Fresh tea leaves, after initial processing, become raw tea, which generally cannot yet be sold as commercial tea. Commercial tea requires standardized quality, that is, it must have a unified standard grade. Export tea has even higher requirements, not only for stable and good internal quality, but also for neat and regular appearance.
Except for high-end premium teas, the quality of raw tea often fails to meet the requirements of commercial tea. Especially when the appearance is mixed, it is necessary to conduct fine processing of raw tea, tidy up the appearance, and turn it into finished tea to meet the requirements of commercial tea and satisfy the needs of both domestic and international markets.
Features of Crude Tea
The characteristics of Crude Tea include its morphology and properties (such as specific gravity, moisture absorption and adsorption, dispersibility and automatic grading, thermal conductivity, etc.), which are closely related to the quality of tea refining. Tea refining involves using appropriate machinery and reasonable process technology based on these properties to separate the mixed raw tea and then blend and combine it into finished tea that meets the requirements, achieving the purpose and requirements of refining.
1. shapes of crude tea
Crude Tea is processed from fresh leaves and has complex and diverse forms, including long and short ones; Coarse or fine; Thick or thin; Curved, straight, hooked; Round or flat; Tight, loose, as well as various low-quality teas, tea seeds, tea stems, and various impurities such as wood and bamboo chips, sand and stones, and iron filings mixed in during the initial processing, need to be screened, air sorted, cut and rolled, picked and sorted to standardize their appearance and meet the quality specifications of the finished tea. The main reasons for the complex and diverse forms of crude tea leaves are:
① Different tea tree varieties result in differences in fresh leaf traits;
② The quality of fresh leaves varies depending on the level of tea garden management;
③ The picking requirements are not strict, the old and tender are mixed, and there are impurities;
④ Different production regions, seasons, initial processing machinery, and processing methods can all cause differences in tea morphology;
⑤ The different levels of tea inspection and purchasing habits of the acquiring personnel are important reasons for the complexity of the morphology of raw tea.
2.Characteristics of crude tea
The characteristics of raw tea are not only the basis for the quality of refined tea, but also the basis for formulating refined processing technology measures. The difference in appearance of hairy tea is not significant, and the processing technology is relatively simple; On the contrary, processing is more complex.
1) Scatteriness of raw tea
Due to differences in shape, size, and absolute weight, raw tea has varying degrees of free mobility under the influence of Earth’s gravity. This characteristic is called the scattering property of hairy tea.
The strength of the scattering of raw tea is represented by the natural slope and friction angle. The natural slope angle is the angle formed by the diameter of a cone and the slope line on a horizontal plane, where tea leaves fall naturally without any restriction or pushing. This angle is called the natural slope angle or static angle.
The natural slope angle is equal to the friction angle at which the tea automatically slides down, and the friction angle is the angle at which the tea can start sliding along a surface. A large friction angle indicates weak scattering, making it difficult for tea leaves to slide down the slope; Small friction angle results in low frictional resistance when the tea leaves move, making it easy for the tea leaves to slide down the slope and have strong scattering properties.
The friction angle varies depending on the shape, quality, and surface condition of tea leaves. Round tea has a temperature of 48 °, flat tea has a temperature of 54 °, strip tea has a temperature of 63 °, and sheet tea has a temperature of 73 °. The sliced tea has a large rubbing angle and weak scattering.
The static angle of different grades of raw tea also varies. According to the test results of Pan Zuyue et al. on stir fried green tea, the static angle of raw tea increases as the grade decreases. That is to say, as the grade of raw tea decreases, the dispersibility becomes weaker.
The surface smoothness and moisture content of raw tea are also related to its dispersibility. The scattering property of raw tea increases with the smoothness of the tea surface, and decreases with the increase of the moisture content of raw tea. Therefore, raw tea has a high moisture content and must be dried before separation to enhance its dispersibility and improve screening efficiency.
The scattering property of tea is an important physical characteristic utilized in the tea screening process. When tea leaves are sieved, when the raw tea automatically falls onto the sieve surface from the conveyor belt, a static angle of varying sizes is formed. The larger the static angle, the weaker the scattering and the lower the sieving efficiency. On the contrary, the stronger the scattering, the higher the screening efficiency. For example, crude tea with coarse and loose strands, rough surface, and high moisture content has high frictional resistance and angle during movement, resulting in weak scattering. On the contrary, crude tea with tight and thin strands, smooth surface, and low moisture content has less frictional resistance and smaller friction angle when moving, resulting in stronger scattering.
