Harvesting and Processing Techniques on Black Tea Quality

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This entry is adapted from the peer-reviewed paper 10.3390/foods12244467

Tea (Camellia sinensis) has grown for over 300 years and is recognized worldwide as among other well-renowned crops. The quality of black tea depends on plucking (method, standard, season, and intervals), withering and rolling (time and temperature), fermentation (time, temperature, and RH), drying (temperature and method), and storage conditions, which have a high influence on the final quality of black tea. At the rolling stage, the oxidation process is initiated and ends at the early drying stage until the enzymes that transform tea polyphenols into thearubigins (TRs) and theaflavins (TFs) are denatured by heat. By increasing fermentation time, TRs increased, and TF decreased. Each is liable for black tea’s brightness, taste, and color.

black tea processing technique phytochemical profile

1. Introduction

Tea, scientifically known as (Camellia sinensis L.), is a widely consumed beverage worldwide due to its satisfactory aroma, taste, and health benefits ^[1]^[2]. Tea consumption originated in ancient China, where it was served as a beverage and medicine ^[3]. Approximately 75% of tea consumption in the world comprises black tea, which accounts for the highest production and consumption ^[4]. The ambient temperature range required for a tea plant is 18 °C to 25 °C to grow well. Shoot growth is observed to be slowed down if the temperature is under 13 °C or above 30 °C. Tea plants require at least 1200 mm of rainfall annually. The optimal soil pH range for tea plant growth is 4.5–5.6. The best soil condition for the growth of tea plants is deep, having greater than 2% organic matter, well aerated and drained ^[5]. Usually, it is made from young shoots with one unopened bud and two or three leaves. Tea has been a very trendy beverage for recreation and pleasure for centuries and is also regarded as a nutritious beverage due to its therapeutic and antioxidant properties ^[6].

The production of black tea is carried out by tea leaves experiencing several physicochemical reactions during different processing steps ^[7]. There are various kinds of teas in the market, such as yellow, red, green, black, and so on with a distinct fermentation, oxidation, and processing method ^[8]. As a fermented tea, black tea is typically produced in two ways orthodox and crush, tear, and curl (CTC). The orthodox involves four stages: withering, maceration or rolling, fermentation, and drying, whereas CTC black tea processing involves withering, maceration, cutting, tearing and curling, fermentation, and drying. The CTC process has a horizontal or vertical Rotorvane maceration machine for the maceration of tea leaves instead of rollers ^[9]. The most important of these stages is fermentation, which affects the smell, color, mouth feel, and, most importantly, the bio functions of black tea ^[10]^[11].

Black tea’s quality is widely determined by the physical and chemical procedures used in its production since tea leaves undergo many chemical transformations. Therefore, depending on the technique followed during the processing of black tea, the phytochemical profile may vary significantly. Tea quality is primarily influenced by leaf variety (Camellia sinensis L./Camellia sinensis assamica), growth conditions, plucking protocol, interval and season, manufacturing processes, infusion preparation, and ground tea leaf size. Leaf appearance briskness, aroma, color, flavor, and liquor brightness are considered for quality determination ^[12]^[13]^[14].

2. Black Tea’s Health Benefits and Chemical Composition

Tea is abundant in amino acids, caffeine, polyphenols, and other functional elements that have been connected to a variety of health benefits, including anti-inflammation, antioxidation, anti-mutagenic, anti-cancer, and enhancement of psychomotor performance ^[15]^[16]^[17]^[18]. Any changes in the aforementioned composition of phytochemicals affect the quality of black tea ^[19]. To determine tea consumption’s potential in avoiding different oxidative-stress-related chronic diseases like diabetes, Alzheimer’s disease, cardiovascular conditions, and some cancers, numerous in vitro, in vivo human, and animal intervention studies have been conducted ^[20]^[21]. Tea phytochemicals, notably polyphenols, are primarily responsible for these protective effects ^[22].

Among the many chemical substances in tea shoots, the high concentrations of polyphenols and caffeine are particularly notable. Proteins, carbohydrates, amino acids, lipids, caffeine, polyphenols, fiber, and minerals are the primary chemical components of the green leaf ^[23]. Tea leaves contain chemical constituents, including methylxanthines, vitamins, and more than 600 volatile compounds. It comprises 25 to 35% polyphenols by dry weight ^[24]. The most significant antioxidants, catechins, make up 25% of their dry weight ^[9]^[25]^[26]. Besides theanine, fresh tea leaves include methylxanthines (theophylline, caffeine, and theobromine) ^[20]^[27], and the main TFs include theaflavin, 3-gallate, 3′-gallate, and 3,3′-gallate (Figure 1) ^[28].

