Tea originates during the "Shen-Nong" ear of ancient China, which is around 5000 to 6000 years ago(Chen, 1994). At the beginning, tea was used only as medicine to cure illness. But in nowadays, tea has become one of the most popular beverages in the world. Tea is effective for quenching thirst, its unique sensory properties, rich tradition and culture; the apparent health benefits for human have gained its popularity today.
Drinking tea has become an almost indispensable part nowadays in many parts of the world. When the tea liquor is sucked into the mouth and swilled around the tongue and gums while the aroma is drawn to the back of the mouth and the olfactory nerve in the nose where tea is tasted, felt and smelled(C.R.Harler, 1964).
Taste of food is mainly composed of five basic sensations, sweetness, astringency, sourness, bitterness and umami(Tamura et al.1969). For tea, astringency is a drying, puckering sensation that can affect the entire tongue a little uniformly(Lea&Arnold, 1978). Bitterness, in a usual case, is unpleasant, but however, in tea, if at a moderate level, will be enjoyed as well(Moncrieff, 1967).
2 Tea flavour compounds
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Teas are classified into three category, non fermented tea, such as green tea; semi-fermented oolong tea and fermented teas, like gongfu black tea. The greatest significance is the degree of fermentation. Green tea normally is non-fermented while black tea is fully fermented. The green tea manufacturing normally will go through steps like spreding-out, fixing, rolling and drying while the black tea will go though steps like withering, rolling, fermentation and drying. Different level of fermentation will cause the different flavour volatile and non volatiles compounds, some of the compounds are from the tea plant, some of the compounds are created by the later fermentation (Kit Chow & Ione Kramer,1990).
2.1 Volatile compound
Through Chromatography-mass spectrometry (GC-MS), the volatile compound of tea has been extensively studied.
Tea leaves are very rich in volatile compound that contribute a lot to its unique aroma. Up till now, more than 600 hundreds of aroma compounds have been found in tea(NingXu&Zong Mao Chem, 2002), these aroma compounds can be classified into two categories, the primary or secondary products. The primary ones are biosynthesized in the tea plant and can be found in the green tea leaf and the secondary ones are created in the manufacturing process of the tea leaf (Sanderson&Graham, 1973).
The primary compounds include carotenes, amino acids, lipids and terpene glycosides and etc. Others like alcohols, aldehydes, ketones, acids, esters, lactones, phenols, nitrogenous compounds, sulphur compounds and mixcellaneus oxygen compounds are also found in tea aroma(Robinson&Owuor,1992).
Besides the impact on aroma, some of these aroma compounds also have biological effects on human. For example, geraniol can play a role as a mevalonate biosynthesis inhibitor which can reduce cholesterol level. Geraniol also shows an ability to inhibits the tumor cells proliferation and the growth of transplanted hepatoma in rats and melanoma in mice(Yu et al. 1995).
After all, the content of each aroma compound in the tea leaves varies a lot based on the environment it grows. Changes in the weather, altitude, temperature, soil quality can all contribute to the different taste and aroma in the end(Ningxu&Zongmao Chen, 2002).
2.2 Non-volatile compound
There are hundreds of volatile compounds in the tea aroma, while in general, there are several main non-volatile compounds, they are polyphenol, amino acids, caffeine, organic acids.
The concentration of Ployphenol varies in different types of tea. Black tea usually has higher concentration than the green and white tea which give tea the dark color and bitter and sharp flaovr. Normally, Ployphenol is a reason for the astringent taste of tea. The total polyphenols in tea flush can range from 20% to 35%. There are several groups of compounds included in tea polyphenols. The flavonols, mainly catechins, are most important and takes up about 60%-80% of total amount. Those catechins who can dissolve in water contribute to the astringency and bitterness in green tea. It can also act as the precursor of theaflavins in black tea. About 90-95% of the flavonols undergo enzymatic oxidation to products which are closely responsible for the color of tea infusion and its taste(Zongmao Chen&Huafu Wang, 2002).
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In the fresh leaf, there are three major flavonal aglycones including kaempferol, quercetin, myricetin. These substances occur both as free flavonals and as flavonal gylycosides. The glycosidic group can be glucose, rhamnose, galactose, arabinose or rutinose. They are considered to play a role in the bitterness and astringency taste in green tea (McDowell&Taylor, 1993).
Amino acids constitute about 4% in tea flush. It is reported to be significant in the taste of green tea(Nakagawa,1970,1975) and aroma of black tea(Ekborg-Ott et al.1997).
