Black

Black-tea manufacture is carried out in several stages known as withering, rolling, fermenting, firing, and sorting. An understanding of the changes that take place during these processes has been gradually accumulated since the 1930s.

Withering

Plucked leaf arrives at the estate factory within a few hours of its harvest. It is handled carefully to prevent premature bruising and to promote dissipation of heat generated during continued respiration. Moisture level is reduced from 75-80% to 55-65% without heat buildup, usually in troughs designed for effective airflow through leaf beds up to 30 cm thick. Moisture reduction converts the turgid leaf to a flaccid material that is easily handled without excessive fracture. The process takes place over a period of 6 to 18 h, depending on factory equipment and weather conditions. Important chemical changes occur during withering: cell membrane permeability is increased, which allows for more efficient disruption of cell structure during the next stage of manufacture; amino acid and organic acid levels increase; and polyphenol oxidase activity is enhanced. These changes have been referred to as chemical wither. Degree of physical wither (moisture loss) is estimated by leaf texture or by checking the weight loss of an isolated leaf batch. The withering process affects final product value (51).

Rolling

Rolling serves to establish proper conditions for the en-zymic oxidation of the flavanols by atmospheric oxygen. This is accomplished by disrupting cell structure to effect enzyme-substrate contact. The leaf mass must also be maintained in a physical state conducive to oxygen diffusion. Rolling processes are designed to meet these requirements. The conventional roller consists of a circular platform, 1 to 1.3 m in diameter, equipped with battens. Above this, a smaller circular sleeve moves eccentrically over the surface of the platform. Withered leaf is charged into the sleeve and a pressure cap is lowered into it. The rates of movement and the pressure applied are chosen in accordance with leaf properties and atmospheric conditions. Intermittently, sufficiently rolled leaf (dhool) is removed by sieving and the remainder is rerolled. Leaf juices become well distributed on the surface of the cut, twisted leaf. Teas manufactured by this process are known as orthodox. They include almost all Ceylon teas.

More modern equipment has come into use in most of the other tea-growing areas. The McTear Rotorvane consists of a cylinder 20 or 37.5 cm in diameter, lined with vanes, through which a vaned rotorshaft propels the withered leaf. Constriction at the end applies back pressure and increases the work done on the leaf. Cutter blades are sometimes added to the shaft. From the Rotorvane, tea is usually passed through a CTC (crush, tear, curl) machine, which consists of a pair of ridged cylindrical rollers revolving at different speeds. There is a narrow, adjustable clearance between the rollers. The Rotorvane-CTC combination provides efficient and uniform maceration and is well suited to the establishment of a continuous manufacturing process. The macerated leaf is fluffed up by passage through a high-speed Rotorvane, with no back pressure, to eliminate matting and facilitate air diffusion. It is desirable to maintain leaf temperature below 35°C during maceration to prevent flavor deterioration. The Legg Cutter, originally designed for processing tobacco, is used in India to process unwithered or lightly withered leaf. The Lawrie Tea Processor (LTP) is essentially a hammer mill. It is used in Africa for lightly withered leaf.

The final physical state of the leaf is dependent on the maceration conditions employed and affects the course of the oxidation reactions. This, in turn, affects the chemical composition and some aspects of the quality of the final product, especially the theaflavin-to-thearubigin ratio (52).

Fermentation

The term fermentation arose from the erroneous concept of black-tea production as a microbial process (53). Around

1901 the conversion of green tea to black tea became recognized as an oxidative process initiated by tea enzymes (54). The process actually starts at the onset of maceration and is allowed to continue under ambient conditions in most instances, but leaf temperatures are preferably controlled so as not to exceed 25 to 30°C. Lower temperatures (15-25°C) improve flavor (34). Temperature control and air diffusion are facilitated by spreading the macerated leaf in layers 5 to 8 cm thick on racked trays in a fermentation room maintained at high humidity and at the lowest feasible temperature. Depending on the nature of the leaf, maceration technique, ambient temperature, and the style of tea desired, the fermentation time ranges from 45 min to 3 h, although the shorter process times are more common. Completion of oxidation is usually judged by the development of the characteristic coppery color and the fermented tea aroma. More highly controlled fermentation systems have been described. They depend on the time-controlled conveyance of rolled leaf on mesh belts through which there is forced air circulation. In a few factories the fermentation belt is part of a closed system that allows for control of temperature, aeration, and humidity (55).

Firing

Fermentation is terminated by firing in large ovens with hot forced-air circulation. In most driers, loaded trays traverse the drying zone countercurrent to the airflow. Incoming air temperature is ca 90°C and emergent air is ca 50°C. Time-temperature profiles are controlled to effect drying in 18 to 20 min with little energy wastage. Fluid-bed driers are sometimes utilized (56). During the firing process leaf moisture is reduced to 2 to 3%. Proper drying control is crucial to product quality. Many organochemical processes are accelerated during this period, as are the enzymic reactions before total thermal inactivation occurs. Incomplete enzyme inactivation can cause accelerated deterioration during storage. One noticeable effect of firing is the color change brought about by the conversion of chlorophyll to pheophytins and pheophorbide, which are dark brown and black, respectively. Small amounts of caffeine are lost through sublimation. A high proportion of the aroma of black tea is generated during the firing process.