Figure 1. Structure of tea methylxanthines, theanine and major theaflavins.

3. Black Tea’s Manufacturing Cycle and Its Influence on Quality Characteristics

Black tea’s quality highly relies on the physicochemical processes involved in its production. Soon after tea leaves are plucked, they undergo various processing steps, including withering, rolling, fermentation, drying, sorting, and storage ^[29]^[30]^[31]. Figure 2 shows different processing steps involved in the manufacturing of black tea and factors influencing the quality of black tea during processing and storage.

Figure 2. Factors affecting black tea quality and changes that occur in chemical composition during processing.

3.1. Plucking/Harvesting/Picking

Plucking, often known as picking, is identical to harvesting for all other crops ^[31]^[32]. Black tea leaves can currently be picked using a variety of techniques. The three primary techniques for plucking tea leaves are manual plucking, knife, and machine plucking. Mostly, black tea is hand-picked. The plucking method, standard, interval, and plucking season greatly influence the quality of the final output ^[33].

3.1.1. Plucking Method

Tea leaves are manually plucked by tea pluckers ^[32]. Long-term tea producers developed a tiger’s mouth plucking technique ideal for picking green leaves. When choosing fresh leaves, the thumb and fingers are primarily separated, and the bud tip is inserted from the center ^[34], successfully avoiding crushing and scalding by holding them all in the palm. In particular, the machine and knife picking approach is unable to achieve selective picking and cannot guarantee that fresh tea leaves quality and the size of chosen leaves are essentially the same. However, the benefit is that it can increase picking productivity and lower manual picking costs, which is advantageous for low-grade, mass-produced tea ^[35]. Black tea harvesting is conducted primarily by hand and has not been mechanized. Therefore, the correct harvesting of fresh leaves must be performed manually; however, there are many obstacles, such as a lack of knowledge, improper technical practices among tea producers, and inefficiency. During crown cultivation, the new shoots of tea trees are damaged, the old and tender ones are uneven, and the leaves and branches are severely damaged and mixed up, which not only affects the growth of tea trees but also quality improvement.

Shearing reduced the crude fiber content, an unfavorable parameter whose maximum value was set at roughly 16%. Professional tasters rated hand-plucked tea higher than shear-plucked. They noted that hand-plucked tea had a better flavor and color than shear-harvested tea. After considering all factors, shear harvesting reduced production costs. Shears lengthened the interval between plucking and decreased productivity or yield.

3.1.2. Plucking Standard

Plucking standards are essential factors in determining black tea’s quality. The standard of plucking has a significant impact on tea quality and yield. Many high-quality teas require uniform and tender leaves. Plucking standards are often classified as fine, medium, or coarse, with the given percentages of 75%, 60–70%, or less than 60%, respectively (Figure 3). Plucking with 25% coarse and 75% fine leaves balances yield and quality. Coarse leaves and immature shoots are undesirable because standard flush was dependent on standard leaves ^[31]^[36]^[37].

Figure 3. Percentage of standard flush for standard leaf plucking and effect of fine and coarse plucking on black tea’s quality.

Good crush tear and curl production requires high-quality plucking, which is important not only for the tea liquor’s quality but also to keep the crush, tea, and curl machine from blunting by cutting woody material, stalk, coarse leaf, and branch. To balance black tea quality while ensuring the plucker’s productivity is not compromised, it has been suggested to pluck a bud and two leaves ^[9]^[31].

Fine plucking removes a bud and two leaves, while coarse plucking removes a bud and three to four leaves ^[38]. In all tea cultivars, a bud and two leaves’ plucking standards are considered to be an ideal compromise among plucker productivity, quality, and yield. Therefore, regardless of the manufacturing process or tea leaf variety, most tea processing industries comply with this plucking standard ^[31]^[39]. However, to achieve more outstanding biomass production in a single plucking round, some producers opt to use less delicate shoots. However, the coarse plucking standard reduces plucking frequency because new shoots need more time to mature.

