Oolong Tea Manufacturing Processes

by SANWAR M. CHANGOIWALA
Plucking Standards

Different plucking standards have been mentioned, by different sources, for oolong tea manufacture in China and Taiwan. These include:

• Three leaves & a bud
• Two leaves & a bud with bud removed
• Plucking standard coarser than 3 leaves & a bud
• The leaves must be picked older than those picked for black tea. (The practical method is to pick 4 leaves and a bud.)
• There are in Taiwan six plucking seasons -- what in India would be called "flushes." The different type of teas manufactured from each harvest are as follows:
1. Spring (Darjeeling's first flush): baozhong tea (i.e. green oolong) & green tea.
2. Summer (Darjeeling's second flush): black tea ("red" in Chinese) & oolong
3. 2nd summer (Darjeeling's rainy season): all type of teas, including oolong
4. Autumn (Darjeeling's autumnal flush): all type of teas including oolong
5. 2nd autumn: all type of teas including oolong
6. Winter (Darjeeling's winter flush): oolong & green teas

This means that oolong is produced from all the harvests in Taiwan, including the spring season (or "first flush"), when baozhong (green oolong) is made. But we were not able to ascertain how many times a bush is plucked in the course of a particular flush. Do they pluck the tea bush once only in a particular flush, or more than once? We at Gopaldhara Tea tried to discover this recently when one of our directors visited Taiwan, but were unsuccessful due to the secrecy maintained by the concerned persons. We are hopeful nonetheless that we will find out the answer to this mystery during our next visit. Furthermore, we have imported oolong tea-processing machinery from Taiwan, and are doing different tryouts ourselves in Darjeeling. Eventually we will be able to give our answers regarding plucking standards also from our own experience.

Different Manufacturing Processes

Let us have a look at the different oolong processes used in China, in Taiwan, and also by ourselves in Darjeeling, with special reference to stress (which is my pet subject on tea: I have coined some word for some of the stresses also). For the sake of clarity, the complicated series of events is categorized here according to five "processes" and their sub-processes.

A. First Process: Withering and Agitation.

This process consists of two sub-processes, namely the withering (or "wilting") and agitation of the leaf.
The physiological and biochemical processes observed in the living tissues of a green leaf before plucking -- metabolism, respiration, photosynthesis, and such -- continue after plucking, and even during the first process of sunwithering (indoor withering and agitation). But the withering process changes the pattern and rate of these processes, altering the physiological and biochechemical composition of the tea leaf, as well as its physical properties. The main changes can be listed as follows:
• The physical changes in tea leaf are: a reduction in elasticity, turgor, size, weight, & volume; and an increase in cell wall permeability.
• The microbial changes in the tea leaf are: a decrease in micro-organisms (in sunwithering), or an increase in same (in indoor withering).

Biochemical Changes Occurring During Withering

A number of significant biochemical changes occur during the withering/wilting process. These include the following:
1. New PPO is formed, particularly at the end of the stalk. The spot where a leaf-stem is detached from the bush must undergo a type of wound-healing process.
2. Breakdown of proteins (1-2% of the total green-leaf weight) into amino acids by the enzyme protease.
3. Breakdown of chlorophyll and formation of chlorophyllide by the enzyme chlorophyllase. Loss of magnesium from chlorophyllide or chlorophyll is caused by acids produced in oxidation. But magnesium is saved from the action of acids by a protective lipoprotein. This lipoprotein, in turn, gets coagulated under the action of heat in the dryer; resulting in the formation of pheophytin or pheiphorbide. After a period of oxidation, say 2.5 hours, the protective covering of lipoprotein may be partially eroded, so that there will be some conversion of chlorophyll into pheophtin, and chlorophyllide into pheophorbide, during oxidation as well. During drying, the maximum of chlorophyll and chlorophyllide is converted into pheophytin and pheophorbide.
4. Increase in caffeine. This is due to enzymic synthesis from the amino acid S- adenosyl methionine, and also to the breakdown of caffeine-containing complexes within the leaf. Caffeine is further produced by the breakdown of the ribonucleic acids. Caffeine formation increases in direct proportion to withering time.
5. Oxidation of carotenoids.
6. Increase in simple carbohydrates -- enzymatic by breakdown of complex carbohydrates. But also, a decrease in sugars due to transformation into amino acid. Again due to respiration in withering, some of the plant sugars are metabolised to organic acids.
7. Increase in soluble inorganic phosphorus.
8. Formation of volatile components.
9. Decrease in level of organic acids (enzymatic).
10. Degradation of lipids during withering. Lipids in the tea-flush occur in multiple forms, and are located at various places in the plant. Phospolipids are constituents of the cell wall (membrane). Fatty acids -- also a kind of lipid -- are located in the chloroplasts in good amount. The enzyme lipoxygenase is located in the lamellae fraction of the chloroplast; hence an increase in the permeability of the membranes (cell walls) of chloroplasts brings the corresponding fatty acids in contact with lipoxygenase. Out of an increase of around 10 times in the aroma compounds in withering, the increase of hexenol and linalool is most marked.

