Rough processing of black tea – rolling and twisting of tea leaves

The so-called kneading refers to the use of mechanical force to knead, squeeze, shear, or roll withered leaves into the required strip shape for Gongfu black tea, or to knead and cut them into the required particle shape for red broken tea. Fresh leaves are hard and brittle due to their physical properties, and it is difficult to shape them directly by rolling without withering. The rolling (cutting) process is the result of mechanical force, and if not properly controlled, it cannot shape the withered leaves into shape. Below is a brief introduction to the influence of rolling on the formation of the shape and quality of black tea.

The quality of rolling first depends on the physical properties of the leaves, including softness, toughness, plasticity, viscosity, etc. Rubbing force is applied to the leaves to shape them into shape, which requires good softness of withered leaves and easy deformation under stress; Secondly, it is required that withered leaves have good toughness and can deform under stress without breaking; The third requirement is that withered leaves have good plasticity and are not easily restored to their original shape after deformation under stress. In addition, if the rolled leaves have good viscosity, they can enhance plasticity.

 

Rolling and physical properties of leaves

There is a curvilinear relationship between the moisture content of withered leaves and their physical properties. Fresh leaves have high moisture content, causing cell swelling, brittle and hard leaf texture, and poor physical properties such as softness, toughness, plasticity, and viscosity. As the evaporation of fresh leaf water decreases during withering, these physical properties gradually become better.

When the moisture content of withered leaves drops to around 50%, the physical properties of the leaves are at their best. If the moisture content of withered leaves continues to decrease, the physical properties of the leaves will also decline accordingly. However, due to the uneven process of leaf dehydration during withering, the stem has a higher water content than the leaves, while the leaf tips and edges have a lower water content than the base of the leaves.

Therefore, in actual production, the mastery of the moisture content standard for withered leaves is higher than 50%, and generally around 60% is appropriate. Therefore, the withering process is known as “withering of old leaves”, where “tender” refers to controlling the moisture content of old leaves to be slightly higher than that of tender leaves during withering, in order to facilitate rolling and shaping.

There is also a certain correlation between the leaf temperature during rolling and the physical properties of the leaves. When the leaf temperature is high, the molecular structure of the substances inside is relaxed, and the softness, toughness, and plasticity of the leaves are enhanced. Especially for old leaves, which have a high content of cellulose and poor softness and plasticity, the leaf temperature is moderately higher during rolling, which has a significant effect on improving the physical properties of old leaves.

The process of rolling leaves into strips

Rubbing and twisting leaf clusters move uniformly in a flat circular motion in a kneading bucket. Under the combined action of the kneading bucket, pressing cover, kneading disc, ribs, and the multi-directional force of the leaf cluster itself, the leaves inside the leaf cluster are compressed from all sides, causing them to rub and knead along their respective main veins into tight, round, and smooth strips. At the same time, the leaf cell tissue is rubbed and crushed, increasing the softness and plasticity of the leaves. Simultaneously squeeze and mix the tea juice to increase the stickiness of the leaves. All of these have created more favorable conditions for the formation of leaves into strips. The more wrinkles and patterns on each leaf, the more likely it is to be rolled into tight strips.

In the first stage of black tea rolling, the leaf clusters need to obtain pressure, but the pressure should not be too high. Due to excessive pressure, the leaves are folded under unilateral vertical pressure, and leaves with poor toughness are prone to breaking into fragments at the folds. It is very difficult to curl folded or broken leaves into strips. Therefore, during the initial stage of rolling, it is important to master light pressure. As the rolling process progresses, the wrinkles and patterns of the rolled leaves gradually increase, the softness, plasticity, and viscosity increase, and the volume decreases. At this point, gradually increasing the pressure, on the one hand, causes more wrinkles and patterns on the leaves, forming thicker stripes; On the other hand, increasing the friction between leaves results in different frictional forces acting on different parts of the leaves and varying speeds of movement, leading to the generation of torque. As a result, the thick strip gradually twists into a tight strip through the action of torque.

Due to the softness and high viscosity of tender leaves, they may not need to go through too many processes to form wrinkles and can be directly twisted into tight strips. The tighter the rope, the greater the viscosity, the greater the friction, and the greater the torque generated. If the pressure is continued to knead and twist, the strands of the leaves may be crushed by compression. At this point, rolling and twisting should be stopped, and the tightly woven leaves should be separated using the method of splitting and sieving. For older leaves with still coarse and loose cords, a second round of rolling and twisting can be carried out, with increased pressure to adapt to the more elastic older leaves, further forming wrinkles, deformation, and twisting into tight strips.

During the rolling process, leaves with good softness and high viscosity are prone to sticking together and gradually rolling into clumps, which become tighter and tighter under pressure. These clumps are not easily evaporated during drying, and are prone to mold and deterioration during storage, affecting the quality of the entire batch of tea. If the clumps are dissolved again during drying, it will make the tightly kneaded strands become coarse and loose or not in a strip shape, affecting the appearance of the tea leaves. Therefore, in the process of rolling and twisting, a combination of pressure and loose pressure should be adopted, that is, after a few minutes of pressure, if lumps may form, the pressure should be removed in a timely manner to dissolve the loose lumps under the impact of the rolling bucket movement. After a few minutes of loose pressure, if the loose pressure measures still cannot completely dissolve the lumps, sometimes it is necessary to combine screening with rolling for a certain period of time to dissolve the lumps.