Coarse plucking reduces black tea’s plain and aroma quality ^[31]. The findings were attributed to the greater levels of polyphenols present in young tea shoots ^[39], making tea beverages’ plain quality characteristics decline as leaves older. So, the catechins (flavan-3-ols) responsible for making TFs and TRs, which are black tea’s quality parameters, are reduced ^[39]. But as the leaves age, the amount of chlorophyll, which lowers black tea’s quality, increases ^[40]^[41]. Plucking standards varied widely, as according to ^[40]^[42], fine plucking exhibited massive water-soluble solids, caffeine, and TF content but low crude fiber and ash. Catechin, polyphenol oxidase activity, and TFs were maximum in a bud and two leaves. Because mature shoots have more polyphenol oxidase, coarse plucking produces high TRs and low TF.

The plucking standard also results in the variation of catechin level. When plucking was coarser, catechin levels decreased. Each plucking standard changed catechin concentration. Even though epigallocatechin gallate was the most prominent and (+)-catechin the least prominent flavanol, the decline in individual catechins varied with the plucking standard. Due to plucking standards, epigallocatechin gallate and epigallocatechin exhibited the most variation, whereas (+)-catechin exhibited the least ^[43]. Caffeine and polyphenols, which are plentiful in the bud and decrease with leaf coarseness, are the tea shoot’s primary components ^[9]^[31]. Flavon-3-ols catechins are plentiful polyphenols in young tea shoots and are essential for black tea production. The primary catechins found in tea leaves are catechin, epicatechin, epicatechin gallate, epigallocatechin, and gallocatechin ^[10]^[25].

3.1.3. Shooting Period/Plucking Interval

Plucking intervals affect black tea’s quality, green leaf standard, and chemical composition. Long intervals between plucking contribute to inferior leaf quality with low TFs and more mature leaves, the total amount of Group II volatile flavor compounds, level of caffeine, and black tea’s taster ratings. However, the total of Group I volatile flavor compounds—which give tea its poor, grassy, green flavor—increases with longer intervals between pluckings ^[44]. The decrease was primarily due to an unsaturated fatty acid increase ^[45], leading to a rise in black tea’s Group 1 VFC ^[44]. So, plucking with short intervals increases crop quality and yield. The plucking interval can affect shoot distribution, crop quality, and quantity ^[46]. A short plucking round for black tea was preferred over long intervals because it had higher caffeine, chemical aroma quality indices, brightness, TF, and sensory ratings. Plucking with long intervals produces coarser leaves and reduces tea yields as compared to short plucking intervals ^[33].

It has been reported that brisk, bright, and colored teas have been highly valued for caffeine, TF, and thearubigins content. Shorter plucking intervals resulted in more excellent caffeine and TF content in teas. With longer plucking rounds, these values declined. Insignificant changes in TRs were found. While Tasters A and B gave the teas no order, Taster C gave them a systematic drop as the plucking rounds increased. Taster C found that plucking with short intervals resulted in teas with excellent ratings ^[44].

The Group I volatile flavor compounds contribute a lower flavor to the tea’s quality; consequently, as the picking durations were reduced, the tea’s flavor enhanced. However, as plucking rounds were longer, the sum of Group II volatile flavor compounds declined. Although the VFC of Groups I and II rose and declined, respectively, with an increase in plucking length periods, Even though the plucking length rounds changed, the total volatile flavor compounds did not vary.

3.1.4. Plucking Season

The seasonal changes in temperature and weather, humidity, rainfall, and soil water deficiencies affect annual yield as well as annual yield distribution ^[47]^[48] and also black tea’s quality ^[49]. The performance of most crops, including tea, varies from season to season and from locality to locality ^[50]. These variances result from a difference in growth parameters ^[51], which causes a change in the overall quality and chemical composition of black tea ^[52]. Tea quality depends upon seasonal variations in terms of moisture content, TFs, and TRs, which also link well with tea color ^[53]. TFs and TRs, two significant polyphenols used as black tea quality indicators, make up 50–70% of the phenolic components in tea water extract ^[54]. Black tea’s quality is mostly determined by the phenolic chemicals that are found in young tea shoots ^[55]. Low-quality black teas have a low total polyphenol concentration ^[56].

It has been examined previously how harvesting seasons affected the physicochemical properties of “Yinghong 9” (Yh) and its mutant “Huangyu” (Hy) black tea with large leaves. The results showed spring through summer-processed black tea showed significant soluble sugar and caffeine increases, while free amino acids decreased significantly. This variation was noticed in both Yh and Hy ^[57], and the outcomes follow previous findings ^[58], which defend it from UV radiation across the three harvesting seasons ^[59].

