The principal difference between black teas and other forms of teas like green tea and oolong tea is the presence of condensed catechins, i.e. polyphenols of higher molecular weight formed through enzymatic oxidation with the help of enzyme polyphenol oxidase (PPO) and peroxidase (PO). The next process objective is, therefore, to allow intimate contact of the catechins with the respective enzymes, which oxidize these catechins in presence of oxygen. The temperature and Relative Humidity also have a role in these oxidation reactions and should be kept at a levels at which the enzyme activity is at the peak.

Process chemistry

The six catechins present in tea shoots are catechin (C), epicatechin (EC), epicatechin gallate (ECG), gallocatechin (GC), epigallocatechin (EGC), and epigallocatechingallate (EGCG). The "epi" forms have the same structural formula as the catechin, but have a different orientation when seen in three dimension models for which their chemical behaviour i.e. capacity to get oxidized is different. These catechins are oxidized by PPO, first forming intermediate compounds called orthoquinones that are very reactive and then combine in pairs to form theaflavins (TF), which are larger molecules and are unique in chemistry. The catechins react in pairs to form six theaflavins.

EGC + EC       - Theaflavin
EGCG + EC    - Theaflavin-3 monogallate
EGCG + ECG - Theaflavin 3'3' digallate
EGC + ECG    - Theaflavin -3'- monogallate
GC + EC         - Isotheaflavin
GC + C            - Neotheaflavin

From the above it can be seen that out of the pairs, at least one has to be in `epi' form to produce theaflavins.

Theaflavin monogallates are more brighter and brisker than theaflavin and theaflavin digallate is much more brighter and brisker than theaflavin monogallate. Therefore, the extent to which theaflavin digallate has been formed determines the brightness and briskness and not merely the total polyphenol content, as was earlier thought of. It must, therefore, be remembered that composition of catechins and the extent of availability of PPO in the shoots are the limiting factor for tea quality. 

Thearubigins (TR) are formed by action of PO from the same catechins, but the reaction is much more complex. TRs are also formed fromTFs. The chemistry of thearubigins is highly complex and their structures have not yet been established. When the TFs start getting oxidized to TRs, it has deleterious affect on quality.  Thus the skill in carrying out the oxidation leads to formation of a right combination of TFs and TRs to give brightness, body and colour of black tea, but unless the epigallates are present in sufficient quantities, the desired TFs cannot be formed.

Process requirements

Adequate supply of oxygen is a must to achieve oxidation. Processed leaf in absence of adequate oxygen gets heated up and the chemical oxidation is impeded leading to dull liquors. Since the oxidation process starts as soon as the catechins and enzymes come in contact with cell maceration, it is essential that the turbulence received by the rolled leaf during orthodox rolling is adequate to enable the air to mix with the rolled leaves. Prolonged period of rolling with hard pressure can, therefore, have detrimental effect on the liquors. For the same reason, the leaf should not be allowed to be held up in between the Rolls for long periods. Similarly for CTC manufacture the rotorvaned or rolled leaf should be immediately processed in the CTC machine and subjected to oxidation on the floor or continuous fermenting machine (CFM) without any delay. This has profound effect on the liquors of made tea. Therefore, leaf should not be left undisturbed in rung trolleys for longer than necessary between rolls and should be taken for oxidation promptly. Further, various liquoring qualities of tea are formed depending on to what extent the catechins have been oxidized to individual TFs, particularly the theaflavin gallates, as well as, on to what extent conversion of TFs to TRs take place. Briskness, quality, strength and colour change with time and temperature and each character is at its peak at different times.

Flavour is developed much more rapidly than other quality attributes and may slowly disappear if oxidation is unduly prolonged. Similarly, strength is a measure of the soluble matter in the liquor; TRs in particular, depend on the degree of cell rupture and expression of the juice over the leaf particles.

Method of oxidation

• Floor and Rack
• Deep Bed/Forced Air.

Floor

Oxidation on cement floor is the oldest and most popular method. This method produces the best results especially during the quality period. Spreading thickness vary depending on the method of manufacture.

Completion of oxidation is usually assessed by studying the colour change and increase in aroma/flavour. But such a test is visual and needs considerable experience. Kits are available for assessing the oxidation. However, these methods assess the total polyphenols and do not reflect the composition of individual TFs or TF : TR ratio.  It is important to note that there is not one optimum time for oxidation - it keeps on varying all the time depending on various factors.

Deep bed/forced air oxidation

Trough and most of the continuous oxidation machines fall into this category. In this method air is forced through a bed of leaf about 13-20 cm (5-8 inch) thick.

Trough

Trough is a better-controlled system because the quantity of airflow and the pressure can be adjusted. It also cut down the total surface area required. A 15 cm deep trough can hold up to 16 kg processed leaf when filled to the top. However, the troughs are not filled up to the brim to facilitate turning of the leaf, if required.

