Black tea is a type of tea that is more oxidized than oolong, green and white teas. All four types are made from leaves of the shrub (or small tree) Camellia sinensis. Black tea is generally stronger in flavor than the less oxidized teas. Two principal varieties of the species are used – the small-leaved Chinese variety plant (C. sinensis subsp. sinensis), used for most other types of teas, and the large-leaved Assamese plant (C. sinensis subsp. assamica), which was traditionally mainly used for black tea, although in recent years some green and white have been produced.
Black tea is usually graded on one of four scales of quality. Whole leaf teas are highest quality followed by broken leaves, fannings, and dusts. Whole leaf teas are produced with little or no alteration to the tea leaf. This results in a finished product with a coarser texture than that of bagged teas. Whole leaf teas are widely considered the most valuable, especially if they contain leaf tips. Broken leaves are commonly sold as medium grade loose teas. Smaller broken varieties may be included in tea bags. Fannings are usually small particles of tea left over from the production of larger tea varieties, but are occasionally manufactured specifically for use in bagged teas. Dusts are the finest particles of tea left over from production of the above varieties, and are often used for tea bags with very fast, very harsh brews. Fannings and dust are useful in bagged teas because the greater surface area of the many particles allows for a fast, complete diffusion of the tea into the water. Fannings and dusts usually have a darker colour, lack of sweetness, and stronger flavor when brewed.
"The additional fluoride from drinking two to four cups of tea a day won't harm anyone; it's the very heavy tea drinkers who could get in trouble," said Dr. Gary Whitford, Regents Professor of oral biology in the School of Dentistry. He presented his findings at the 2010 International Association of Dental Research Conference in Barcelona, Spain.
Most published reports show 1 to 5 milligrams of fluoride per liter of black tea, but a new study shows that number could be as high as 9 milligrams.
Fluoride is known to help prevent dental cavities, but long-term ingestion of excessive amounts could cause bone problems. The average person ingests a very safe amount, 2 to 3 milligrams, daily through fluoridated drinking water, toothpaste and food. It would take ingesting about 20 milligrams a day over 10 or more years before posing a significant risk to bone health.
Whitford discovered that the fluoride concentration in black tea had long been underestimated when he began analyzing data from four patients with advanced skeletal fluorosis, a disease caused by excessive fluoride consumption and characterized by joint and bone pain and damage. While it is extremely rare in the United States, the common link between these four patients was their tea consumption -- each person drank 1 to 2 gallons of tea daily for the past 10 to 30 years.
"When we tested the patients' tea brands using a traditional method, we found the fluoride concentrations to be very low, so we wondered if that method was detecting all of the fluoride," Whitford said, noting that the tea plant, Camellia sinensis, creates a quandary when measuring fluoride. Unique among other plants, it accumulates huge concentrations of fluoride and aluminum in its leaves - each mineral ranges from 600 to more than 1,000 milligrams per kilogram of leaves. When the leaves are brewed for tea, some of the minerals leach into the beverage.
Most published studies about black tea traditionally have used a method of measuring fluoride that doesn't account for the amount that combines with aluminum to form insoluble aluminum fluoride, which is not detected by the fluoride electrode. Whitford compared that method with a diffusion method, which breaks the aluminum-fluoride bond so that all fluoride in the tea samples can be extracted and measured.