Journal of Tea Science Research, 2024, Vol.14, No.4, 238-248 http://hortherbpublisher.com/index.php/jtsr 241 Amino acids, particularly theanine, are also affected by elevated temperatures. Theanine, which contributes to the umami flavor of tea, decreases under moderately high temperatures due to the suppression of theanine biosynthetic genes (Li et al., 2018). This reduction in theanine content results in a less distinctive umami taste in tea harvested during warmer months. Furthermore, caffeine levels are influenced by temperature variations, with higher temperatures generally leading to increased caffeine concentrations. This is attributed to the upregulation of genes involved in caffeine biosynthesis, mediated by hormone signal transduction factors (Lin, 2023). 4.2 Impact on aroma profiles The aroma profile of tea is significantly influenced by temperature, as it affects the production of essential oils and volatile compounds. High temperatures can lead to the degradation of certain volatiles that contribute to the pleasant aroma of tea, while increasing the concentration of others that may impart undesirable odors. For instance, heating green tea liquor results in the epimerization of catechins and a decrease in volatiles such as linalool oxide and β-ionone, which are associated with floral and fruity notes. Conversely, compounds like indole and α-terpineol, which have less pleasant odors, increase in concentration (Kim et al., 2007). Moreover, the production of specific aroma compounds is closely linked to the biosynthesis of secondary metabolites, which are regulated by temperature-sensitive pathways. For example, the roasting of green tea at different temperatures can optimize the formation of desirable odorants while mitigating the degradation of catechins. Roasting at 160 °C for 30 minutes has been shown to produce a favorable aroma profile while minimizing the loss of catechins. Additionally, the fermentation temperature of black tea significantly affects its aroma, with lower fermentation temperatures (around 28 °C) resulting in higher sensory quality scores for aroma and taste (Qu et al., 2020). 4.3 Influence on nutritional value The nutritional value of tea, particularly its antioxidant properties, is also affected by rising temperatures. Catechins, which are potent antioxidants, decrease in concentration with increasing temperatures, leading to a potential reduction in the health benefits associated with tea consumption. Studies have shown that higher temperatures result in the degradation of catechins, thereby diminishing the antioxidant capacity of tea (Mizukami et al., 2008). This reduction in catechin levels can negatively impact the tea's ability to scavenge free radicals and protect against oxidative stress. In addition to catechins, other bioactive compounds such as polyphenols and amino acids are influenced by temperature variations. For instance, moderate high temperatures can increase the total polyphenol concentration in tea, which may enhance its antioxidant properties. However, this is often accompanied by a decrease in free amino acids, altering the overall nutritional balance of the tea (Li et al., 2020). Furthermore, the fermentation temperature of black tea affects its bioactivity, with lower temperatures (around 28 °C) enhancing its antioxidant activities and inhibitory effects on enzymes related to glucose metabolism (Qu et al., 2020). 5 Impact on Sensory Attributes 5.1 Changes in taste profiles Rising temperatures significantly influence the taste profiles of tea, particularly affecting bitterness, astringency, and sweetness. Bitterness and astringency are primarily attributed to phenolic compounds, caffeine, and tannins, which can be modified by temperature and brewing time. For instance, a study on coffee leaves tea demonstrated that higher brewing temperatures (91 °C-99 °C) and longer brewing times (3-5 minutes) enhanced the bitterness and astringency, which were well accepted by consumers when brewed at 95 °C for 5 minutes (Fibrianto et al., 2021). Similarly, in green tea, catechins such as (-)-epigallocatechin gallate and (-)-epicatechin gallate are major contributors to bitterness, while flavonol glycosides contribute to astringency. These compounds' concentrations and their sensory impacts are highly temperature-dependent (Xu et al., 2018).
RkJQdWJsaXNoZXIy MjQ4ODYzNA==