Aspartic, glutamic, serine, glutamine, tyrosine, valine, phenylalanine, leucine, isoleucine and theanine (5-N-ethylglutamine) were identified to be the main amino acids in tea leaf. Theanine is the most abundant one in these amino acid which takes about 60% of total content. The amino acids, such as leucine and isoleucine, if high level, are detrimental to the quality of black tea(Tirimanna,1967; Wickremasinghe, 1978). Besides its aroma function, amino acids also play an important role of bringing down blood-pressure of human in vitro and in vivo(Omori,1991). It may also act as biochemical modulator to increase the antitumor effect of doxorubicin(Sadzuka et al,1996).
Caffeine is a purine alkaloid present in tea, which is 1,3,7-tri-methyl xanthine. It takes about 2-5% in tea flush on dry weight basis. It is the main stimulating property and can help get rid of the mental fatigue. Caffeine can increase the briskness property by forming a complex with polyphenols. Briskness is a very refreshing sensation.
During the withering process, the level on caffeine may increase, because other components will relatively decrease, however, the total amount of caffeine will decrease during the firing process.
The pharmaceutical function of caffeine have been identified as a stimulates which works on the central nervous system and has been applied therapeutically(Aranda et al, 1977). Caffeine can also relax the chronic bronchitis's smooth muscle which makes it quite valuable for the curing of asthma and the bronchospasm of chornic bronchitis. It can also increase the workability of human muscles which may be due to its stimulating effect on central nervous system.
Several organic acids are identified in the tea flush such as dicarboxylic acids and tricaboxylic acids which includes succinct acid, oxalic acid, quinic acid, malic acid citric acid etc. fatty acids are also found in tea, such as linoleic acid, palmic acid, hexanoic acid, pentoic acid etc. Some of the organic acids act as aroma compounds and some of them do not. But however, these non aroma compounds can still transfer to aromatic through oxidation and other reactions.ï¼ˆZongmao Chen&Huafu Wang, 2002ï¼‰
3 Effects of Long shelf life over tea flavour
In this report, Puer tea will be used as an example to unveil the effects of long storage time over the change of flavour compounds and quality. The reason puer tea is picked is because as a saying goes for puer tea, "The more aged, the better quality". Customer believe that the flavour and nutrition of puer tea will goes upward with the day pass by which gave the aged puer tea a very high price, some are even as expensive as gold.
Normally, the tea polyphenols in puer tea is lower than green tea because of the long fermentation period of tea processing. In the tea polyphenols of puer tea, catechins accounts for 65%-80%, during the fermentation process, most of the catechins are oxidised. It is reported that polyphenols and catechins during storage decreased rapidly for the first two months and then slow down. Since catechins is a very important source for the bitter and astringent taste, hence the long shelf life storage can actually improve the taste of puer tea because the taste will go milder when time goes by.
Based on the variation of tea polyphenols, catechins, it is reported that the transformation of the main tea polyphenols are divided into two main stages. The quick conversion of tea polyphenols and catechins into teaflavin, thearubigin and theabronins predominates for the first two months, after that, the process will slow down because deactivity of enzymes with time.
It is similar of the volatiles in puer tea stored for long time, both the class and the content of volatile compounds in the tea stored for years decreased. Levels of some types of volatile compounds exist in newly processed tea will go down or even disappeared(Tongxun Liu et al, 2009).
4 Flavour and nutrition effects of milk teaÂ
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Drinking tea with milk has been a very popular way UK and other places in the world. For a lot of tasters, the smoothness and sweetness in milk can help to cover the bitterness and astringency to make the entire beverage milder. But in general, adding milk in tea or not is mainly due to personal habits.
The flavor aspect may be quite beneficial to tasters, but speaking of health function of this combination, the outcome may goes otherwise. As have been accepted, drinking tea is helpful to improve the function of the artery and prevent cardiovascular problems, but adding milk in tea, the improvements will vanish. This is mainly due to the most abundant proteins in milk, Î²-casein. Î²-casein is extremely liked by epigallocatechin-3-gallate (EGCG) because the proline it is studded with. EGCG will combine with the proline which will lead to the decrease of catechins in tea, one of the main function chemicals who has beneficial effects on the heart. It is said that EGCG level in milk tea is only 20% of the plain tea level(Jeremy Warner,2009).
Tea aroma is mainly decided by the volatiles and the effects on the human health are mainly decided by the non-volatiles. Apart from a nutritional, flavoured beverage, it is also medicine plant that is even used to cure disease in china. The various contained in tea plays multiple functions including anticaries, blood pressure lowering, blood-glucose depressing, blood-lipid reducing, prevention of atherosclerosis and thrombus formation, corpulence reducing, antisenescent, anti-rediation damage, antioxidative, immune function improving, deodorizing, gemicidial and antiviral, diuretic, liver-protecting, antiulceric, antimutagenic and anticarcinogenic acitivies(Chen, 1991,1998; Blot et al. 1996; Dou, et al. 1997).