Tea is cultivated from the equator to the subtropics, where seasonal changes may be extreme ^[47]^[60]. Seasonal temperatures in the Kericho District, Kenya, range from 15 to 17 °C compared to the Tea Research Foundation of Central Africa at 18–24 °C ^[61]. Early research in Central Africa found that fresh apical shoots have the highest level of flavanols in the winter season. Tea shoots harvested under conditions of slow growth, such as during the cool season, exhibited higher levels of simple catechins compared to catechin gallates, and the most significantly affected compound is epigallocatechin gallate (EGC). Contrarily, total flavanol concentration is highest during the summer (growing season) and lowest in late autumn in the northern hemisphere. Cool, slow-growing seasons result in black teas with high quality but low production ^[62]. Dry or severely cold seasons reduce yields ^[63], which is in accordance with a previous study ^[64] that declared that slow shoot growth in dry and cool seasons increases black tea quality, while fast tea flush growth in wet seasons, especially rainy periods, lowers black tea’s quality. If the soil is moist, high temperatures promote rapid shoot growth, increasing production but reducing black tea quality.

4. Withering Stage

Withering, as the initial step in manufacturing black tea, is crucial to produce high-quality black tea. Tea processing begins with withering, where freshly harvested tea leaves are spread out in a proper withering system to lose moisture before processing ^[32]. For the subsequent steps, tea leaves go through a series of physical and chemical changes ^[31]^[65]. In tea processing, leaf moisture is crucial; tea leaf withering partially dries the surface and core moisture of tea leaves ^[66]. Black tea leaves are usually withered at room temperature to reduce moisture. Tea shoots undergo withering from being harvested until they are macerated or rolled ^[31]. Turgid tea shoots become flaccid as moisture levels decrease on a wet basis from ~70–80% to 60–70% during the withering process ^[67].

Chemical and physical withering are the two main kinds of withering. During the procedure, tea shoots experience chemical and physical changes ^[31]. Immediately after harvesting tea leaves, chemical withering occurs. This process breaks down complex chemical compounds into volatile flavor compounds, simple sugar, and amino acids. The enzymatic activities and caffeine content rise with withering. Additionally, it has been found that during the process of withering, lipids degrade into simpler molecules, and the catechin amount drops ^[67]. The withering phase’s dehydration shock induces enzymatic ripening and gives the shoots a floral flavor. On the other hand, physical withering is the process of moisture removal from tea leaves and changing tea leaves’ cell membrane permeability. Both methods of withering are essential, and it is difficult to achieve ideal attributes (color, flavor, aroma, and taste) from uneven or un-withered leaves.

Another major determinant of tea quality is withering temperature. Excessive heat during withering destroyed the leaf cell matrix, resulting in earlier uncontrolled fermentation-like responses ^[40]. It has been evaluated how temperature and moisture loss affect TFs, TRs, and volatile flavor components development during withering. Results show that restricting moisture loss during the early withering stage improves black tea quality.

Some unfavorable enzymatic reactions occur at high temperatures, leading to an undesirable amount of TFs and TRs, which results in an increase or decrease in the flavor index, brightness, and black tea’s sensory quality. The two primary kinds of VFC are E-2-hexenal and hexanal (Group I) and geraniol and linalool (Group II). Group II volatile flavor compounds contribute to a good effect on aroma, and Group I volatile flavor compounds grant inferior aroma ^[56]. Withering decreases the total amount of Group I volatile flavor compounds while increasing the total of Group II volatile flavors ^[68]^[69]. The Group II VFC was raised when tea shoots were restricted in their moisture loss for a prolonged period prior to physical wither, which improves the aroma’s overall quality. However, stressed shoots have the most Group I volatile component. The flavor index needs to be optimal to improve quality. It has also been concluded that a low temperature of 22 °C increased TF, total color, and brightness ^[70]. The over-withered tea shoots harden and blunt the roller or maceration machine. As a result, tea fiber content rises, as do the possibilities of over-firing and over-fermentation. In contrast, a high content of moisture in withered leaves leads to clog crush, tea, and curl (CTC) and the Rotorvane machine during operation ^[71].

