained the chemical phenomena of tea film, including chemical composition and various factors that affect the formation of tea film, and more importantly, they also provided chemical kinetic explanations for "sometimes the tea film can be seen in the tea cup, sometimes not seen".
For scientists, the lab is a great place to make tea — a glass beaker of a few hundred milliliters is an excellent cup, and a constant temperature water bath pot allows them to control the temperature of the "tea" relatively precisely. Spiro and Jagani put the black tea bag in a beaker, brewed it in 80 °C water for 5 minutes, then removed the tea bag and let the tea sit for a while.
Previously, some scientists believed that the tea film was brewed with boiling water, and the waxy layer on the tea "floated" to the surface of the water. But when Spyro and Jagani made tea with distilled water in the lab (very few impurities such as inorganics and organic matter), no tea film appeared. This shows that relying only on tea leaves and higher water temperatures cannot produce tea films, and some components in the water must play a key role. Through further experiments, they confirmed that calcium ions and bicarbonate ions are the key to inducing the formation of tea membranes, but only calcium ions or bicarbonate ions alone cannot make the tea membrane "appear", it must be a combination of the two.
In addition, acidity and alkalinity and oxygen concentration will also affect the formation of tea film. For example, the greater the alkalinity, that is, the higher the hardness of the water, the easier it is to form a tea film. Moreover, if the air is replaced by nitrogen, it is difficult to see the tea film, so the formation of the tea film must involve oxygen and oxidation reactions, which is also one of the differences between tea scale and scale.
At the same time, through scanning electron microscopy, mass spectrometry, trace analysis and other test methods, Spyro and Jagani further analyzed the composition of tea film: tea film is actually composed of organic matter (mainly containing carbon, hydrogen, oxygen) and inorganic matter (including carbonates and hydroxides). Among them, almost all calcium and sodium ions come from water, and potassium, manganese and aluminum ions almost all come from tea. They also specifically stated that the carbonates and hydroxides in this membrane exist independently in the form of insoluble compounds, while organic matter provides physical support for these insoluble inorganic substances — from the scanning electron microscope (SEM), calcium carbonate particles "stay" on the surface of organic matter.
Spyro and Jaganyi also tried to write the molecular formula of the tea membrane. They speculate that after 1 hour of standing, a "tea membrane molecule" will be composed of about 45 carbon atoms, 50 hydrogen atoms, 40 oxygen atoms and 2.7 divalent metal ions, and the molar mass can even reach about 1400 grams.
To achieve a chemical process, it is necessary to cross too many energy barriers, that is, how much energy is needed to make the reaction process go smoothly, which is a problem that chemists are extremely concerned about. By precisely regulating the tea temperature at rest, and based on the Arrhenius equation, Spireo and Jagani calculated the activation energy that formed the tea film: 34 kJ/mol. This is a relatively high barrier , greater than the activation energy required for diffusion ( 15.8 kJ/mol , calculated from stokes–Einstein relations). Diffusion includes the diffusion of ions in solution, and the diffusion of gases from air into solution. Only after the reactants have been diffused "meet" and collide, that is, a chemical reaction, it is possible to form a tea film.
Image source: Michelle Francl
However, when making tea in a teacup, sometimes the water temperature drops down quickly, so before the tea film has time to form, a large amount of heat is lost. On the contrary, if you use a ceramic teapot with better heat preservation to brew tea, the tea water heat dissipation is slower, so the tea film can usually be seen, and more tea scale will be left in the teapot, and these mineral-rich (such as calcium ions, magnesium ions) tea scale can also induce the formation of the next tea film. This coincides with Jacobin. Jakomin quipped that if you want to see the tea film in the teacup all the time, it is best not to wash the cup.
"See" the tea membrane through rheology
Based on the research of the above 2 chemists, Jakomin wants to observe the membrane from a rheological point of view and analyze the mechanical properties of the membrane, rather than the chemical properties.
As early as 1678, Robert Hooke proposed Hooke's law - for solids, under a certain pressure, the stress in the material has a linear relationship with the strain (degree of deformation), such materials are called Hooke elastic solids. Nine years after Hooke published his paper, Isssac Newton solved the problem of the flow of shear fluids and proposed Newton's law of viscosity (also known as Newton's law of internal friction). A fluid is a liquid or gas that, when flowing under the action of an external force, creates a stress that resists an external force. Newton states that there is a linear relationship between the shear stress of a fluid and its flow rate, and a fluid that conforms to this law is called a Newtonian fluid, such as water and alcohol.
But in fact, not all material motion can be explained by Hooke's law or Newton's law of stickiness. There is a class of materials that exhibit the characteristics of Hooker solids under certain conditions, such as elastic deformation (transient, revertable deformation), while under other conditions, behave like a fluid, that is, viscous flow (continuous, non-returnable flow). Rheology studies this kind of weird material. According to the American chemist Eugene Bingham, rheology is a new branch of discipline that studies the deformation and flow of materials.
