Wednesday, April 11, 2007

The Flavor of Tea: The Polyphenol Connection


The enjoyment of drinking tea is a complex phenomenon. On a specifically physiological level, what we enjoy most about drinking a particular tea is the result of the interaction of various biochemicals that are present in the tea. These, in the right proportions of quantity and quality, give the resultant pleasant flavour -- just as, in a good symphony, the different musical pitches are combined in right proportion.

In an earlier essay I discussed the presence and importance of polyphenols in tea. The partial oxidation of polyphenols in the tea-leaf is a vital aspect of the production of Darjeeling and oolong teas. Indeed it has a decisive effect on what the final flavour of these teas will be. In this connection, it is worth examining just how -- and to what extent -- polyphenols are (or should be) brought to a process of oxidation.

The Physiology of Aroma

Flavor sensed by nose is called "Aroma." Aroma is perceived by two different mechanisms. It can either be sensed nasally (via smelling the tea infusion through the nose) or retronasally. Retronasal perception occurs when the tea liquor is either present in the mouth or has been swallowed, and aromatic volatile compounds drift upward into the nasal passage.

Polyphenols in Darjeeling & oolong Teas

Polyphenols in the tea brew give astringency/ briskness/ body and strength to the tea brew, and these characteristics tend to overlap or shadow the flavor. Hence:
• A lower level of polyphenols in the brewed tea yields more/better flavor.
• The lower the percentage of polyphenols in the tea shoot, the lower the percentage of polyphenols in the brewed tea. Hence the better will be the flavour of the liquor. For this reason the C. sinensis cultivar, having lower polyphenol levels than the C. assamica cultivar, in general leads to better flavour.
• In oolong processing, only a limited number of cells are bruised before inactivation of the enzymes. Hence there is more limited oxidation of polyphenols in the course of the oolong process, which in turn yields its excellent flavor.
• Substantial amounts of polyphenols are polymerized during the processing of the tea, to the extent that they become insoluble. This yields better flavor, for the same reason as detailed above.
Less bruising of the leaf cells during processing yields better flavour, for the same reason as detailed above.
• Clones having lower levels of polyphenol oxidase (PPO) activity give more flavor.

Shading Tea During Growth

• Tea that is shade-grown prior to plucking yields more theanine and less polyphenols, besides small size. And less polyphenol content in such leaves is one of the major factor to give better flavor in oolong teas and Darjeeling teas which are processed from such leaves. as is known to all. Shading is used in Japan for the production of some of its best green teas, but this practice also improves the quality of oolong teas, as is clear from the following information.

The effects of different levels of shade (0, 40, 60, & 80%) & different durations (5 & 10 days) were tested in Taiwan to determing how these influenced variations in the quality of oolong tea. The best results were obtained with 60% shading, maintained for 10 days prior to plucking; this yielded smaller and more tender green leaves than the unshaded control. The tea made from such leaves was low in polyphenols, high in caffeine, & had a good appearance. It is concluded that shading treatments could be used to improve the quality of summer tea generally.

Sunday, April 1, 2007

Notes on the Biochemistry of Tea


"Life" Span of the Plucked Green Tea Leaf

A most interesting (and maybe the least-known) important point in oolong tea processing is the desired and the required amount of oxidation during the period when the leaf is alive. In black tea production, the maximum amount of desired/required oxidation takes place on the fermentation floor, after the leaf is killed during rolling.

As with human beings, green tea leaf is alive as long as it is able to breathe. And the plucked tea green leaf is alive even after being detached from the tea bush. It continues to respire till it is subjected to heavy damage.

Types of Heavy Damage

The types of heavy damage employed to the plucked tea shoots to cause the death in various types of tea manufacture are as follows:

[1] Mechanical damage: Heavy Mechanical damage is done to the tea leaf structure, causing in the death of cells by [a] orthodox rollers in orthodox manufacture, or [b] CTC rollers in CTC process [[the 'crush/tear/curl' procedure]].

[2] Thermal damage: The leaf is killed when high-temperature wet thermal damage is applied by steaming (in Japanese green tea production), or when dry thermal damage is applied by panning (in Chinese green tea manufacture), or when dry thermal damage is applied for fixing (in oolong tea manufacture). In one of our gardens in Darjeeling, we at Gopaldhara were the first to use the dry heat of hot air in our normal dryers to inactivate the enzymes (to kill the flush) for our Darjeeling green tea production. This is a unique method of green tea manufacture to inactivate the enzymes, and so far is still practised nowhere in the world except in a few Darjeeling and other Indian tea factories.