It has been examined how withering affects fermentation ^[72]. During fermentation, (PPO) polyphenol oxidase activity affects TF and TR production. TFs and TRs give tea liquors briskness, brightness, and “body”. It has also been found that more extreme withering and greater moisture loss decrease PPO activity. Therefore, black tea loses briskness and brightness. Tea leaves, which are shortly withered, result in more briskness and brightness due to a rise in TF formation. Extreme moisture loss, such as long duration or high temperature, reduces green leaf PPO activity during withering ^[26]. It has been reported optimum withering time is 14 h ^[72].

It has been shown that the duration of withering has a direct effect on volatile compounds content. The leaves following hard withering have more hexenal, linalool, and oxides, which explains why such tea fragrances more ^[73]. Withering temperature affects the brightness of black tea, and withering at low temperatures can result in brighter tea ^[70].

Allowing withering for 8 ± 0.5 h to 10 ± 0.5 h can enhance quality and nutritional value. It reveals that withering time duration is essential for biochemical properties. This stage can be controlled and monitored to produce high-quality black tea ^[29].

Good-quality black tea requires optimized chemical withering. There is no set standard for withering. It varies with leaf quality, requirements, and ambient conditions. For maceration, withered leaves should have 68–72% moisture is recommended. Tea shoots to wither properly depends on plucking, transportation, time, temperature, and environmental conditions. Thus, withering must be regulated to ensure high-quality tea ^[67].

5. Rolling/Maceration Stage

Post-withering tea leaves are macerated or rolled to rupture plant cell structures, allowing catechins, polyphenol oxidase, and peroxidase to interact during fermentation ^[74]. Rolling extracts and twists leaf juice ^[36]. The main goal of rolling is size reduction and cell destruction to expose new surfaces to air during fermentation. It also presses out juice and coats leaf particles with a thin juice coating to accelerate chemical reactions ^[75]. This process is crucial for black tea production since most TFs are generated during rolling ^[76]. During rolling, enzymes are released from the leaf when it is broken and exposed to oxidation. Crush particles (dholes) were crushed by the rotor vane machine after rolling. After crushing, the material was run through a CTC crush tear and curl machine to make particles finer. The material then experiences a roll breaker, which breaks the twisted balls and reduces fermentation ^[40]. In Sri Lankan black tea production, orthodox rolling is used to gently roll tea leaves. The rolling method produces black orthodox tea. For tea leaf pulverization in Central Africa, a Lawrie tea processor is used, which can also be used to treat them harsher ^[77], whereas Indian and Kenyan tea leaves are crushed and macerated using crush tear and curl machines ^[77]^[78], which are referred to as black crush tear and curled teas. The maceration methods that influence black tea’s chemical composition and quality were investigated by ^[79]. While rolling tea shoots, polyphenol oxidase oxidizes catechins and leads to the formation of TFs and TRs. TFs provide tea liquor’s brightness and briskness, while thearubigins contribute to taste and orange-brown color ^[80]. With an exhaust temperature below 49 °C for a long time, the post-fermentation process will soften the liquor. Exhaust temperatures above 57.2 °C accelerate moisture removal, resulting in hardened tea with hard particles but incomplete drying. Such types of tea do not store well and have harsh liquors ^[75]. Catechins and gallates oxidize to TFs and TRs. Black tea’s TFs and TRs not only rely on the green leaf’s oxidase activity, the content of flavanol and protein but also the processing method and leaf resistance to mechanical damage. This resistance affects CTC processing less than orthodox. Crush, tear, and curl and orthodox teas differ greatly in TF and TR content. TFs and TRs are higher in crush tear and curl teas compared to orthodox teas. ^[81]. These two processing methods have less effect on TRs than on TF. The impact on flavonoids is associated with the variation in the concentration of volatile compounds generated by each distinct processing technique ^[82]. This may explain why CTC tea was less aromatic than other teas, as it had less volatile compounds with floral notes like linalool and its oxides than orthodox black teas ^[83]. Rolling speeds (35, 45, 55, 60 r/min) affected gongfu black tea flavor and composition. Electronic tongue and color difference examinations showed that Congou black tea had the best taste and aroma at 45 r/min. Rolling speed affected tea pigments (TFs, TRs, and theabrownins), phenolic and organic acids, but not other metabolites. Carbohydrates, quality index TRs, and aspartic acid were found to be highest in Congou black tea prepared at 45 r/min, while theabrownins and organic and phenolic acids were lowest, indicating that this rolling speed was better for flavor formation. It also showed that rolling times affect black tea quality ^[84]. Rolling times (50 min, 75 min, 100 min, 125 min) also affected black tea’s chemical composition and sensory quality. TFs, TRs, theabrownins, and the dominant TF contents significantly differed over four rolling times of Congou black tea ^[85]. Prolonged rolling times enhanced tea liquor color and pigments, i.e., TFs, TRs, and theabrownins. The maximum umami and lowest bitter intensities and higher index of quality (10 TFs + TRs)/TBs were achieved with 100-min-rolled black tea. Those findings revealed that rolling times affected amino acid oxidation, catechin oxidative polymerization, and flavonol O-glycoside hydrolysis.

