Tea preservation remains a major challenge in the world today. How to prevent tea leaves from deteriorating is about doing our best, not achieving perfection, because long-term freshness of tea is not easy; we can only try to extend its保鲜期 as much as possible.
Due to differences in tea processing techniques, the保鲜期 of different teas varies. Green tea, white tea, and yellow tea, being non-fermented or lightly fermented, are the most difficult to preserve. They generally start to lose their original flavor about three months after the new tea hits the market.
The reason for tea deterioration is its strong hygroscopicity and odor absorption. It easily absorbs moisture and odors from the air. If the storage method is slightly improper, it will lose its flavor in a short time. Moreover, the more lightly fermented and highly fragrant premium teas are, the harder they are to preserve. Usually, after being stored for a period, the aroma,汤色, taste, and color of tea will change. The fresh taste of new tea disappears, and a stale taste gradually emerges.
Chlorophyll is an important component forming the color and luster of green tea. Chlorophyll consists of bluish-green chlorophyll a and yellowish-green chlorophyll b. When the former is high, the leaves appear emerald green; the latter is expressed as yellowish-green and is higher in tender buds and leaves.
Because the content of chlorophyll in made tea and the ratio of the two largely determine the color of the tea, a higher preservation amount makes the color appear green and vibrant. However, chlorophyll is a very unstable substance. Under the influence of light and heat, it easily decomposes, especially when exposed to ultraviolet light.
An important reason for green tea losing its green color and turning brown is the formation of pheophytin as chlorophyll loses magnesium during storage. Generally, when the proportion of this pheophytin reaches over 70%, significant browning occurs.
Higher temperatures make the external color of tea leaves more prone to turning brown. Low-temperature refrigeration can effectively slow down the browning and aging of tea. When the moisture content in tea leaves exceeds 5%, it accelerates quality deterioration and promotes the oxidation of residual enzymes in the tea, causing discoloration.
Tea polyphenols are the components most closely related to the汤色 and taste of tea. Their content in tea determines the taste concentration, astringency, and briskness of the tea liquor. These originally colorless substances, during black tea processing, undergo a series of reactions catalyzed by enzymes to generate theaflavins, which significantly influence汤色 and taste. Further polymerization produces thearubigins, which play a crucial role in汤色 color.
However, theaflavins and thearubigins are prone to further oxidation during storage, polymerizing into high-molecular compounds detrimental to both汤色 and taste. Green tea is characterized by a high retention of tea polyphenols, which are also prone to oxidation during storage, forming quinones, thereby turning the tea liquor brown. Moreover, these oxidation products can further react with amino acids, deteriorating the taste.
Vitamin C is not only one of the health components contained in tea but also closely related to the quality of tea. Especially in high-quality green tea, its content is very high. Vitamin C is also a substance easily oxidized, which is one reason why higher-grade green teas are more difficult to preserve.
When oxidized, vitamin C can generate dehydroascorbic acid, which easily reacts with amino acids to form氨基羰基 compounds. This not only reduces the nutritional value of tea but also causes browning. At the same time, as the amino acid content decreases, the taste becomes less fresh and brisk.
If the residual amount of vitamin C in green tea is above 80%, the quality of the green tea hardly changes. Once it drops below 60%, the tea quality明显变质.
The above are the reasons for tea deterioration. Knowing the causes, we can appropriately find ways to solve the problem of how to prevent tea from deteriorating. That is, try to keep tea in a place with low temperature, little light, sealed, and not in contact with air. This can help ensure that the time before tea deteriorates is延长.