Grading and Packaging

Fired tea is graded by the use of a series of oscillating screens. The grades most frequently produced, in descending particle size order, are orange pekoe (OP), pekoe (P), broken orange pekoe (BOP), broken orange pekoe fannings (BOPF), fannings, and dust. Fine dust, debris, and some fiber are removed by winnowing. Stalky materials tend to retain moisture and may be removed from the dryer tea fractions by use of an electrostatic sorter. It is essential to protect tea from heat, moisture, and light to insure reasonable shelf life. Color, taste, and aroma deteriorate if poor storage conditions prevail. Theaflavin decrease is marked (57). Aroma quality also deteriorates. Multilay-ered natural Kraft bags lined with aluminum foil have come into general use in shipment.

Blending

Most branded black tea is blended by the packer. This is essential to insure constancy of taste, appearance, and, to the greatest extent possible, cost. Seasonal, climatic, and market changes would make a constant product impossible to achieve unless the tea blender had access to many different teas for use in a single blend. This allows for necessary substitutions without departure from product image. The special talents and experience of the tea taster are still required. Instrumental analysis is not yet adequate to define and control all of the attributes important to the consumer.

Aroma Generation

Black-tea aroma is a complex system in which well over 500 components have been identified (58). The continuing improvement of analytical techniques results in the proliferation of known tea volatiles, for example, one research paper reported the identification of 133 new components, all of which are present in quantities <10 /ig/kg of beverage (59). Aroma ingredients may originate either as components of the fresh leaf, as products of enzymic or organ-ochemical activity during the manufacturing processes, or as products of the high-temperature firing step. Almost all aroma components develop during postharvest processes. It is probable that only a few dozen are significant to tea aroma quality.

Aldehydes are generated in fresh leaf by the oxidative breakdown of unsaturated fatty acids catalyzed by lipoxygenases. The short-chain unsaturated aldehydes are generally deleterious to tea flavor. Leaf alcohol dehydrogenase systems catalyze their partial reduction to alcohols that exhibit a characteristic green leaf flavor not desirable at high levels (60). The more volatile of these compounds are partially lost during the firing stage. Aldehydes are also formed during black-tea manufacture by a Strecker-type synthesis resulting from the reaction of catechin quinones with amino acids. Phenylacetaldehyde is formed from phenylalanine in this manner (61).

Terpene alcohols, phenylethyl alcohol, methyl salicylate, and other important aroma components are generated by enzymic hydrolysis of their glycosides (62). Their reported presence in fresh leaf may be partially the result of initiating hydrolysis during analytical procedures. These compounds contribute significantly to the character of tea aroma. Several aromatic compounds present in fine teas are formed by the oxidation of the tea carotenoids by catechin quinones. These include cjs-jasmone, theaspi-rone, /?-ionone, and dihydroactinodiolide (61). Carotenoids are also thermally decomposed to produce these and other aroma components during firing.

Other thermal reactions are responsible for the development of a large group of cyclic aroma components. These include pyrazines, pyrroles, pyridines, and quinolines (63). Their significance is not known. Some compounds that impart the most desirable flavor characteristics to black tea are shown in Table 2. Tea tasters use the term quality to refer to tea with desirable aroma character. Black-tea aroma also contains the group of short-chain (5-7 carbon atoms) aldehydes and alcohols that arise from lipid oxi-

Table 2. Desirable Black-Tea Aroma Components

Benzyl alcohol

Dihydroactinodiolide

Geraniol cis-3-hexenyl hexanoate

/J-ionone cis-jasmone

Linalool

Linalool oxides

Methyl salicylate

Nerolidol

Phenylacetaldehyde 2-phenylethanol 1-terpineol Theaspirone dation. An abundance of these substances is undesirable. Aroma quality is determined by the relative quantities of the two groups of components (58,64).

BLACK TEA BEVERAGE Preparation and Properties

Black-tea beverage is characterized by its amber color and distinctive aroma. In the United States it is generally prepared from tea bags containing 2.27 g of tea (200 servings/ lb). Tea bag paper must be sufficiently permeable to provide rapid infusion but also retain small leaf particles. It should not impart taste to the beverage, it must be consistent in weight and thickness to insure machinability, and it must have adequate dry and wet strength. Abaca fiber obtained from a plant grown in Ecuador and the Philippines is commonly included. Tea bagging machines are able to produce up to 1,000 bags/min.

Infusing tea bags with boiling water in a 6- to 8-oz cup for 3 min provides a beverage containing 0.25 to 0.35% solids. (The use of packaged tea in the United States has declined greatly and now accounts for only approximately 3% of retail sales). Beverage concentration is highly dependent on extraction time, temperature, and water-to-tea ratio. Caffeine is extracted proportionally with the other soluble components. Excessively hard water produces a murky beverage. Black-tea beverage color is one of the useful indicators of value in that it reflects the quality of the cultivar, the success of the manufacturing procedure, and the control of storage conditions. Desirable tea beverage color requires the use of fresh leaf that contains a sufficient level of polyphenols and oxidizing enzyme held under conditions favorable to achieving adequate oxidation, and proper theaflavin-to-thearubigin ratios. Inferior aroma may result from suboptimal manufacturing or storage conditions. Brewed tea beverage is relatively unstable. Storage for more than 30 to 60 min results in noticeable flavor deterioration. Theaflavin levels decrease during long-term holding at elevated temperatures (65).

When black-tea beverages are allowed to cool, cloudiness slowly develops, and eventually a precipitate becomes visible. The tea taster refers to this as cream and considers it a normal characteristic of most acceptable teas. The com position of cream and its significance to instant tea manufacture will be discussed under that topic.

Composition

The composition of a characteristic black-tea beverage is shown in Table 3.

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