6. Fermentation/Oxidation Stage

After leaf disruption, fermentation is another crucial step in black tea production due to chemical changes ^[86]. Therefore, fermentation is the most crucial factor in the determination of the processed black tea’s quality ^[9]^[87]; oxidation of catechins during fermentation leads to the formation of TFs and TRs ^[88] liable for briskness, brightness, color, strength, and black tea liquor ^[89]. In pairings, catechins generate different TF compositions ^[9]^[68]^[90]. In black tea leaves, enzymes oxidize and partially polymerize around 75% of the catechins ^[91]. Polyphenol oxidase and peroxidase oxidized catechins ^[92]. When catechins are exposed to oxygen, these enzymes develop oxidized polyphenolic substances like TFs and TRs ^[25]^[93]^[94]. The fermentation environment’s temperatures, relative humidity, time, pH, and oxygen affect these compounds greatly. When black tea ferments properly, it turns green to coppery brown. It also smells fruity, brisk, rich in flavor, and tastes strong. Fermentation needs a set temperature, humidity, and time. During fermentation, good aeration increases TFs and TRs, whereas less aeration results in a reduction of those compounds. In the same way, high temperatures also reduce TF formation ^[32]^[95].

6.1. The Influence of Fermentation Process Parameters on Black Tea’s Quality Attributes

Postharvest techniques and raw materials determine tea liquor quality. High-quality black tea has a rich flavor, brisk, brighter reddish-brown color, and strong taste. Fermentation conditions like time, temperature, relative humidity, and oxygen affect these quality attributes ^[96]. Thus, fermentation parameters must be controlled to make high-quality black tea liquor ^[97].

6.2. Time

The fermentation period greatly affects black tea quality ^[10]. There is no set amount of time it relies on plucking standard, degree of rolling or maceration, type of tea, and degree of withering. Liquor quality attributes include brightness, astringency, strength, and briskness peak at different times. Thus, to achieve the optimal result, process parameters must be optimized ^[68]. In general, fermentation is carried out at around 20–30 °C for 30–120 min, although 25 °C for 60 min is considered the optimum ^[25]^[98].

The quality of the prepared tea depends on when the fermentation process is stopped ^[10]. As shown in (Figure 4) as fermentation time increases, TFs and TRs concentrations and desirable quality features approach optimum levels and then degrade if prolonged ^[9]^[68]^[99]. An astringent, brisk taste and golden color come from TF, while a brown-red color and rich mouth feel are contributed by TRs ^[100]. Crush tear curled and orthodox teas ferment for about 55–110 min and 2–4 h, respectively ^[101]. Fermentation time significantly affects the content and changes of tannins, TF, TF/TR characteristics, and brightness, which rely on the genetic potential of plants ^[102].

Figure 4. Variation in quality characteristics of black tea during fermentation.

TFs degrade to TRs and thicken tea liquor if fermentation is prolonged ^[103]^[104]. Despite its body, over-fermented liquid lacks quality. For optimal results, maintain the (TF:TR) 1:10 ratio to achieve the overall best result ^[9]^[104]. For fermentation at 20 °C, total TFs, total TRs, total color, brightness, and briskness peak at fermentation durations of 90 min, 120 min, 120 min, 60 min, and 60 min are in the given order ^[96]. To oxidize catechins and obtain the appropriate TF content, macerated tea leaves are fermented shorter than rolled tea leaves.

