During the initial production process of black tea, the product undergoes a series of complex changes, forming the unique color, aroma, taste, and shape quality characteristics of black tea.

Withering

Withering is the first process in making black tea. Under normal climatic conditions, fresh leaves are spread thin for a period of time, mainly due to water evaporation. As the withering time prolongs, the self decomposition of substances in fresh leaves gradually strengthens. With the continuous loss of fresh leaf moisture, the leaves gradually shrink, the leaf texture changes from hard to soft, the leaf color changes from fresh green to dark green, and the internal quality and aroma also change. This process is called withering.

The withering process involves both physical and chemical changes during withering. These two changes are interrelated and mutually restrictive. Physical changes can promote chemical changes, inhibit chemical changes, and even affect the products of chemical changes.

On the contrary, chemical changes also affect the progress of physical changes. The changes, development, and mutual influence between the two vary greatly depending on external conditions such as temperature and humidity. To master the degree of withering and meet the requirements of tea quality, reasonable technical measures must be taken.

1. Physical changes of withering

The loss of fresh leaf moisture is the main aspect of physical changes in withering. Under normal climatic conditions, indoor natural withering under artificial control results in a “fast, slow, fast” pattern of fresh leaves wilting and losing water. In the first stage, free water in the leaves evaporates rapidly; In the second stage, during the self decomposition of internal substances and the dispersion of leaf stem water to the leaves, water evaporation slows down; In the third stage, the water and internal substances transported from the stem to the leaves undergo self decomposition to form compound water, as well as some bound water released by colloid solidification, and evaporation accelerates again. If the climate is abnormal or artificial control is not strict, the speed of evaporation of fresh leaf water during withering may not be certain. Withering technology is the artificial control of the evaporation process of fresh leaf moisture.

Most of the water in withered leaves evaporates through the stomata on the back of the leaves, while a portion of the water evaporates through the leaf epidermis. Therefore, the evaporation rate of fresh leaf water is not only influenced by external conditions, but also by the structure of the leaves themselves. The degree of keratinization of old leaves is high, making it difficult for water to dissipate, while the degree of keratinization of young leaves is low, making it easy for water to dissipate.
According to research, more than half of the water in young leaves evaporates through the underdeveloped cuticle layer, so older leaves lose water at a slower rate and leaves lose water at a faster rate. The stem contains more water than the leaves, but the evaporation of water from the stem is slower and some of it evaporates through transport to the leaves.

As the moisture content of withered leaves decreases, leaf cells lose their swollen state, leaf mass becomes softer, and leaf area decreases. The younger the leaves, the greater the reduction in leaf area. According to Manskaya data (Table 8-1), after withering for 12 hours, the first leaf shrinks by 68%, the second leaf shrinks by 58%, and the third leaf shrinks by 28%. This is related to the different cellular tissue structures of leaves with different degrees of tenderness. If withering continues, the water content decreases to a certain extent, and the leaf quality changes from soft to hard and brittle, especially the tips and edges of buds and leaves become hard and brittle.

The difference in water loss between buds and leaves leads to uneven withering. There are two situations: one is due to poor picking uniformity of fresh leaves, resulting in differences in tenderness between buds and leaves, which is not conducive to improving tea quality. Fresh leaf grading measures can be taken to overcome this. Secondly, even if the tenderness is the same, there may still be differences between different parts of the buds, leaves, and stems. In short, the degree of dehydration is relative, and unevenness is absolute.

The change in moisture content of withered leaves is a sign of water dispersion loss caused by a series of tea withering technical conditions such as temperature, leaf spreading thickness, time, and air circulation.

2. Withering conditions

All technical measures taken during withering are aimed at achieving uniform and moderate physical and chemical changes in withered leaves to meet the conditions required for fermentation. The external conditions that affect the quality of withered leaves are first the evaporation of water, then the influence of temperature, and finally the length of time. Among them, temperature has the most significant impact on the quality of withered leaves.

a.Water evaporation

The first step in withering is to evaporate water, and the evaporation of water is closely related to the relative humidity of the air. Low air humidity leads to rapid evaporation of moisture from withered leaves; If the air humidity is high, the evaporation of moisture will be slow. The result of the evaporation of wilting water is the formation of a saturated layer of water vapor on the surface of the leaves.