After CTC cut the leaf temperature rises because of the chemical reaction. In deep bed oxidation it is much higher, as the heat cannot escape rapidly. It is normally observed that the temperature continues to rise for the first quarter of oxidation and then falls.    

The troughs made usually of aluminium or FRP are placed on saddles to facilitate uniform distribution of air. Rubber or felt linings are used as gaskets to prevent air leakage. Two types of troughs are used : one with valve and the other with 4 nos. 10-16 mm (3/8 to 5/8 inch) holes at the bottom. Air pressure is maintained at 51 mm (2 inch) water gauge. The airflow should be between 2.5-3.7 m/min (8-12 fpm).  Troughs do not work well with under-withered leaf.

In troughs, a temperature rise even up to 43°C (110^o F) has been found not harmful if adequate quantity of humidified air is passed through and the oxidation period is cut down.

Requirement of fermenting troughs:

For 1000 kg green leaf throughput per hour, the requirements of troughs are:

   CTC                         100 troughs (or gumlas)
   3-roll Orthodox        150 troughs (or gumlas)
   2-roll Orthodox        150 troughs (or gumlas)

Continuous fermenting machine

Limitation of space, spiralling cost of labour, and the need for making tea processing a continuous one have lead to the mechanization of the oxidation system resulting in the development of the Continuous Fermenting Machine (CFM). The CFM uses the best of both floor and gumla fermentation. Introduction of CFM has been able to achieve the following objectives:

Elimination of batch process

• Allow variability in thickness of bed, period of oxidation and volume of air
• Achieving optimum ranges of temperature and humidity
• Sustain quality

Principle

The macerated leaf is dropped on to a moving perforated tray forming a moving bed. The speed of the moving bed is regulated to allow variation in the period of oxidation. The continuous feeding to and discharge from the fermenting machine have been able to eliminate the batch system.

In the CFM a higher bed thickness, as in the case of gumla fermentation, can be used. Present day CFMs are designed mostly on modular concepts, each module having its own fan, which helps in pushing air from the bottom or draw in air from the top through the bed, thus making the operation of the machine highly flexible.

The CFM can produce equally good quality tea as in floor fermentation as handling of leaf is minimized and parameters used can be brought under control.

Factors influencing oxidation

Temperature

Temperature has a greater influence on the rate of initial oxidation of catechins and their subsequent condensation. The rate of consumption of oxygen increases with temperature up to around 29°C (84°F) and is most active at temperature at 27°-29°C (80-84°F), after which the rate declines. It is therefore desirable to keep the temperature during rolling at   around 29°C.

Degree of wither

Hard wither causes slower rate of oxidation. If the wither is only moderately high, the effect may not be that detrimental. Damaged leaf leads to uneven oxidation. Such leaf should be processed separately at the earliest.

Rolling

During rolling tender leaf parts are bruised first followed by the coarser ones. Therefore, the finer particles formed from tender leaf start oxidation reaction instantly. Oxidization requirements of the fine particles are, accordingly, different from those of the coarse particles. Coarser particles are rolled for a longer period to achieve oxidation. The various fractions obtained from 2-rolled or 3-rolled systems are oxidized separately for different periods. In case of hard twisted leaf in orthodox manufacture penetration of oxygen into leaf cells becomes difficult and as such oxidation takes longer time. On the other hand, severity of leaf maceration with CTC rollers considerably reduces oxidation time.

Degree of oxidation

Leaf plucked from different sources has different capacity for oxidation. Planting materials have, therefore, been classified on the basis of their ability to oxidise into three broad categories as fast fermenters, medium fermenters and slow fermenters.

Mixed leaf may therefore lead to uneven oxidation. Planning of plucking and segregation of leaf at the withering stage, therefore, reduces heterogeneity and helps in producing better quality tea.

Period of oxidation

During oxidation, the colour of the processed leaf changes from green to coppery red and liquor characteristics develop. The important characteristics that develop are briskness, brightness, astringency and strength. These characteristics reach their optima at different times.  It is therefore apparent that a compromise has to be arrived at on the oxidation time so that the overall effect is the best.

Environment for oxidation

The room where oxidation is carried out should be cool and humid with ambient temperature maintained at 27° - 29°C (80° - 84°F). The room therefore, must have provision for availability of humidified air to keep the macerated leaf cool and fresh. The temperature should be maintained around 29° ± 2°C (84° ± 4°F (with hygrometric difference of less than 1.5°C (3°F). Adequate ventilation must be provided to effect 15 to  20 changes of air per hour.

Besides the surfaces are to be always kept clean and free from stale juice deposits. Joints between tiles, cracks, small holes and rough surfaces are sources of bacterial contamination.