Kinetics and equilibria of tea infusion. Part 11—The kinetics of the formation of tea scum
1994; Elsevier BV; Volume: 49; Issue: 4 Linguagem: Inglês
10.1016/0308-8146(94)90005-1
ISSN1873-7072
Autores Tópico(s)Pesticide Residue Analysis and Safety
ResumoA detailed study has been made of the rate of scum formation in hard London mains water at 80°C. From a given black tea infusion the scum grew continuously and, in the first hour, proportionately with time. The amount formed depended on the exposed surface area and not on the volume of the tea brew. Longer infusion of a given mass of tea leaf first increased and eventually decreased the mass of scum. Surprisingly, the more tea leaf that was used the less scum was produced, a phenomenon largely caused by the concomitant decrease in pH. Tea scum only formed in water which contained both calcium (or magnesium) ions and bicarbonate ions. No scum developed if the former were removed by complexing agents or the latter by lowering the pH. Boiling the water reduced but did not eliminate scum. More scum formed if the period of scum development took place at a higher temperature and the activation energy of scum formation was found to be 34 kJ mol−1. This relatively high value, as well as the absence of any effect when the infusion was stirred, showed that the overall process is chemically- and not diffusion-controlled. Decaffeinated tea gave a similar amount of scum as normal black tea while green Chun Mee leaf produced about 80% as much. Moreover, scum formation was inhibited by passing nitrogen over the surface but enhanced by passing oxygen. Taken together, these experiments suggest that scum is produced by oxidation of tea solubles mediated by calcium carbonate formation.
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