If the air humidity is low, that is, there is more water vapor that can be contained in the air, and the water vapor on the leaves can quickly diffuse into the air, there will be no steam saturation state on the leaves, and the physical changes of withered leaves will proceed faster. Of course, the saturation of water vapor in the air is closely related to the temperature of the air. The higher the temperature, the more water vapor the air absorbs, making it difficult to form a saturated state of vapor on the surface of leaves.
Therefore, with the same amount of water vapor in the air, if the temperature is high, the relative humidity will be low; When the temperature is low, the relative humidity is high. So high temperature will accelerate the evaporation of water.

Ventilation is an important condition for normal withering. If the withering chamber is sealed and not ventilated, during the initial stage of heating withering, the low relative humidity of the air accelerates the vaporization of moisture in the withered leaves. As the withering time prolongs, the amount of water vapor in the air increases, the relative humidity rises, the vaporization and liquefaction of water gradually reach equilibrium, the leaf temperature relatively increases, the permeability of the withered leaf cell membrane increases, the activity of enzymes strengthens, chemical changes accelerate, and the self decomposition and oxidation changes of contents change from slow to intense, causing the chemical changes of withering to develop along a deteriorating path, and in severe cases, red discoloration of withered leaves may occur.

So, indoor tea leaves withering, especially heating withering, must be accompanied by a certain amount of ventilation. The flowing air blows through the withered leaf layer, carrying away the water vapor on the leaf surface, forming a low humidity environment around the leaves, further accelerating the evaporation of leaf moisture. The evaporation of water from withered leaves requires the absorption of a certain amount of heat, which slows down the increase in leaf temperature. The larger the air volume, the faster the evaporation of water, the slower the rise in leaf temperature, and the slower the chemical changes in withered leaves.

In order to overcome the influence of natural climate on withering, artificial withering equipment is widely used in production, such as withering machines, withering tanks, etc., all of which are equipped with hot air generators and can adjust temperature and air volume. The air volume of the withering trough is generally based on the principle of not blowing “holes” in the scattered leaf layer.

Otherwise, air will concentrate through the “holes” in the leaf layer, causing an increase in wind pressure and scattering of buds and leaves around the withering bed. The air volume is closely related to the air permeability of the blade layer. If the air permeability of the blade layer is good, the air volume can be larger, and vice versa, it should be smaller. If the fresh leaves are tender, the buds and leaves are small, the leaf layer is compact, and the breathability is poor; The breathability of leaves in the later stage of withering will also decrease, and the air volume should be smaller. The air volume is small, and the temperature must decrease accordingly. The principle of withering operation is to first increase the air volume and then decrease it, and to first increase the temperature and then decrease it. Therefore, there are certain requirements for the blade thickness of the withering groove, which generally should not exceed 15-20 cm. At the same time, in order to achieve uniform withering of leaves in the upper and lower parts of the leaf layer, manual mixing is also necessary during withering.

b.Withering temperature

Temperature is the main condition for withering. During the withering process, the physicochemical changes of fresh leaves are closely related to the temperature. With the increase of temperature, leaf temperature rises rapidly, water evaporation increases, withering time shortens, and the process of physical and chemical changes accelerates. If the temperature is too high, it will cause an intensification of chemical changes in the contents of withered leaves. Therefore, it is advisable to control the wind temperature below 35 ℃ during withering, preferably 30-32 ℃, especially for fresh leaves of large leaf species, as high leaf temperature can cause dry and burnt shoot tips.

The withering temperature affects the activity changes of endogenous enzymes in withered leaves, which in turn affects the chemical reaction rate of the contained substances. Except for the base acid, other compounds have little variation within the range of 23-33 ℃. When the temperature rises above 33 ℃, the content of the main compounds gradually decreases with the increase of temperature, which is not conducive to the quality of withered leaves.

The temperature and air volume are closely related to the physical and chemical changes of withering, with a greater correlation between temperature and chemical changes, and a greater correlation between air volume and physical changes. By adjusting the temperature and air volume, the progress rate of physicochemical changes in wilting leaves can be controlled. It is advisable to adopt the operating principle of “increasing air volume first and then decreasing” and “increasing temperature first and then decreasing”. Mastering a certain amount of time can achieve the desired level.

3. Withering time

The effect of withering time on the physicochemical changes of withered leaves varies due to different conditions such as temperature and leaf spreading thickness. Within the same time, the weight loss rate of withered leaves varies with different temperatures, and the impact on their chemical changes and quality is also different.