Orthodox Black Tea Manufacturing Process

Tea Manufacturing 

Tea is a product with different colour, taste, smell as well as different shapes in visual appearance based on its type or variety, but its processing methods are almost similar with minor variations. However, the quality and the characteristics of the final product basically depend on the contents of the leaf where large number of chemical constituents is present in fresh leaf. The most important group of chemical compounds present in the leaf is polyphenols which undergo series of modifications during black tea manufacturing process to impart the specific colour and flavour of tea brew. In addition, caffeine, amino acids and carbohydrates are also include in the tea leaf, but all the these chemical compounds are stored separately in different specific cells or within the leaf without mix-up to prevent chemical reactions (Roberts, 2008) such as generation of dimeric or polymeric phenolic compounds by polyphenols in the presence of polyphenolic oxidase and oxygen. Tea will deteriorate in no time by fungus attacks, if the moisture and temperature is not carefully controlled during the manufacturing process as well as delivery to the end consumer.  

Tea Plucking

First process step of the tea manufacturing is tea plucking, where tea leaves and flushes including terminal bud with two top young leaves are picked from tea (camellia sinensis) plantations and it was transported to manufacturing facilities in ventilated trucks under loosely packed conditions (Sen et al., 1983). The younger tea leaves contain higher concentrations of relevant chemical constituents which is ideal for the production while coarse leaves contain lower amounts where it is very important to have tender two leaves and the bud for production to manufacture good quality black tea (Roberts, 2008). The plucked tea leaves are subjected to leaf count at the receiving where B – 60 or "Randhalu" method is applied and if the receiving green leaves had more than 75 – 80% of tender shoots with two young leaves and bud will ensure the better quality of the final product (Wijerathne, 2008). The plucking is manual and it was carried out by hand which is a labour intensive process where picking is performed by pulling the flush with a snap of forearm, arm or even by using shoulders while gasping the tea shoot using the thumb and forefinger with middle finger or combination (Sen et al, 1983).

Withering

The first stage of black tea manufacturing is withering, which refers to the changes in green tea leaf from the time it is detached from the plant to the time of maceration (Owuor and Orchard, 1989), while chemical withering involves biochemical changes, which solely depend on time (Das, 2006). Tea leaves were subjected to weighing and scrutinization for quality and moisture on the arrival and taken into withering section of the factory which is normally built on top floors of the tea manufacturing facilities. The leaves were loosely stacked in withering troughs for controlled withering with free flow of air as well as electric fans which can provide heated air flows under controlled conditions where frequent turning and mixing with supervision are mandatory. Normally withering requires 16 to 18 hours depending on the weather condition and the moisture content of the tea leaves (Samaraweera and Mohamed, 2008). Tea leaves are subjected to wilting just after picking from the plant with gradual onset of oxidation where withering is applied to remove excess water from the tea leaves while minimizing the enzymatic oxidation within the leaf (Guang, 2007) which was carried out on the withering troughs in orthodox manufacturing model and it was considered as physical wither. On the other hand, minimum of six hours is required for the desirable chemical changes (Samaraweera and Mohamed, 2008) which have to be minimized or delayed till the leaf is ready for the rest of the processing with exposure to oxygen.  The leaves are subjected to reduce more than quarter of its weight in certain processes which very important to catalyze the breakdown of leaf proteins into free amino acids while increasing the amount of freely available caffeine, that change the flavour of tea (Roberts, 1958). This process was basically considered as chemical wither which starts with the breakdown of larger molecules in to smaller molecules while result in increase of amino acids, flavour compounds, caffeine as well as increase in cell wall permeability for rest of the chemical withering (Roberts, 2008). During this process, approximately 63% moisture is extracted from the leaves, making them soft and pliable for further processing (http://www.tea.lk). Thus it was necessary for the operator to have control of all those important chemical and physical changes, in order to have effective control of quality of the final product (Samaraweera and Mohamed, 2008).

