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The rates of infusion of caffeine into water at 25.5°C were measured for green and medium roast Kenyan arabica coffees in the particle size range 0.85–1.00 mm. Each type of coffee was employed in three forms: dry (as received), water-filled but still containing the original amount of solubles, and water-filled and free of solubles other than caffeine. Intra-bean diffusion coefficients of caffeine were calculated from the rate constants and compared with the known diffusion coefficients of caffeine in water to provide hindrance factors. These were much larger than the corresponding hindrance factors obtained at 80°C (Spiro et al, J Sci Food Agric46, 349–356 (1989)). Examination of these results and of partition constants determined at both temperatures enabled the factors contributing to slow internal diffusion to be separated and identified. Factors which were more pronounced at 25°C were the finite rate of caffeine dissolution within the bean, complexation of caffeine with chlorogenic acid and other coffee solubles, and the adsorption of caffeine molecules within the bean matrix. © 1997 SCI

Dilute (1–10 vol. %) solutions of milk in hot London mains water formed films on the surface which increased with time. No film formed in solutions of milk in distilled water unless calcium chloride had been added. The mass of the surface film depended on the total mass of milk used, not on its concentration. These facts are consistent with an interaction between calcium (and other metal) ions and milk components within the bulk solution, the product then rising to the surface. Surface oxidation did not seem to be involved. This mechanism is quite different from that which forms films on infusions of black tea. The mass of the milk film increased in the order skimmed milk

Added milk greatly increased the mass of film formed on the surface of black tea brewed in hot London mains water. The amounts of film collected after adding different types of milk (skimmed, homogenised full cream, non-homogenised full cream, and UHT single cream) to black tea brews increased in the sequence listed. No film formed when distilled water was used, and very little even in hard water if the tea concentration was large. Chemical microanalysis and Fourier transform infrared examination of the films indicated that they were composed of milk constituents modified by tea polyphenols and aggregated by calcium ions. The role of the tea polyphenols is consistent with their being sequestered in casein micelles as well as bound to the protein membranes of milk fat globules. The presence of milk prevented the formation of most of the dark tea scum observed on black tea infused in hard water.

Experiments carried out on tea infusions with blown-over films of calcium carbonate (CaCO3) have shown that it is the presence of calcium and bicarbonate ions in the water, and not the concomitantly produced CaCO3, which mediates the formation of tea scum. Other experiments have refuted Lewin's (1993) theory that tea scum is mainly composed of epidermal waxes loosened from the tea leaves by hot water. Addition of fresh lemon juice greatly decreased the amount of tea scum formed in temporary hard water. This effect was explained by the resulting drop in pH and by complexation of calcium ions by the citric acid contained in the juice. Addition of sucrose equivalent to two cubes of sugar per cup significantly reduced the mass of scum, far more than could be accounted for by the increase in solution viscosity.