For Jacobin, she wanted to know both the elasticity of the membrane and its viscosity. In addition, this membrane is located between water and air, so she chose a double-cone interface rheometer.
It is worth mentioning that to describe the mechanical properties of tea film, Jakomin has to use the "modulus" indicator. Similar to hooke and Newton's laws, modulus is a measure of the relationship between stress and strain. In addition, for complex materials such as tea film, the corresponding modulus is the elastic modulus (G′) and the viscous modulus (G)," which can also be called energy storage modulus and loss modulus.
In order to determine the effect of calcium ions, Jakomin prepared 6 different concentrations of calcium carbonate solutions (0, 10, 25, 50, 100 and 200 mg/L), which contained almost no other metal ions, and used these solutions instead of water to make tea. To Jia Keming's surprise, she did not see the tea film, but the membrane, which was invisible to the naked eye, was "seen" by the rheometer.
Image source: original paper
Jakomin found that when the shear stress amplitude is fixed (0.3%), when doing dynamic time scanning, for solutions with calcium carbonate concentrations of 50, 100 and 200 mg/L, the elastic modulus of the tea film is greater than the viscous modulus (G′ > G), that is, it is solid; when the calcium carbonate concentration is less than 50 mg/L, the tea film is fluid (G" > G′). That is to say, the lower the concentration of calcium carbonate, the more it can make the tea film flow. In addition, similar to the results obtained by Spyro and Jagany, when making tea with ultrapure water (almost no metal ions), not only can the tea film not be seen, but the rheometer cannot detect it.
As mentioned earlier, we tend to see cracked tea membranes, so Jakomin wants to see how much stress amplitude can make this membrane crack. If the "modulus" is used to indicate when the membrane will crack (the strength of the membrane), it is when the modulus of loss is greater than the modulus of energy storage. Here, compared to elasticity and viscosity, the terms energy storage and loss can give us a more intuitive sense of "why the membrane is rupturing".
When calcium carbonate concentrations are higher (100 and 200 mg/L), a lower shear stress amplitude (0.5%) can cause the tea film to fragment, i.e. G" > G′. However, when the calcium carbonate concentration is reduced to 50 mg/L, a higher stress amplitude (0.8%) is required to cause the tea film to crack, so the energy storage-based (G′ > G") tea film is more flexible and not fragile. However, for 10 and 25 mg/L, no matter how the stress changes, the modulus of loss has always been greater than the modulus of elasticity, and the tea film at this time is like a fluid, which is difficult to form.
In Jakomin's eyes, tea film is a shiny and beautiful thing. Therefore, in order to see this layer of tea film, she suggested at the end of the paper: "Do not wash the teacup."
How about lemon tea?
One of the most common types of tea on the market is lemon tea. This is not only because of the flavor of lemons, but also because of the scientific principles behind them. Spiro and Jagani had already discovered that citric acid inhibited the formation and growth of tea membranes. This is because citric acid can complexate with metal ions such as calcium ions, thereby reducing the concentration of free metal ions, and calcium ions and the like are the key to the formation of tea films. Jia Keming found that after the addition of citric acid, although the tea film cannot be seen, the rheometer "indicates" that the tea film still exists, but at this time the modulus of the tea film is reduced, that is, citric acid can soften the tea film, making the film easier to stretch, and at the same time increasing its mechanical strength.
Jakomin said that such stronger films play an important role in bottled drinks.
Image credit: pixabay
In fact, in our bottled tea drinks, it is unlikely that we can see a layer of membrane floating above with the naked eye, mostly because it contains citric acid or other complexes, which can inhibit the formation of tea film. Moreover, when the tea film inevitably appears, for example in milk tea drinks, a little citric acid can also stabilize the tea film by increasing the mechanical strength of the tea film.
Franzer writes that every time she squeezes lemon juice into black tea, it takes her back to her college basic chemistry class and reminds her of midterm exam questions that have made her shudder. At the time, her chemistry professor, Sherry Rowland, asked them, "Why do lemons make tea lighter?" Please write out the corresponding chemical equations. ”
Cover image source: Carolyn Jacobmin
Reference Links:
https://aip.scitation.org/doi/10.1063/5.0059760
https://doi.org/10.1038/364581a0
https://doi.org/10.1016/0308-8146(94)90005-1
https://doi.org/10.1016/0308-8146(94)90004-3
https://tea-biz.com/2021/09/19/the-physics-of-black-tea-film/
Rheology: An Historical Perspective. By R.I.Tanner and K. Walters
Source: Global Science
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