[3] Dehydration damage: The leaf is able to respire and remain alive as long as its moisture remains more than the minimum critical moisture. Once the moisture level dips below this minimum critical level, the leaf will die. In some Darjeeling tea manufacture, the plucked tea shoots are hard-withered: 100kg of green leaf, having say 75 kg of moisture and 25 kg of solid matter, are withered to say 35 kg and so this resulting 35 kg of withered leaf has 25 kg of dry matter and 10 kg of moisture. As a result of this heavy removal of moisture, some cells die after hard-withering. In some cases, a tender leaf will die due to the death of all of its cells.

[4] Starvation damage: The leaf uses its food materials (carbohydrates, fats, etc) for the release of the energy needed for its physiological functions. Once this food-stuff is fully consumed, the leaf dies. After plucking from the tea bush, the tea shoot enters into its "senescence" phase.

From Tea Tree (Bush) to Tea Brew:
Polyphenols in Tea


Tea Flush: The tea leaves are plucked from the tea bush as tea shoots having an apical bud plus a number of leaves. Such tea shoots are termed "tea flush."

Processed Teas: The tea flush is processed in the factory and the processed teas are termed "made teas," "manufactured teas," "processed teas," or simply "teas."

Tea Liquor: The teas are brewed in water and the resultant liquid which we drink is termed "tea liquor."

Infused Leaf: The spent tea leaves are called the "infused leaf."

Biochemicals in Tea

These have been very much fascinating to me. To understand them, we should have a look at their presence in the three main stages of tea, which are as follows:
[i] Tea Flush
[ii] Processed Tea: Biochemicals present in the processed tea shoots, i.e. the biochemicals present in the manufactured tea. The processing methods and the processing environment have a big impact on the conversion of biochemicals present in the tea flush. So different categories of processed teas (green tea, black tea, oolong tea, white tea etc) from the same flush, will have different biochemicals in quantity and quality.
[iii] Tea Liquor: This brew is the result of the extraction in water of water-soluble solids present in the processed tea.

Not all the solid compounds in the processed tea are water soluble; some are not soluble at all, and the solubility of the water-soluble compounds also is different at different water temperatures. Furthermore, there are some changes in biochemicals during the process of brewing; and different brewing processes will result in different biochemicals in the brew.

It is the brew that we drink, and so the biochemicals present in the tea brew are most important to us -- not the biochemicals present in the tea flush or in the processed tea. Let us have a look at the biochemicals present in the typical tea flush of Darjeeling.

Chemical Composition of the Fresh Tea Shoot

The chemical composition of tea shoot varies with agroclimatic condition, season, cultural practice and the type of material, type of soil, etc. The following table shows the biochemicals present in a typical tea shoot plucked from a Darjeeling garden, on the dry weight basis.

A plucked tea shoot may consist of two leaves and a bud, three leaves and a bud, four leaves and a bud, and so on; and the amount of biochemicals present will not be the same in all the leaves or in the stem portions. The apical bud is the youngest and we get more and more matured leaf as we move from the bud down the branch. The biochemicals present in the different leaves and stem increase or decrease as shown below in Table 2, as we go from the apical bud down wards to first leaf, second leaf, and so on.

In the manufacturing process advantage is taken of this difference. For example, we may pluck (say) five leaves and a bud, taking advantage of some of the biochemicals present in the stem, and after processing discard the stems in sorting before final packing of tea.

As mentioned earlier, the leaf is alive up to a particular stage of the manufacturing process followed in the factory for the different types of tea, and when leaf is alive, it performs all its normal physiological functions -- photosynthesis, respiration, etc. The resultant changes in its biochemical content will depend upon the processing environment to which green leaf is subjected at every stage of the processing.

But even after the leaf is dead at a particular stage of the manufacturing process, the enzymatic reactions will continue as long as the substrate comes in contact with the enzymes, till the equilibrium is reached between the substrate and the resultant compound, or the enzymes are inactivated. Other chemical reactions besides enzymatic reactions also take place.
So many major biochemical changes occur during the processing, but here we will confine ourselves to changes due to the oxidation of polyphenols only.

