Journal of Tea Science Research, 2024, Vol.14, No.5, 249-261 http://hortherbpublisher.com/index.php/jtsr 251 The final steps are drying and grading, which develop its strong flavor and dark color. Each tea type's specific processing method significantly influences its taste, aroma, and health benefits. 2.2 Key compounds influencing flavor and quality The key compounds that influence the flavor and quality of tea during fermentation include catechins, theaflavins, thearubigins, amino acids, and volatile organic compounds (VOCs). Catechins, which are abundant in fresh tea leaves, undergo oxidation to form theaflavins and thearubigins, imparting the characteristic color and astringency to black tea (Hua et al., 2021; Chen et al., 2021). Theaflavins contribute to the briskness and brightness of the tea liquor, while thearubigins add depth and body to the flavor (Rahman et al., 2020). Amino acids, such as theanine, enhance the umami taste and overall flavor profile of the tea (Xiao et al., 2021; Chen et al., 2021). VOCs, including terpene alcohols and esters, are responsible for the aroma and are significantly influenced by the fermentation conditions (Xiao et al., 2021; Chun et al., 2021). The balance and concentration of these compounds are critical for achieving the desired sensory qualities in the final tea product. 2.3 Role of enzymes in fermentation Enzymes play a pivotal role in the fermentation process of tea, driving the chemical transformations that determine the tea's flavor and quality. Polyphenol oxidase (PPO) and peroxidase (POD) are the primary enzymes involved in the oxidation of catechins to theaflavins and thearubigins (Xu et al., 2019; Hua et al., 2021). The activity of these enzymes is influenced by factors such as temperature, humidity, and oxygen levels during fermentation (Chen et al., 2021; Saikia et al., 2023). For instance, optimal fermentation temperatures can enhance enzyme activity, leading to a higher formation rate of theaflavins and thearubigins, which are essential for the quality of black tea (Qu et al., 2020). Additionally, enzymes such as α-amylase, proteinase, and β-glucosidase, which are secreted by specific fungi during fermentation, contribute to the modification of chemical properties and the enhancement of flavor (Xu et al., 2019). Understanding the role of these enzymes and optimizing fermentation conditions can significantly improve the sensory attributes and overall quality of tea. 3 Factors Affecting Fermentation Conditions 3.1 Temperature and its impact on fermentation Temperature is a critical factor influencing the fermentation process of tea, significantly affecting its chemical composition and sensory properties. Research has shown that the optimal fermentation temperature for black tea is around 28 °C, which results in the best sensory quality, including the highest scores for aroma and taste. At this temperature, the tea exhibits strong antioxidant activities and inhibitory effects on enzymes such as α-glucosidase (Qu et al., 2020). As the temperature increases, the levels of polyphenols, polysaccharides, and catechins decrease, while amino acids and soluble sugars increase, which can alter the flavor profile of the tea. Additionally, a study on the fermentation of tea juice indicated that a temperature of 35 °C, combined with specific time and pH conditions, resulted in the highest content of theaflavins and the best sensory quality (Tang et al., 2018). 3.2 Humidity and its role in fermentation Humidity during fermentation also plays a significant role in determining the quality of tea. Different levels of humidity can affect the appearance, aroma, and taste of the tea. For instance, fermentation at lower humidity levels (75% or below) tends to produce tea with a greenish, astringent, and bitter taste, while higher humidity levels (85% or above) result in a sweet and mellow flavor (Zhang et al., 2023). The content of various chemical compounds such as flavones, tea polyphenols, and catechins decreases with increasing humidity, whereas the levels of soluble sugars, thearubigins, and theabrownins increase, contributing to the development of a sweeter taste. This indicates that maintaining an optimal humidity level is crucial for achieving the desired tea quality. 3.3 Fermentation time and its effects The duration of fermentation is another crucial factor that impacts the quality of tea. The fermentation time affects the chemical composition and sensory attributes of the tea. For example, a study on the fermentation of tea juice found that a fermentation time of 75 minutes at a specific temperature and pH resulted in the highest content of theaflavins and the best overall acceptability (Tang et al., 2018). Additionally, the quantitative prediction and
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