Disruption/Rolling

Most of the Sri Lankan tea factories manufacture orthodox tea because country is renowned for these teas while both producer and the buyer prefer to continue orthodox character of Ceylon teas which was basically carried out using orthodox or rotorvane orthodox rollers (Samaraweera and Mohamed, 2008).  The disruption which is also called as leaf maceration by westerners is carryout to bruise or torn the tea leaves for the promotion of quick oxidation (Guang, 2007) which is very important in tea manufacturing process and it was carried out using manual means or mechanical means based on the requirements. The light bruising is achieved manually while mechanical methods such as kneading, rolling, tearing and crushing are used for more extensive disruption (Varnam et al., 1994) which breakdown the structures inside and outside of the leaf cells allowing to blend oxidative enzymes with various substrates to begin oxidation (Chen and Lin, 2008). The leaf maceration releases some of the leaf juices inside the cells which may aid in oxidation while changing the taste profile of the tea. The rolling process is used to form damp tea leaves into wrinkled strips where tea leaf is wrapped around itself. On the other hand, rolling action further causes some of sap comes out with essential oils and juices inside the leaves to ooze out where taste of the tea is further enhanced (Guang, 2007). In orthodox method, various types of rollers are used to macerate leaf where first roll is mostly used as preconditioning roll with very gentle pressure which promotes distribution of leaf juice on to the surface of twisted particles and it dry up on the surface to enhance blackness of final product. The subsequent rolling program is planned to achieve through breakdown of leaf cells (Roberts, 2008)

The orthodox rolling process is complex and the leaves are rolled by applying mechanical pressure to break up the cells and extract the cell sap at the orthodox roller with approximately around 20 minutes, the macerated  leaves; still damp from the sap are sieved on roll breakers to separate the finer leaves which is called the first dhool. The rolled tea leaf is sieved due to further breaking up of twisted leaves will disappear its twisted-up appearance. Thus prompt separation of fine twisted particles from larger sized bulk is required while controlling of the initiated chemical reactions due to cell breakage is a necessity where roll braking is applied to effectively separate them. The extracted uniformly fine particles are called dhools (Samaraweera and Mohamed, 2008).  These are spread out or stacked on racks immediately for fermentation, while the remaining coarse leaves are rolled for a further 20 minutes under higher pressure. The rolling and roll breaking process is continued up to 3rd dhool or 4th dhool and rest of the coarse leaves which are not pass through the last sieves called the big bulk. As a matter of fact, short rolling time produces larger leaf grades, while longer rolling produce smaller leaf grades. During and after the rolling process, the cell sap runs out and reacts with oxygen, thus triggering the fermentation process while the essential oils responsible for the aroma are released (http://www.tea.lk).

Considering the orthodox black tea fermentation, during the rolling process, cell breakages causes the mixing of enzymes with other chemical compounds while polymerization primarily starts and continues through rolling and roll braking processes. Further, the fermentation is allowed to polymerize on racks or on floor such time as desirable. The completeness of the polymerizing reaction is best judged by the characteristics of the final product. The most important polymerization reaction that is responsible for liquor character is the oxidation of polyphenols by the enzyme polyphenol oxidase and this reaction is highly temperature dependent (Samaraweera and Mohamed, 2008). The optimum temperature range for this reaction is 25 - 30°C (Mohamed and Dahanayake, 2003).

Oxidation/Fermentation

The oxidation of tea represents a series of complex chemical reactions which begins just after the cell maceration in orthodox roller where mixing up of enzymes with other chemical compounds within the cell generates number of reactions (Roberts, 2008). Certain teas require oxidation while some of them need less oxidation due to the specific production methods, however orthodox black tea require 100% oxidation to convert phenolic compounds in to polyphenolic compounds. The oxidation process in tea manufacturing is also referred as fermentation where chlorophyll pigments in the tea leaves are enzymatically broken down while releasing the tannins or transforming into other compounds. The amount of fermentation required for a given tea sample is decided by the process owner based on the finish product quality required and the weather conditions because oxidation is the most important factor in the development of aroma compounds, which gives the resulting tea’s liquor colour, strength and briskness (Roberts, 1958).

Due to the maceration of tea leaves cells and the subcellular compartments disrupted where cytoplasmic polyphenol oxidase (PPO) allows oxidizing the flavan-3-ols in the vacuoles. Thus majority of the monomeric flavan-3-ols are oxidized and polymerized to form thearubigins (TRs) and theaflavins (TFs) (Bailey et al., 1992; Bailey et al., 1993). Nevertheless, the first step of the process is oxidation of polyphenol in the presence of polyphenolic oxidase where polyphenol is converted to very unstable oxidized polyphenol which further react with polyphenols to create more stable polyphenolic compounds using polymerization.

There are two type of polymers produced due to these reactions; the orange red compounds are called Theaflavins (TF) which is generated by dimerization while dark brown compounds or the Thearubigins (TR) are generated due to polymerization of three or more compounds. The percentage formation of TR and TF is related to the fermentation time and the temperature of fermenting dhool where high temperatures will lead to rapid formation of TR groups rather than TF groups which will impart a dull colour to the liquor. Thus control of temperature is of a paramount importance. There are other reactions such as oxidation of carotene and amino acids with oxidized polyphenols which contribute to the flavour of the made tea (Roberts, 2008). Thus optimum temperature range for the reaction is 25 – 30⁰C whereas the completeness of fermentation reaction is best judged by the characteristics of final product (Samaraweera and Mohamed, 2008) which intern find the appropriate price in the tea auction.     

