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 [
Post-withering tea leaves are macerated or rolled to rupture plant cell structures, allowing catechins, polyphenol oxidase, and peroxidase to interact during fermentation [
97]. Rolling extracts and twists leaf juice [
54]. 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 [
119]. This process is crucial for black tea production since most TFs are generated during rolling [
112]. 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 [
60]. 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 [
120], whereas Indian and Kenyan tea leaves are crushed and macerated using crush tear and curl machines [
120,
121], 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 [
113]. 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 [
122]. 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 [
119]. 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. [
123]. 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 [
124]. 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 [
125]. 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 [
126]. 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 [
68]. 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 [
131]. Therefore, fermentation is the most crucial factor in the determination of the processed black tea’s quality [
12,
132]; oxidation of catechins during fermentation leads to the formation of TFs and TRs [
133] liable for briskness, brightness, color, strength, and black tea liquor [
134]. In pairings, catechins generate different TF compositions [
12,
107,
135]. In black tea leaves, enzymes oxidize and partially polymerize around 75% of the catechins [
136]. Polyphenol oxidase and peroxidase oxidized catechins [
106]. When catechins are exposed to oxygen, these enzymes develop oxidized polyphenolic substances like TFs and TRs [
40,
137,
138]. 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 [
49,
139].
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 [
140]. Thus, fermentation parameters must be controlled to make high-quality black tea liquor [
141].
6.2. Time
The fermentation period greatly affects black tea quality [
13]. 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 [
107]. 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 [
40,
142].
The quality of the prepared tea depends on when the fermentation process is stopped [
13]. As shown in (
Figure 6) as fermentation time increases, TFs and TRs concentrations and desirable quality features approach optimum levels and then degrade if prolonged [
12,
107,
143]. An astringent, brisk taste and golden color come from TF, while a brown-red color and rich mouth feel are contributed by TRs [
144]. Crush tear curled and orthodox teas ferment for about 55–110 min and 2–4 h, respectively [
145]. Fermentation time significantly affects the content and changes of tannins, TF, TF/TR characteristics, and brightness, which rely on the genetic potential of plants [
146].
Figure 6. Variation in quality characteristics of black tea during fermentation.
TFs degrade to TRs and thicken tea liquor if fermentation is prolonged [
147,
148]. Despite its body, over-fermented liquid lacks quality. For optimal results, maintain the (TF:TR) 1:10 ratio to achieve the overall best result [
12,
148]. 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 [
140]. 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 [
153]. 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 [
40]. Fermentation at 25 °C for 60 min is considered best for Chinese and promising 100 cultivars [
142]. Generally, 24–27 °C temperature is considered best for fermentation, but different kinds of teas have different optimum conditions [
145]. On the other hand, [
107] 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 [
154]. Teas with high fermentation temperatures have higher color and TR values but lower sensory values, brightness, and TF content [
96]. 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 [
107]. Low-oxygen and high-temperature fermentation reduces TF content and increases TR content [
107,
145]. 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 [
72], assuming that 28 °C is ideal [
116]. Maintaining high relative humidity (95–98% is vital during fermentation [
12,
107]. 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 [
107]. 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 [
156]. 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 [
41]. Tea particle drying stops oxidation and enzymatic activity, decreases moisture to 3–4% (wb), facilitates handling and transportation, and enhances shelf life [
159]. 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 [
60]. During the drying process, the fermented tea color turns from coppery brown to blackish brown [
47]. 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 [
47]. 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 [
160].
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 [
161]. Hot air drying may decrease volatile flavor elements and quality [
162], 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 [
161,
163,
164].
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 [
64]. 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 [
64]. These findings are consistent with [
61]. 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 [
162].
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 [
165,
166]. 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 [
167]. 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 [
160].
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 [
174]. 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 [
175]. 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 [
176]. 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 [
177]. 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 [
178]. 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 [
179]. It has been observed that black tea that was stored improperly for an extended period lost its qualitative properties [
175]. 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 [
180]. 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 [
181].