Green Tea: Classification, Processing, Biochemistry, and Quality Factors

Green tea is a type of non-fermented tea originating in China and is known for its health benefits. Different types of green tea exist based on their processing and appearance, with Japan being the second largest producer and primarily producing steamed green teas. Processing green tea involves plucking tender tea leaves, fixing the leaves to prevent fermentation, rolling the leaves to break the cells and liberate the juices, and drying them. This process involves a range of biochemical transformations, including enzyme activity, chlorophyll content, tea polyphenols, protein and amino acids, carbohydrates, and aroma. These biochemical changes play a crucial role in developing the characteristic taste, flavor, and color of green tea.

Classification of Green Tea

Green tea is a type of non-fermented tea that originated in China and is still primarily produced and consumed there. Other countries like Japan, Indonesia, and Taiwan also produce green tea. Green tea is known for its health benefits, and there are different types of green tea based on their processing and appearance. Japan is the second largest producer of green tea and primarily produces steamed green teas.

Processing of Green Tea

Green tea can be made from various cultivars and its quality is based on the tenderness of the fresh green leaf. The shape of green tea can be needle, twisted, flat, round, compressed, or ground powder. The process of making green tea involves plucking tender tea leaves, spreading them out for a few hours, fixing the leaves to prevent fermentation, rolling the leaves to break the cells and liberate the juices, and drying them. The drying process is usually done twice and can be conducted in a pan, basket, machine, or by sunlight. Steamed green tea is mostly made in Japan and is fixed by steaming in perforated drums. Steaming green tea produces a needle-shaped tea with high amounts of vitamin C.

Biochemistry of Green Tea Processing

The crucial step in processing green tea is to rapidly and effectively halt the activity of leaf enzymes through high-temperature fixing. After this, the fixed leaves can then undergo rolling and drying. Properly fixed leaves will maintain their green color throughout the entire process. The processing of green tea involves a range of biochemical transformations.

Enzyme activity

Living tea shoots contain many enzymes responsible for various metabolic pathways. When tea leaves are picked, these enzymes remain highly active. The main enzymes in tea leaves are polyphenol oxidase (PPO), catalase (CAT), peroxidase (PO), and ascorbic acid oxidase (AAO). During the fixing process, high temperature deactivates these enzymes.

Different enzymes in tea leaves respond differently to temperature. For instance, CAT and PO activity increase within a range of 15°C to 25°C but decrease when the temperature is higher than 35°C. The PPO activity increases steadily within a range of 15°C to 55°C, and deactivation occurs when the temperature rises to 65°C.

Therefore, the green tea fixing process is completed only when PPO activity is entirely deactivated. The fixing temperature and duration vary depending on the plucking standard, leaf position, seasons, cultivars, and other factors. For example, tender shoots require longer and higher temperature fixing than mature shoots because they have higher PPO activity.

Chlorophyll

During the green tea processing, the thermo-physiochemical action causes chemical changes. Chlorophyll is the primary pigment that gives green tea its color. However, the chlorophyll content decreases as the tea leaves undergo the manufacturing process. The relative chlorophyll content in newly plucked tea leaves is 100, which decreases to 87 in fixed leaves, 74 in rolled leaves, and 52 in dried leaves. High temperature and pH changes lead to the loss of Mg from chlorophyll during the fixing process. Additionally, chlorophyll can break down into folic acid, phytol, and Mg-free chlorophyll due to hydrolysis.

Tea polyphenol

During green tea processing, tea polyphenols (TP) undergo oxidation, hydrolysis, polymerization, and transformation, causing a decrease in total TP content compared to fresh leaves, usually around 15%. This chemical change is important in developing the characteristic taste and flavor of green tea. When TP are auto-oxidized under heat, they produce yellow substances that contribute to the color of green tea infusion. These substances can also form water-insoluble compounds with proteins in tea leaves, contributing to the bright color of infused leaves. TLC analysis shows that gallated catechins are converted to nongallated catechins due to hydrolysis under humid and heating conditions during processing, which leads to significant changes in the composition of tea catechins in the final product.

Protein and amino acid

During green tea processing, proteins in tea leaves are hydrolyzed into free amino acids due to high temperature and moisture, resulting in a higher amino acid content in the final product compared to fresh leaves. Amino acids are essential for creating the freshness of tea infusion. The decrease in tea polyphenols during processing changes the ratio of polyphenols to amino acids, and a coordinated ratio of the two components is necessary for producing a fresh and grassy taste. Moreover, some amino acids are transformed into volatile substances, which contribute to the quality of green tea, such as isoleucine oxidizing into isopentaldehyde and phenylalanine into phenylaldehyde (Zhou, 1976).

Carbohydrates

During green tea processing, starch is hydrolyzed under high temperature and moist conditions, producing more soluble sugars. As a result, the soluble carbohydrate content is increased in made tea compared to fresh shoots. This increase in soluble sugars varies depending on green tea type, fixing temperature, and drying method. Baked green teas tend to have higher levels of soluble sugars. Studies show that soluble sugars play a role in enhancing the flavor of green tea.

Aroma

During green tea processing, a wide range of biochemical changes occur that affect aroma. Over 600 tea aroma compounds have been identified, with most coming from processed leaves rather than fresh leaves. Lower boiling point volatile compounds are released during the early stages of processing, while higher boiling point compounds increase. Fresh leaves contain lower bp compounds such as Z-3-hexen-1-ol, which accounts for 60% of total volatile compounds and has a strong grassy odor. Most of this compound evaporates during processing, with only a small amount changing into E-3-hexen-1-ol, which has a fresh odor.

Factors Affecting Green Tea Quality

Several factors influence the quality of green tea, including processing technology, tea plant cultivars, made tea shapes, fertilization, seasons, and plucking standards. Among these, processing factors have the most significant impact on quality. Spreading out green leaves for a short time before fixing is an essential step in producing high-quality green tea. Fixing temperature should be higher at the start and then lower. Tender leaves require more fixing than mature leaves, and the rolling time and degree of pressure are essential technical parameters. Tea plant cultivars, proper shading, and higher nitrogen application rates also influence green tea quality. Fine plucking standards are also favorable.