The chemistry of clear plaques on the surface of tea.
FOODS AND BEVERAGES contain so many amphiphilic molecules—one part of which are water soluble and another part insoluble—that foams are common in the kitchen: stiffly beaten egg whites, champagne, beer, cream, and so on. In cooking one sometimes tries to minimize contact with antifoaming agents, taking care not to spill any drops of egg yolk into whites that are about to be whisked, for example, because the fatty molecules in the yolk bond with the hydrophobic part of the proteins in the white, removing them from contact with the air and thus stabilizing the water–air interface. Similarly, people who like the fizz of champagne are careful not to wear certain kinds of lipstick that contain antifoaming molecules. Under other circumstances, however, foam is something to be avoided. When making apricot jam, for example, one can cause the froth created by cooking to subside by daubing the surface with melted butter.
Films Disturb Connoisseurs
A pair of chemists at Imperial College, London, both great lovers of tea, set out to investigate the thin clear plaques that appeared on the surface of their teacups. Michael Spiro and Deogratius Jaganyl initially supposed the plaques to be the residue of a foaming phenomenon that had been arrested by the hardness of the water, but they were surprised to find something entirely different, which they described in an article in Nature.
The films that one observes in teapots and teacups are irregularly shaped plaques that to the naked eye look like surface stains. Spiro and Jaganyl were able to study them quantitatively once they had devised a method for collecting them from the surface of large containers that they had infused with tea. Examination of these films with a scanning microscope revealed the presence of small, clear particles on their surface that turned out to be calcium carbonate. Microchemical analysis confirmed that all of the calcium, as well as traces of magnesium, manganese, and other metals coming partly from the municipal water supply and partly from the tea, could be eliminated by treatment with chlorhydric acid. The remaining particles were organic in nature and insoluble in all the solvents tested but soluble in concentrated bases. Mass spectrometry indicated that this residue was composed of molecules of different mass, in the neighborhood of 1000 daltons.
Spiro and Jaganyl also studied the rate of formation of these films by infusing black Typhoo tea in water heated to exactly 80°C (176°F) for definite periods of time. After the infusion phase they removed the tea bags, skimmed any froth from the surface, and then measured the formation of films over time. They observed that films formed for several hours, their quantity increasing proportionally with the number of minutes elapsed during the first hour; after four hours, the mass of the thick film that had formed was proportional to the surface of the container. The rate at which the film formed depended on the atmosphere above the tea: The mass of film that appears in atmospheres of pure oxygen is greater than that which develops in ordinary air or under a nitrogen atmosphere. It was clear that the film was produced by the oxidation of the tea’s soluble compounds, certain polyphenols among them (the bitterness of tea results from such molecules).
The British chemists did not observe any film at the surface of tea infused in distilled water or in distilled water to which calcium chloride had been added. A film appeared only if the water contained both calcium (or magnesium) and bicarbonate ions. Nor did any film form when the calcium ions were sequestered by a chelating compound such as ethylenediaminetetraacetate or when the tea was acidified. No film appears in tea containing lemon, for example, because its acidity is greater than that of unadulterated tea, its calcium ions being sequestered by citrate ions. Similarly, very strong teas have little film, for the abundance of polyphenols increases their acidity. By contrast, the addition of milk greatly increases the amount of film, as does raising the temperature of the infusion.
The complex organic compounds found in such films, which result from the oxidation of soluble molecules in the presence of calcium ions and bicarbonate ions, seem to form by a process analogous to the enzymatic oxidation used by tea producers to transform green tea into black tea. Further research will be needed to confirm this hypothesis.