6.3. Temperature

The temperature involved in fermentation significantly affects the enzymatic activities and, consequently, the process of fermentation. A low or high fermentation temperature can inactivate enzymes and stop enzymatic processes. Enzymes are protein in nature and high temperature leads to denaturing ^[105]. Enzymes (PPO and PO) break down tea leaf chlorophyll during fermentation. Superior quality tea requires a controlled temperature during the process of fermentation. A 20–35 °C air temperature during the process of fermentation affected crush tear and curl CTC black tea quality, resulting in peak TFs, TRs, ratio, and brightness at 20 °C ^[25]. Fermentation at 25 °C for 60 min is considered best for Chinese and promising 100 cultivars ^[98]. Generally, 24–27 °C temperature is considered best for fermentation, but different kinds of teas have different optimum conditions ^[101]. On the other hand, ^[68] suggested 27–29 °C temperature and a time range of 2 h 30 min to 3 h 45 min or 55–110 min for orthodox tea or crush tear and curl black tea, respectively. PPO and PO enzymes are most active at this temperature, resulting in good-quality tea. Different clones of macerated tea leaves (dhool) fermented for 0–180 min at 15–30 °C, and fermentation at 20 °C resulted in the highest black-quality tea for all clones. Low-temperature fermentation creates good-quality black tea, while long fermentation durations and high temperatures produce intense color and high TF content ^[106]. Teas with high fermentation temperatures have higher color and TR values but lower sensory values, brightness, and TF content ^[107]. Due to its more robust taste and aroma, at 28 °C, fermented black tea has the highest sensory ratings.

6.4. Oxygen and Relative Humidity

Monitoring relative humidity RH and oxygen is crucial to making good-quality black tea. Enzymatic reactions require enough oxygen. Low oxygen levels cause leaf heat and hinder chemical oxidation, which leads to a dull liquor ^[68]. Low-oxygen and high-temperature fermentation reduces TF content and increases TR content ^[68]^[101]. Non-fermented tea contains less volatile flavor components than fermented tea. The essential oil from fermented leaves contains linalool oxides, but fresh leaf homogenates do not. Rapid polyphenol oxidation appears to prevent the production of volatile flavor compounds in tea leaves ^[108], assuming that 28 °C is ideal ^[73]. Maintaining high relative humidity (95–98% is vital during fermentation ^[9]^[68]. The ruptured leaves must be humidified to stay fresh and cool during the fermentation process in the afternoon when relative humidity is low, and temperature is high. Avoid dry air passage over the leaves, as this interferes with the oxidation rate and causes blackening ^[68]. Adjusting the fermenting tea pH from 5.5 to 4.5–4.8 decreased thearubigin levels and increased TF levels, which is possibly due to the lower turnover of produced TFs to thearubigins ^[109]. Excess dhool moisture could hamper aeration and temperature regulation, causing uneven fermentation.

7. Drying/Firing Stage

Drying stops fermentation by inhibiting enzyme activity, producing dried black tea ^[26]. Tea particle drying stops oxidation and enzymatic activity, decreases moisture to 3–4% (wb), facilitates handling and transportation, and enhances shelf life ^[110]. After thermochemical reactions at high temperatures, drying causes dehydration in tea to decrease its moisture level and enhance the taste and aroma of tea. Therefore, measuring tea’s moisture content is crucial to making high-quality tea since it affects both physical and chemical reactions in tea processing and determines its shelf life ^[40]. During the drying process, the fermented tea color turns from coppery brown to blackish brown ^[31]. Regulating drying temperature, tea moisture content, and evaporation is crucial for effective drying. Case hardening occurs when tea is dried rapidly enough at high temperatures. The surface of the tea particle dries faster than the core, retaining some moisture and affecting storage quality. However, if a too-low temperature is used for drying the fermented tea, the fermentation will continue. Stewing causes the tea not to dry correctly, affecting its liquoring qualities ^[31]. It has been reported that heat, but not enzymes, caused the chemical changes during drying. Many of these chemical changes are suitable for the quality of tea, while others are undesirable. When the enzymes (PPO and PO) are deactivated, almost all of the biochemical processes stop. At the initial stage of drying as the drying temperature rises, the enzymes get active, and the reaction proceeds faster, but as the drying temperature rises to the point where enzymes cease to function, the reactions stop. TR formation is likely to persist if the drying temperature is gradually increased. Drying at high temperatures degrades chlorophyll to pheophytin, making tea black. Polyphenols interacting with proteins generate complex compounds at higher temperatures, reducing astringency. Carbohydrates and amino acids react at high temperatures to generate flavor compounds. After a certain level of drying (dry tea), more exposure to heat will degrade quality and cause a burnt taste ^[111].

