The Chemistry Of Tea
Part of the enigma of tea is it can both stimulate and relax the body, characteristics that the earliest patrons of tea, Buddhist monks, embraced wholeheartedly. In their efforts to meditate for long periods of time, they were greatly assisted by tea's ability to keep their bodies relaxed in meditative poses while keeping their minds alert along the path to meditative calm.
From the ever-expanding understanding of chemistry, we now know the scientific reasons for tea's impact on monks...and the rest of us. Yet even the most learned of chemists cannot identify the thousands of chemical molecules in Camellia sinensis. To further compound the mystery, the chemical elements of tea are different when processed as green, oolong or black. When tea lovers add milk or lemon to tea in the cup, the chemistry of tea can change once again.
To summarize the chemistry of tea, it's all about the "Pac men" of chemistry, polyphenols, which eat away free radicals; the quinones which contribute to aroma, and thearubigens which contribute both to the taste and the color of tea. The volatile oils in tea, which are elevated during processing, also contribute to color and taste and are fragile; age or poor storage conditions can greatly compromise these and make the tea tasteless and faded in color.
One common misconception is that there is tannin or tannic acid in tea. Tannins, a word used erroneously in the 19th century in early books on tea, are actually polyphenols, antioxidants that are healthful and, more importantly, give tea its taste profile.
"Fermentation" is another word popularized during the 19th century to explain the processing of tea leaves. We now understand that tea processing is done by oxidation, which assists in enzymatic transformation. This transformation results in aroma compounds being bound together so that they do not escape into the air, and large molecules are built up from smaller ones modifying not only flavor and color but the very body "feel" of tea.
All processed tea, no matter which varietal or process, has both soluble and insoluble elements. The insoluble elements include crude fiber, cellulose, lignin, proteins, fats, chlorophyll and pigments, pectins, and starches. The soluble elements include vital oxidizable polyphenols, other polyphenols, amino acids, minerals (ash), gummy matter and the stimulants theine, theobromine, theophylline and caffeine.
The polyphenols are beneficial elements along with Vitamin C, a vitamin typically impacted by heat. However, heat does not appear to destroy vitamin C in tea when brewed in green or oolong teas infused under 176 degrees F. There are small, nearly negligible amounts of other vitamins including B1 and B2, K and P, niacin, folic acid and manganese. Black tea, however, contains little or no vitamin C because the complete oxidation destroys it during processing. Black tea is a good source of potassium and contains about 58mg per cup.
GREEN TEA, because it is not oxidized, contains more of its chemical elements, particularly its six main polyphenols, which include flavanols, flavandiols, flavonoids and phenolic acids. The bulk of polyphenols in green tea are flavonols (aka catechins). These catechins, which make up 30-42% of the elements of green tea, work to attack free radicals which cause disease. Of these catechins, EGCG has the strongest capacity to fight off free radicals. Catechins in green tea contain Epicatechin (EC), 17mg; Epicatechin gallate (ECG), 28mg; Epigallocatchin gallate (EGCG) 142mg, Epigallocatechin (EGC) 65mg, which are the most evident. Two others, catechin (C) and Gallocatechin (GC) contain trace amounts. These catechins (which make up 30-42% of the elements in green tea) work to attack free radicals which cause disease. A cup of green tea also includes 76mg of stimulants including caffeine (3.5%), theobromine (0.1%), theanine (4-6%), and theophylline (.02%). Other elements in green tea include theogallin (2-3%), quinic acids (2%), minerals (6-8%); carotenoids (.02%) and monomeric flavonols (5-10%). These percentages are based on fresh leaves from Chinese greens; those from Japan or any other country will be similar but not exactly the same.
OOLONG TEA, which can be oxidized from 2 to 80%, has a unique flavonoid, oolonghomobisflavins, and like green tea, catechins are high: monomeric catechins, theaflavins and about 30% thearubingins. Other elements found in oolongs are epigallocatechin esters, theasinensins, dimeric catechins and dimeric proanthocyanidins. Other polyphenols in oolongs include flavanols, depsides, theogallins and theanine (an amino acid unique to tea which contains 20 amino acids.)
BLACK TEA, which is fully oxidized, contains about 10-20% thearubigens; other elements include theaflavins (1-2%). The full oxidation process also changes the chemical elements and one result is the creation of bisflavanols, a result from the flavanols undergoing polyphenol oxidative polymerization, and oligomers.
CHEMISTRY OF FLAVOR ADDITIVES
Adding sugar or artificial sweeteners does not appear to change the chemistry of tea, however lemon and milk can. The polyphenols in all tea bind with iron and calcium and thus prevent these minerals from being absorbed, however, adding lemon juice inhibits this binding and makes the minerals available for the body to use; lemon juice added to black tea will increase iron and calcium solubility and absorption in the body. Lemon slices or lemon juice should not, however, be used when drinking tea from Styrofoam cups because the polystyrene will be dissolved into the tea liquid.
Milk in green and oolong teas have not been thoroughly studied, however milk in black teas has. Chemists have discovered that a few tablespoons of milk will not interact with the 70% of oxidized polyphenols, which may explain why milk in black tea calms the stomach. However, if too much milk is added, it will interact with the 30% of unoxidized polyphenols and may irritate the stomach. Like everything in life, chemical changes have many exceptions, so use your own judgment whether milk added to black tea is good or not for you.
SOURCES: For more details on the chemistry of tea, see TEA: Cultivation to Consumption by K.C. Willson and M.N. Clifford. Although published in 1992, it remains a definitive source for clinical and physiological elements in tea. ON FOOD AND COOKING, The Science and Lore of The Kitchen, Harold McGee.