Polyphenols in the Tea Flush

In terms of human consumption, tea represents a major source of dietary polyphenols. A tea drinker typically consumes 180 to 240 mg of polyphenols from a strong cup of tea.

Polyphenols are a group of chemical compounds which contain more than one phenolic groups. There are many subgroups of polyphenols, but in the tea flush the following 5 subgroups are found.

[1] Simple polyphenols are those that are synthesised during the early stages of the polyphenol biosynthesis in the tea flush. Gallic acid, theogallins, chlorogenic, p-coumaryl-quinic acids, theogallin (about 1%), ellagic acid, corilagin, chebulagic acid, and small quantities of two or three other unidentified compounds acids are the simple polyphenols found in tea flush. (Chebulagic acid & corilagin are found more in low-grown tea flush and they give a coarse taste to tea liquors.)

[2] Flavanols: catechins belong to the flavanol subgroup of polyphenols. The catechins form approximately 90% of the total weight of the polyphenols in tea flush, and hence are most important polyphenol present in tea leaves. The different catechins present in the tea leaves are divided into two groups: catechins and gallocatechins.

• The catechins group has three catechins as follows:
a. (+) Catechin (C)
b. (-) Epicatechin (EC)
c. (-) Epi catechin gallate (ECG)

• The gallocatechins group has three gallocatechins as follows:
a. (-) Epigallocatechin gallate (EGCG)
b. (-) Epigallocatechin (EGC)
c. (+) Gallocatechin (GC)

[3] Flavonols in the tea leaf are quercetin, kaempferol, and myricitin, and their glycosides like quercitin-3-rhamnoglucoside(rutin)kaempferol-3- and(4)rhamnglucoside.

[4] Other Polyphenols: Flavones and their glycosides and proanthocyanidin species and leucoanthocynins are also found in tea leaf, but in very much smaller quantity.

[5] Tannins: Though it is commonly stated that there are no hydrolysable tannins in tea, this statement is not strictly true. In addition to the gallic esters of the catechins and their oxidation products (which can be hydrolysed to produce gallic acid readily and precipitate protein), there is a small quantity of a hydrolysable tannin known as Cameliatanin A and pentagalloyl glucose.

The dry-weight percentages of the subgroups of polyphenols are shown in Table 3:

Properties of Polyphenols

• They are colourless.

• They are astringent.

• They do not have any flavour of their own, but are instrumental in producing some flavour, or of causing the experience of less flavour or more flavour.

• They are strong antioxidants with so many health benefits.


The effect of astringency is indeed a mouth-puckering one. Usually it is a slight feeling of drying of mouth skin. To demonstrate to yourself the taste, take a slightly underripe banana -- green and yellow on the skin. Peel, and touch the edge of the banana peel to your tongue -- that’s astringent. As is chewing grape skin (black varieties). Astringency is also known in the tea trade as "pungency": see ISO [[International Organization for Standardization]] Black Tea Vocabulary (ISO 6078-1982): "Pungent: describes a tea liquor having marked briskness and an astringent effect on the palate without bitterness."

Oxidation of Polyphenols

In no type of tea are 100% of the polyphenols in the tea flush oxidised, and also none of the teas have nil oxidation. So all the varieties of tea have some oxidised and some unoxidised polyphenols. The degree of oxidation is minimal in green tea, followed by oolong teas, followed by normal Darjeeling teas, followed by orthodox teas; the maximum oxidation is to be found in CTC teas.

The unoxidised polyphenols released in the beverage, which cause the astringent, “puckery” feeling in the mouth when you drink tea, stimulate the salivary glands. This is why tea is a thirst quencher.

Enzymes Responsible for the Oxidation of Polyphenols

Enzymes work as catalyst in the biochemical reactions. Hence they only increase the speed of reactions.

The enzymes that cause the oxidation of polyphenols are polyphenol oxidase (PPO) and peroxidase. These enzymes and the polyphenols are located in different compartments of the cells. Polyphenols are located in the cytoplasmic vacuoles of the mesophyll cells, whereas peroxidase is located in the peroxisomes, and polyphenol oxidase is located in the cell walls of the epidermal layers. Only when the membranes and/or cell walls surrounding these different compartments break, do the polyphenols and the oxidisng enzymes come in contact. At that point the enzymatic reaction takes place and the following compounds are formed.