Use of Biochemicals in Respiration by Harvested Tea Leaves

• Approximately 3-4 % of soluble solids are used in respiration during tea manufacture.
• Free carbohydrates will be utilized first in respiration, followed by free amino acids and organic acids. Proteins will also break down into amino acids, which will also be used for respiration.
• After amino acids are used for respiration, then lipids will be used for respiration.
• There is no chance of polyphenols being used for respiration.
• The process of tea manufacture, of course, causes the breakdown of the internal membranous structure of the leaf-cells. As a result, the contents of the cell's various compartments (mitochondria, vacuole, etc) are released into free solution. The free catechins will then bind to any protein (including the proteins of enzymes), or to any other polymer (nucleic acids etc), and become insoluble in water.

Hard Withering

With respect to hard withering, the following important biochemical changes take place.
• Hard withering gives less trans-2-hexanals; more linalools, & geraniol, more 1-octen-3-ol (which has a stimulating, fruity odour) and more methyl salicylate. The gradual death of leaf cells as withering proceeds may impede the oxygen uptake by the leaf cells, and oxygen is essential for the oxidation of polyphenols and lipids etc.
• In the case of hard withering, the conversion of the fat in the protoplasm (which is 8% of the total dry weight of the leaf) to hexanals may be less (since some of the protoplasm becomes dry).
• In the case of hard withering, the conversion of hexenal (greenish odour) into hexenol (fruity odour) increases.
• The increase in linalool oxide again may be due to the death of some cells affecting oxygen uptake; and anaerobic conditions (i.e. those occurring without oxygen) again give more linalool. Further, a harder wither may cause rapid and increased hydrolysis of glycosides of flavory compounds.

Solar Withering

During this process, UV radiation stress and thirst stress (a term coined by me to mean "dehydration stress" or "moisture stress") is applied to the living tea leaves. Solar withering is a very important process in Darjeeling and oolong tea manufacture. Indeed, according to one legend, it was accidental prolonged sun-withering that led to the production of a new type of tea in China: oolong tea. The legend is as follows.
The Chinese term "oolong" (pinyin wulong) means "black dragon" or "black snake." In one legend, the owner of a tea plantation was scared away from his drying tea leaves by the appearance of a black snake; when he cautiously returned several days later, the leaves had been oxidized by the sun and gave a delightful brew. The tea was accordingly named "Oolong" in honor of the serpent.

Sunlight: Some Details & Effects

• Sunlight includes visible light, infra-red light, and ultra-violet light.
• The wave-length of visible light is in the range of 400-700 nm.
• Infra-red light is above 740 nm (i.e. with a wave-length of more than the upper limit of visible light).
• Red light is in the range of 640-740 nm.
• UV light is below 400 nm (i.e. with a wave-length of less than the lower limit of visible light).
• Phytochrome is an elongated nonglobular protein occuring in cell membranes. Phytochrome is normally found in the horizontal position, but due to the influence of red & infra-red light, it changes its position to vertical, thereby causing a gap (or pores) in the membrane. Such pores in the membrane allow the hydrolyzing enzymes to come in contact with cell wall, causing the breakdown of the cell wall.
• Sunlight destroys the chlorophyll in the plant.
• With exposure to sunlight, some existing enzymes are inactivated, while some new enzymes are synthesized.
• Irradiation at 254-290 nm will injure the proteins and nucleic acids of the protoplasm.
Bleaching of chlorophyll by sun rays
Anthocynin absorbs UV rays extensively. Since it is located in the epidermal and subepidermal cells of the tea-leaf, it protects the mesophyll cells (which contain the chloroplasts) by filtering out the excess UV rays. But this does not occur in the plucked leaf. So chlorophyll is oxidized in sun withering.

Breakdown of the Chloroplast Membrane

The chloroplast membrane is the most sensitive to heat, followed by the mitochondrial membrane. The plasma membrane is the least sensitive to heat.