The two most common dryers used in tea factories are fluidized bed dryers (FBDs) and endless chain pressure (ECP) dryers. The process of FBD involves exposing tea leaves to inlet air that is approximately 140 °C hot. ECP usually involves drying air temperatures of about 100 °C ^[112]. Hot air drying may decrease volatile flavor elements and quality ^[113], so researchers are investigating new drying methods. Drying tea leaves at low temperatures preserves their volatile flavor compounds. Tea leaves can be dried at low temperatures using radio frequency, microwave, freeze, and vacuum ^[112]^[114]^[115].

Factors Influencing the Drying Process

Tea drying and its qualitative attributes are regulated by drying process parameters such as spread thickness, temperature, air flow rate, and drying period ^[116]. Black tea dried at 110 °C temperature and 1.5 rpm is of excellent quality. Each lot undergoes a second drying process at 80 °C of low temperature to eliminate 95–97% of the moisture, leading to products that have excellent storage and keeping quality. Since fermentation continues after drying, black tea with more than 6% moisture loses quality, as high-moisture processed tea has a short shelf life ^[116]. These findings are consistent with ^[43]. As duration and temperature increase, the biochemical composition and quality of black tea decrease. The most effective combination was found to be 100 °C for 25 min ^[113].

An increase in amino acid concentrations, loss of volatile substances, binding of tea polyphenols to other tea constituents, carboxylic acid elevation, and Maillard reactions are among the additional changes during the drying process in addition to the removal of moisture. Black tea needs high-temperature drying to develop flavor, color, and aroma. Too-wet dhool can clump and make drying difficult, especially in fluidized bed dryers ^[117]^[118]. Compared to the 96 °C dryer temperature, a high inlet drying temperature did not affect the quality of the tea. A higher inlet temperature could enhance the appearance when there are only 40% excellent leaves ^[119]. Quality may be improved by exposing tea constituents to temperatures of up to 120 °C for less than a minute. With a drying time of under 15 min, stewing was not observed ^[111].

8. Storage Method and Duration

Chemical reactions in storage swiftly remove harshness and greenness from the final tea product. Tea stays flavorful and healthy for over a year in cool conditions and away from air and moisture ^[120]. Broadly, black tea quality is the sum of all desired qualities that determine its market value. The quality of black tea is determined by briskness, flavor, aroma, color, and strength, as well as a chemical constituent concentration in the brew that affects tea quality. Catechins in black tea oxidize to TFs and TRs during processing. Tea tastes bitter because of the 1, 3, 7-methylxanthine in it, so it has been concluded that long-term improper storage of black tea significantly reduced its quality ^[121]. The storage impact on the taste quality and chemical profile of Keemun black tea after 1, 2, 3, 4, 5, 10, 17, and 20 years of storage was evaluated ^[122]. During 10-year storage, the significant polyphenols declined, although theobromine and caffeine remained stable. The astringency, umami, and bitterness intensities were inversely correlated with years of storage, but the sweetness was positively correlated. A positive correlation between fatty acid content to sweetness and storage time was found. In Northeast India, the black tea color profile changed significantly during storage. Following one month of production, some pigments were enhanced and sustained for 8 months without considerable change ^[123]. Tea may lose flavor and astringency during several months of transportation and storage, leading to unpleasant attributes. Oxygen is consumed by black tea during this period, suggesting that oxidative deterioration may grant changes in aroma quality after processing ^[124]. Compared to tea stored under normal conditions, the TF level of accelerated-storage black tea slightly dropped. The accelerated-storage black tea showed a slight rise of TRs of 13.71% in the first month and a slight reduction of TRs of 11.81% in the second month. Total color increased in accelerated storage samples (4.73) in the first month and decreased (4.17) in the second month ^[125]. It has been observed that black tea that was stored improperly for an extended period lost its qualitative properties ^[121]. However, prolonged storage, particularly in conditions where moisture and light exist, degrades the quality of the tea, which eventually results in “softness”, or lacking briskness and having a “flat” taste ^[126]. Black tea deteriorates by losing astringency and flavor and sometimes finding undesirable “taints” due to autoxidative reactions and lipid hydrolysis that reduce sugars, TF, photosynthetic pigments, amino acids, and some volatile aliphatic elements and raise volatile phenolic and non-dialyzable pigments. Heat and moisture accelerate these reactions. Even though lipid oxidation is negligible except under dry and hot conditions, it is hypothesized that oxidation of free fatty acids released during storage occurs during brewing and has a significant impact on tea liquor quality ^[127].

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Muhammad Aaqil

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Update Date: 29 Dec 2023

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