Polyphenol Oxidation in Green Tea Manufacture

The oxidising enzymes are inactivated in the first stage of the manufacturing process (whether by steaming, by pan-firing, or -- as in the case of some Darjeeling and other Indian green tea manufacturers -- by the use of dry hot air), so no oxidation of polyphenols takes place in green-tea manufacture. Thus green tea has the same polyphenols as were present in the green leaf.

Polyphenol Oxidation in Black Tea Manufacture

The black tea manufacturing process employed in India is as follows:

[1] Withering: Some oxidation of polyphenols may take place at this point, if
• the leaf is damaged during the handling of the green leaf, or if
• the leaf is exposed to a temperature of more than 110°F due to respiration heat, or if
• the respiration heat is not allowed to dissipate into the atmosphere. Respiration is an exothermic reaction [[i.e. it causes the release of energy in the form of heat]]. The green leaf is alive during withering.

[2] Rolling:
• CTC rolling time is some seconds only, and the CTC process itself causes the death of the withered leaf. So oxidation of polyphenols during CTC rolling is negligible.
• In orthodox manufacture, the rolling time varies and during the first parts of the time of rolling, the leaf is alive; after a particular amount of mechanical damage in rolling the leaf is dead. Oxidation takes place throughout the rolling process.

[3] Fermentation [[i.e. oxidation]]: in this process, in both CTC and orthodox manufacture, the desired amount of oxidation is given. This oxidation of polyphenols by PPO or peroxidase occurs when the leaf is dead. (Not many know that the same green leaf may be processed into green, into black, or into oolong tea by controlling the biological oxidation of polyphenols, which is wrongly termed "fermentation" -- wrongly, because in common parlance the word "fermentation" means the microbial production of alcohol.)

[4] Drying: in this process the oxidation is stopped by the cessation of enzyme activity. But to my mind, a whole lot of the PPO and peroxidase enzymes are not inactivated, but the remaining few active enzymes are not able to act on polyphenols, since an enzyme requires a certain amount of moisture, and dried tea does not have that much moisture. And for this reason (apart from auto-oxidation), some of the inactivated enzymes start oxidation after the tea gets moist heat and moisture from the hot and humid climate. ¶ There is generally an increase in enzyme activity as the temperature rises in the drying of teas till a maximum temperature is reached, after which the enzyme activity declines and enzymes are inactivated at a particular temperature of the teas. And for this reason, in India, oxidation continues during the initial phase of the drying process, and that also at a very high rate, till a particular high temperature is reached in the drying process.
There is auto-oxidation of tea also even at the consumer end, in spite of all the efforts taken by the producers to inactivate the enzymes involved.

Taste Changes during Typical Darjeeling Black Tea Manufacture

Polyphenols in Processed Black Tea

[1] Residual Tea Flush Polyphenols

All the polyphenols present in the tea flush which are not oxidised in black tea manufacture, may be called the residual polyphenols. Black tea has good amount of residual polyphenols.

Due to the oxidation conditions and thermal conditions experienced by the tea leaf during black tea manufacture, it is hypothesised that some of the catechins are also epimerised and/or degallated, which explains the appearance of free gallic acids as well as nonepiisomers of catechins.

Only a small portion of the myricitin and its glycoside are oxidised during black tea manufacture, and hence black tea contains all the flavanols of green tea except this small amount of oxidised myricitin.

[2] Orthoquinones

Peroxidase enzyme cannot cause the oxidation of catechins. But in the presence of the enzyme PPO, one molecule of catechin gets oxidised with one-half molecule of oxygen to form the orthoquinone of the corresponding catechin. Orthoquinones exhibit the following properties:
• A good part of the orthoquinones formed react with proteins & phenols and carbohydrates, due to non-specific oxidation reactions producing products which are complex in nature. (Little is known about the protein & phenol complexes.)
• Orthoquinones also oxidise carbohydrates, amino acids, carotenes, fatty acids, gallic acid & may be some other flavor precursors.
• Made tea will not have any orthoquinones as such, due to its high reactivity.