  

Firing

The next step of orthodox tea processing was drying which is carried out to terminate biological reactions by heat denaturation of enzymes while reducing the moisture content to increase the shelf life of orthodox black tea and to enhance the chemical reactions that are responsible for the character and flavour of orthodox black tea (Mauskar, 2007). On the other hand, firing further influence balancing of flavour of the tea, because firing eliminates some of the less desirable low boiling point compound such as volatile constituents are removed while retaining more useful high boiling point compounds (Roberts, 2008). The polyphenolic oxidase enzyme will convert catechins into theaflavins till it reaches the temperature 55⁰C where enzymatic reactions will not be arrested as soon as the fermented tea reached the dryers. Actually fermentation is accelerated at the beginning of the drying where 10 to 15 percent of theaflavin content in the black tea is formed during first 10 minutes of drying (Mauskar, 2007).

In addition, drying process reduces the most of the moisture percentage in the wet dhools which comes out of the dryer mouth reaching after 3% (Samaraweera and Mohamed, 2008). Nevertheless, the colour of the wet dhools changed to black from green due to the transformation of chlorophyll into pheophytin, which imparts the desired black colour of the orthodox black tea while reducing the stringency of black tea due to combination of polyphenols with tea leaf proteins at drying because of elevated temperatures. On the other hand, volatile compounds are reduced due to drying; however, some of the aromatic compounds are formed. The fermented dhools are dried through hot air circulation where it needs around 20 – 30 minute based on the specific manufacturer in conventional drying units while fluid bed dryers are operated around 20 minute with 125⁰C (Mauskar, 2007). 

Grading

The conventionally dried teas are cooled before issued for grading while fluidized bed dryers release tea after cooling which can be directly used for sifting operations.  The sifting is carried out by sorting the leaf particles into different sizes defending on the market demand as well as buyer requirements according to their popular blended brands. The primary objective of the sorting is to enhance the value while imparting the quality. The process of sorting enhances the appearance of the liquor quality while removing the fiber or flakes of coarse leaf particles. Thus sorting is carried out in four stages which are cleaning of fiber, grading, winnowing and colour sorting (Samaraweera and Mohamed, 2008).

Nevertheless, orthodox black tea generally has four scales for quality where whole leaf tea is considered highest quality followed by broken leaves, fannings and dusts. Hence, whole leaf tea is produced without alteration to the tea leaf or with very little alterations where it will end up in finish product with coarser texture while bagged tea has more smooth appearance. However, whole leaf tea is as the most valuable, especially with high content leaf tips. On the contrary, broken leaves are considered as medium grade tea which is basically sold as loose tea while smaller broken grades may be used for tea bags. The next grade fannings are considered as leftover products of larger tea grades which is smaller in size and it is also suitable for the tea bags. Dust is the finest particles of leftover products of all other three types which are also good for tea bags that can be utilized for very fast harsh brews, as the greater surface area of the many particles allows for a complete diffusion of the tea into the water. Fannings and dusts have a darker colour, stronger flavor when brewed but lack sweetness (Embole, 2011).

Bulking

The bulking of made tea is basically carryout to even the latter dhool particles as well as early dhool particles which is very important to eliminate day-to-day variations in the produce and to increase the quantity of a single grade. Most of the orthodox manufacturers use manual methods due to the small quantities manufactured due to large array of grades (Samaraweera and Mohamed, 2008).

Packing

Tea is packed in paper sacks in current manufacturing facilities, however it was earlier packed in plywood box few decades ago and it was the most popular method at that time. Tea may be consumed after many months of preparation where it needs preservation techniques to improve the keeping quality while preserving its desirable characters which will be deteriorate due to the absorption of moisture where packing needs special attention to resistance against moisture absorption (Samaraweera and Mohamed, 2008).  

Tea Brewing

Tea has to be brewed to get the liquor out where about one teaspoon or 2.25 grams of orthodox black tea is used per 180ml of water in 6 ounce cup where it must be steeped in freshly boiled water unlike green teas which turn bitter when higher temperatures were used. On the other hand, whole leaf black teas need to be steeped around 4 to 5 minutes, but broken leaf grades require less brewing times because they have more surface area than whole leaf grades. In addition, Darjeeling tea requires 3 to 4 minutes of steeping to have a better brew (Embole, 2011).