Chlorophyll in Tea Leaves and its Degradation Products

• The upper surface of the green leaf has more chlorophyll than the lower surface.
• Chlorophyll is a lipid. It is greenest in color when its chemical structure is intact. Chlorophyll has a magnesium molecule, and once this magnesium molecule is taken out, the greenness will decrease.
• Known degradation products of chlorophyll are chlorophyllide, pheophytin and phephorbide. In the case of very hard withers, there is further degradation of chlorophyll (besides the formation of chlorophyllide, pheophytin, and phephorbide); not all of these degraded components/compounds of chlorophyll have been fully identified, and it is believed that some of the degraded compounds/components of chlorophyll are bitter. This may explain the bitterness found (for example) in very hard withered First Flush Darjeeling teas.

Fever Withering

• In the absence of surface moisture, the green leaf will turn red if subjected to a temperature of 110°F. This reddening indicates that oxidation has occurred.
• So the tea leaf is subjected to warm-air withering (approximately 104°F -- more than the normal temperature of the human body) for some time. For this process I have coined the term fever withering. This type of warm withering results in increased membrane and cell-wall permeability.
• The permeability of the cell wall increases when the temperature rises from 25c to 31°C. At temperatures above 31°C, the increase in permeability slows down.
• At 44°C, there will be scorching of epidermal layer of leaf; thus no photosynthesis can take place in the tea plant at this temperature.
• There is actual distortion of the cell wall if the green leaf is heated to 49°C (~ 120°F). But even apart form this, major death of tissue takes place if the green leaf is heated at 40°C (~104°F) for 6 hours.
• In a 40°C hot-air wither, there is very rapid breakdown of proteins into amino acids.
• Caffeine formation under a 38°C wither is at a lower level than caffeine formation at a 30°C wither. A reduction in caffeine is desirable, because its bitter taste tends to mask the flavour of the tea. But when caffeine is joined with theaflavins, the brewed tea will be both more brisk and less bitter.
• Chlorophyllase activity at 40°C will be more than at 30°C. It is maximal at 45°C.
• The final part of the withering to 98°F leaf temperature, since hydrolysing enzyme activity is maximum at 98°F.
• Indoor withering further increases thirst-stress in the leaf.

Leaf Agitation

When subjected to agitation, the tea-leaf undergoes wound stress. During this process, some portions of the leaf become red, due to the oxidation of polyphenols. The portions that redden are principally the leaf-edges (perhaps because they are the thinnest part of the leaf, and hence more susceptible to damage), and some portions of the stems and veins of the leaf (possibly because these are the portions of the leaf that protrude the most).

Different Methods of Giving Wound Stress

• Turning over by hand. Several pounds of green leaf may be wrapped together in a cotton blanket and then hand-pressed in such a way that the leaves are not torn.
• Leaf-agitation can be done in machines also.

During this first process, it is clear that the tea shoots have been subjected to various stresses (UV light, thirst, fever, wound, etc) for a number of hours, when the leaf is still alive and its cells are functioning. Although these stresses may not be applied to all the cells of any leaf during processing, they tend to be transferred from cell to cell. All of the reactions taken together in a functioning cell result in the formation of such metabolites as lead to a state of equilibrium in the cell, known as homeostasis.

B. Second Process: Parching, Fixing, De-enzyming, Roasting

After the first process -- that is, of withering and agitation -- the withered leaves are fired either by hand in a pan or in a mechanical roaster at around 300°C for some few minutes.

• The leaf dies after it achieves a certain temperature during the panning process. Only enzymatic and chemical reactions take place after the leaf is dead.
• Although the pan temperature is 300°C or higher, the tea-leaf temperature is lower than 100°C, because of the moisture present in the leaf.
• During this process, most of the enzymes are inactivated, but it is a fact that some flavour forming enzymes are not deactivated. Not much oxidation of polyphenols takes place after this process.

C. Third Process: Rolling

• During this process, the final shape of the "made" tea is given to the leaf. The rolling can be a single rolling, or the leaf may be rolled a number of times. The rolling can be done with leaf inside a cloth, or the leaf can be rolled without being inside a cloth.
• In one type of oolong tea manufacture (Bai Hao or "Oriental Beauty," a Taiwan tea), the leaf is rolled a number of times inside a cloth. After every successive rolling, the tea-leaf is panned and water is added to it in preparation for the next rolling. That is, moisture is being added so that the flavor forming enzymes will continue to be active, as well as to induce proper rolling. After each rolling, the leaf may be spread on the floor so that the desired enzymatic and chemical reactions will continue.

D. Fourth Process: Drying & Firing

Here the rolled leaf is fired at temperatures around 100°C for a particular time, so as to produce the desired moisture level, and to inactivate the leftover enzymes fully.

E. Fifth Process: Sorting and Grading

The tea is sorted, graded, and packed. Some teas are fired again before packing, so as to have the desired characteristics of that tea.