[3] Theaflavins (TF)

Orthoquinones of catechin (or their gallate) & orthoquinone of gallocatechin (or their gallate) combine to form the following three different theaflavins:
• Theaflavin (TF)
• Theaflavin monogallate (TF monogallate)
• Theaflavin digallate (TF digallate )
(In practice, the term "TF" embraces all three of these compounds.) TF monogallate is 2.22 times more astringent than Theaflavin, & TF digallate is 6.4 times more astringent than Theaflavin.

Properties of Theaflavins

[a] Theaflavins are bright and golden yellow (to yellow/brown) in colour. Hence they impart a bright golden colour to the infused leaf (because some TF gets attached to the tea leaf).
[b] Taste receptors of our mouth are proteins in nature & TF in tea liquor combines with the proteins of the taste receptors to give the astringent taste feeling that we get while tasting the tea.
[c] They are astringent but less astringent than polyphenols. So they impart astringency to the liquor.
[d] TF is highly reactive and joins with many proteins.
• It joins with the protein of many enzymes. Hence many enzymes like PPO, chlorophyllase, etc, get inactivated by TF.
• When milk is added to the tea liquor, TF combines with the proteins of the milk & gives the golden colour.
• TF joins with the proteins of the bacteria and fungi. Hence the fermenting tea having bacteria etc. will yield a dull infused leaf and tea liquor, because of lack of theaflavins as theaflavins joined with the proteins of bacteria etc. All live microrganisms get killed in the firing operation.
[e] TF is taken as an indicator of quality for black teas. The more the TF, the better the quality of a black tea. And hence we producers try to control the environment in our factories during processing, so that TF formation is maximised.
[f] TF formation is more at 15°C & at lower pH (4.5 to 4.8).
[g] TF are neutral (pH 7).

[4] Thearubigens (TR)

The compounds known as thearubigens are not as well-defined as theaflavins. The term "thearubigen" embraces all the specific (TR-1, TR-2, TR-3) and nonspecific thearubigens which are formed as follows.
[a] TR-1, TR- 2, TR-3: These thearubigens are formed from catechins and polyphenol oxidase. Polyphenol oxidase uses catechins and nothing but catechins, and forms specific types of TR by polymerisation. These TRs are classified based on their ability to get extracted into organic solvents like Ethyl acetate (TR-1), Butanol (TR-1 and TR-2) and that which stays in the aqueous phase (TR-3). The higher the degree of polymerisation, the higher the molecular weight.
[b] Thearubigens formed by "other than catechins" type of polyphenols in tea flush. Peroxidase can use almost any phenolic compound other than catechins, and -- depending on the substrate(s) used -- gives a variety of products which are red to brown in colour. Since the substrate used in this case is not a defined product, these red to brown compounds will have a very complex picture and are collectively termed as thearubigens. Not much effort has been made to identify the peroxidase products because of the different substrates involved in the tea matrix.

Properties of Thearubigens

• They are brown compounds and impart brown color to dry tea, infused leaf, and tea liquor.
• They are astringent but less astringent and give strength and body to tea liquor.
• The higher the number of polyphenols in TR (the more the polymerisation), the higher is the molecular weight and the lesser is the solubility.
• They are acidic.

[5] Theaflavic Acids

As with TF formation, the quinones of epicatechin, epicatechin gallate, & catechin react with quinone of the gallic acid to form a group of compounds known as theaflavic acids. Gallic acid quinone is not formed by the direct oxidation of the gallic acid but is formed by the oxidation of gallic acid by the quinones of the catechins. Theaflavic acids are highly reactive & are present as red crystals, in negligible quantity.

[6] Theaflagillins arise from the oxidation of gallocatechins and gallic acid.

[7] Theasinensins arise from paired condensation of two gallo catechins.

[8] Theogallilin and theaflavonins are the products of condensation of catechins with theogallinn and myricitin respectively.

Polyphenols in Brewed Black Tea

Not all the polyphenols in made tea are found in the tea brew, because (as a result of polymerisation) some of the polyphenols become insoluble.

The amount of polyphenols coming in the brew will depend upon the brewing temperature, brewing time and the amount of tea put in the brewing water.

The Role of Polyphenols in Forming Cream and Haze in Brewed Black Tea

Lot of polyphenolic changes occur during the processing of black tea, as noted earlier. Though unnoticed, similar polyphenolic changes, on a small scale at least, take place during the brewing of the tea, and also after the tea has been brewed.