Technical requirements for rolling and twisting

The formation of twisted leaf strands is mainly the result of the combined action of pressure and friction forces. Friction forces cause the leaves to roll along the main vein into an elliptical spiral shape, while pressure can increase friction forces and accelerate the process of tightening the leaves into strips. The intensity of pressure, the duration and timing of force application, and the frequency of application are all interrelated and interdependent, and should be determined based on the quality, quantity, and rolling machine of the leaves.

1. Pressure technology

Pressure can vary in severity. Generally speaking, the pressure is heavy and the cables are tightly tied; The pressure is light, and the ropes are thick and loose. But the pressure is too high, and the leaves are flat and not round, with many broken pieces; The pressure is too low, the leaves are thick and loose, and even cannot achieve the purpose of kneading. The leaves are tender, and the amount of leaves should be minimal. The pressure should be light; The leaves are old, so the pressure should be heavier.

Whether under light or heavy pressure, it is related to the duration of pressure application. The pressure time is too long, and the leaves are flattened and broken; The pressurization time is too short, and the leaves are loose and thick. The pressure time for tender leaves is short, while the pressure time for old leaves is long; Less leaves result in shorter pressurization time, while more leaves result in longer pressurization time.

The duration of pressurization is negatively correlated with the number of pressurization cycles. Multiple pressurization cycles and short duration; The pressure is applied less frequently and for a longer period of time. The number of times the pressure is applied is related to the quality and quantity of leaves. If the leaf quality is low and the quantity is small, the number of pressurization times is small, and the duration of each pressurization is longer; The leaves are old in quality and abundant in quantity, with more pressurization times and shorter duration each time. The number of pressurization cycles should be at least two times for light and heavy, and at most five times for light, heavy, relatively heavy, heavy, and light.

There is a difference in pressure time between early and late. Premature pressurization results in flattened and non-circular leaves; Too late, the leaves are loose but not tight. Leaves are abundant and can be pressurized later; Leaves are old but in small quantities, it is advisable to apply pressure earlier. In short, the intensity, duration, and frequency of pressure application, as well as the timing of pressure application, should vary depending on the leaf quality and rolling time. Simply put, the pressure on tender leaves is light, infrequent, short-lived, and delayed; Lao Ye is the opposite.

2. The influence of tea rolling machine

The speed of the rolling machine should follow the principle of slow speed and slow speed. Slow down first, so as not to fold and crush the leaves, nor to generate heat due to hot rubbing or friction, causing the leaf temperature to rise too quickly. Later on, there is a higher possibility of the blade coiling into a spiral shape, which can make the blade coiling tighter. Even slower, it can loosen the clumped leaves and further knead the loose leaves into round and straight ones. The bone structure of the kneading plate is closely related to the kneading into strips. The low and wide curved ribs are suitable for kneading tender and fresh leaves, while thick and old leaves are not easy to form into strips when kneaded; The angular bone is high and narrow, suitable for kneading coarse old and fresh leaves, while kneading fine leaves is easy to crush. It is best to have a movable device for kneading the ribs of the rolling machine to adapt to the different requirements of leaf quality.

Factors affecting rolling and twisting

1. Temperature and humidity

Rolling is suitable for environments with medium temperature and high humidity. The room temperature should generally not exceed 25 ℃, and the relative humidity should be above 95%. Due to the heat generated by rolling and friction, as well as the oxidation of internal components in leaves, the temperature of rolled leaves is generally 3-9 ℃ higher than room temperature. High leaf temperature intensifies the enzymatic oxidation reaction of polyphenolic compounds, resulting in an increase in the formation of highly polymerized substances, which reduces the concentration and redness of the tea soup, weakens the taste, and darkens the bottom of the leaves. In hot summer days, measures such as ground drinks and indoor spray can be taken to reduce the temperature of the rolling workshop and increase the air humidity.

2. Leaf feeding amount

The amount of kneading should be appropriate. If too many leaves are loaded, the leaves are not easy to turn and may form flat strips, which also hinders the heat dissipation of the leaves and causes the leaf temperature to rise too quickly, affecting the quality of black tea. On the contrary, if the amount of leaves added is too small, not only will the production efficiency be low, but the rolled leaves will also stop in the kneading plate, resulting in poor flipping and inability to achieve good rolling effect.

3. Rolling time

The beginning of tea leaves rolling is the beginning of black tea fermentation. If the rolling time is too long, the enzymatic oxidation reaction of polyphenolic compounds will deepen, the retention rate of polyphenolic compounds will be low, and the content of theaflavins and thearubigins will be low, resulting in a weak taste and a lack of red color in the soup and leaves. If the rolling time is too short, firstly, the leaves are difficult to form into strips, and secondly, the rate of damage to leaf cell tissues is not high, resulting in insufficient fermentation degree, leading to a green and astringent aroma of black tea, and the bottom of the leaves turning black. To achieve good quality of black tea, the rolled leaves usually need to be separately fermented in the fermentation chamber for 1-2 hours. Therefore, while ensuring the yield of black tea strips, the fermentation time during the rolling process should be minimized as much as possible.