2) Automatic grading
When tea leaves move, due to their scattering, the various components of the tea pile are redistributed to certain parts according to their specific gravity and size. This characteristic is called automatic grading.
Tea leaves have an automatic grading phenomenon during any screening process, and this phenomenon is more pronounced when floating up. After the tea leaves in motion are rotated, small and heavy ones sink to the lower layer, while thick and light ones float to the upper layer. The technical measure is to use this performance to separate tea leaves of different lengths, thicknesses, and weights. Wind selection and shaking also utilize this property to blow light tea leaves towards the distance and heavy tea leaves towards the nearby area, achieving the purpose of separating light and heavy tea leaves.
(a) The bulk density of raw tea
Tea has a certain volume and mass. The ratio of a certain weight to the true volume of the tea itself is called specific gravity; Under certain volume conditions, the weight of tea leaves is the bulk density. The bulk density of different tea leaves varies, mainly due to the differences in the nature of fresh leaves. Generally speaking, hairy tea made from tender and fresh leaves after reasonable processing has tight and dense strands, high bulk density, and good quality. Low grade fresh leaves are often used to make hairy tea, which is coarse, loose, light, and has a small bulk density.
Tea leaves with different bulk densities have varying degrees of suspension in the same airflow due to the different sizes of their windward surfaces. Heavy tea leaves have a small windward surface and scatter quickly; Light tea leaves have a large windward surface and scatter slowly, which leads to automatic grading. So during processing, air selection technology can be used to distinguish tea leaves with different weights based on their bulk density.
(b) Surface condition of crude tea
Smooth tea leaves have low friction between leaves during the scattering process, resulting in faster scattering speed. Circular tea can quickly slide down from a slope, while sheet tea is less likely to slide down, resulting in automatic grading.
(c) Moisture content and elasticity of raw tea
Dry crude tea has high elasticity, fast flow, and strong scattering properties. Wet tea has low elasticity, slow flow, and weak scattering. Tea leaves have different water contents, and their scattering speed also varies, resulting in automatic grading.
(d) Wind bearing surface
Any object in the air will be subjected to the pressure of airflow. The characteristic of resisting this pressure is called wind carrying capacity. Tea leaves have different shapes, sizes, positions in the air, and wind carrying capacities. The larger the wind carrying surface of tea leaves, the farther they blow, resulting in automatic grading. Both wind selection and shaking techniques utilize this property to separate the weight of tea leaves.
3) Thermal conductivity and heat resistance
Thermal conductivity refers to the ability of tea leaves to transfer heat. It is closely related to water content, and high water content improves thermal conductivity; On the contrary, the thermal conductivity decreases. When processing and drying raw tea extract, its thermal conductivity should be considered.
If the thermal conductivity is poor, it should be spread thinly; Good thermal conductivity, can be appropriately spread. Heat resistance refers to the maximum allowable temperature at which the quality of tea leaves does not deteriorate when heated. It is also closely related to water content, with high water content and good heat resistance; If the water content is low, the heat resistance is poor. Therefore, this characteristic should be fully considered during drying operations, and the appropriate temperature should be controlled to ensure sufficient drying, improve quality, and prevent deterioration.
4) Moisture absorption and adsorption properties
Tea has strong abilities to absorb moisture from the air and adsorb other gases, known as hygroscopicity and adsorption, respectively. Its essence is that tea has a large specific surface area and many tiny capillary structures, which give it strong adsorption properties. Tea can not only absorb moisture and various aromas in the air, but also absorb odorous gases, which has both advantages and disadvantages. So in the process of tea processing and storage, it is necessary to pay attention to moisture prevention and strictly prohibit the mixing of odorous gases. At the same time, the adsorption characteristics of tea can be utilized to produce various floral teas, blend the fragrance of tea flowers into one, and improve the quality of tea.
In addition to the above characteristics, tea also has brittleness, toughness, and friction.