The colour of the hot & concentrated black tea brew is dark brown. When cream is added to such a brew, the color changes from dark brown to a milky-red color.

Without addition of dairy cream also, the onset of cooling in a freshly-brewed cup of tea is accompanied by the production of a dark red/brown cloud whose color changes to milky red (as with the addition of cream), and so the formation of this cloud is known as creaming. The tea cream contains theaflavins and thearubigens (major components of cream), caffeine and lipids like triacontanol and spinisterol, and carbohydrates, along with traces of a number of substances such as very fine fragments of tea leaf.

Complex Formation

The onset of cream formation in tea is caused by the complexity of the black tea polyphenols. Green tea contains mainly simple polyphenols, whereas black tea contains a lot of complex polyphenols. Green tea exhibits the formation of haze, but does not "cream" to the same extent that black tea does.

Polyphenols have a strong precipitating effect on enzyme proteins, denaturing and reducing enzyme activity.

Immobilised polyphenols bind enzyme proteins reversibly, with the restoration of enzyme activity after elution. So during the brewing process, some of the inactivated enzymes become activated, and this is quite an interesting subject for research. The difference in the successive brewing of Darjeeling teas (which are always oolong teas since not fully oxidised) and of oolong teas may offer a clue for the better understanding of the properties of successive brews of such teas.

Secrets of the Oolong Process

Polyphenols in Oolong Teas

The oxidation of polyphenols in oolong tea manufacture is partial. So oolong teas will have the polyphenols of both green tea and black tea. Most oolong teas are made in China -- a country which likes to keep its secrets. Globalisation is leading to better science, but the science of biochemical changes occurring during oolong tea processing, and during successive brewing of the same oolong tea leaf, is yet to be fully known. And so either the biochemicals in oolong teas in China have been studied less, or the results of their studies are not fully known to us. However oolong theanins and oolong theasinensins are said to be the unique oxidation products of polyphenols, in oolong tea oxidation.

A few Darjeeling planters have started manufacturing the so-called oolong teas in Darjeeling. With the present interest of Indian tea research institutes, and with their scientific outlook, the so-called secrets of Chinese oolong tea processing are going to come out soon, resulting in the best oolongs of the world.

If we define oolong teas as intermediate in oxidation between black tea and green tea, then Darjeeling teas have all along been oolong teas, because the hard withers taken in Darjeeling only allowed partial oxidation of polyphenols. Another similarity between famous Chinese oolong teas and famous Darjeeling teas is their unique, flavorful oxidation of polyphenols.

Polyphenols and Darjeeling/Oolong Flavour

Per my experience, the partial oxidation of polyphenols in Darjeeling/oolong teas, the way this is accomplished, and their relation to flavour, is as follows:

• Less quantity of polyphenols in the tea shoot: Polyphenols in the tea brew give astringency/briskness/body and strength to the tea brew, and these characteristics tend to overlap or shadow the flavor. Hence, the lower the percentage of polyphenols in the tea shoot, the better this is for the flavour of the liquor. For this reason the C. sinensis cultivar, having lower polyphenol levels than the C. assamica cultivar, in general yields better flavour.

• Oxidation of smaller amounts of polyphenols: In oolong processing, only a limited number of cells are bruised before inactivation of the enzymes. Hence there is less oxidation of polyphenols in the course of the processing, which yields better flavor.

• Polymerisation of polyphenols: Good amounts of polyphenols are polymerized during the processing, to the extent that they become insoluble. This yields better flavor, for the same reason as detailed above.

• Less amount of cell bruising during processing: This yields better flavour, for the same reason as detailed above.

• Stresses when the leaf is alive -- both when it is as yet unplucked from the bush, and after plucking.


~~~~~~~~~~ · ~~~~~~~~~~ · ~~~~~~~~~~

Much of the information in this article, naturally, has come to me from people related to tea, and since it is not possible to thank them each individually, let me express here my sincere thanks to TEA itself, and to all related to tea. I would like to single out a few by name:

Mr Lew -- whose intelligent questions on tea have prompted me to know more about tea; a good amount of my tea knowledge has come about as a result of his intelligent queries.

Mr JK -- without his support and efforts, this article would not have seen the light of day.

Fumiko Sasaki Robinson -- who in her subtle and sure way guided me to write. Her writings on tea and tea houses have been a source of inspiration to me.

-- SMC