Tea is as good as new, with glossy color, full aroma, bright infusion and refresh taste. But after a period of storage, the quality will be changed or the tea goes bad. It will lost the aroma, the color will become dark, not fresh and even moldy.
The essence of tea quality deterioration is the change of chemical composition related to tea quality. For example, chlorophyll in green tea is a substance that is greatly affected by heat and light. Under high temperature and strong light irradiation, chlorophyll is prone to be degraded and destroyed. Even if the storage time is long at normal temperature, chlorophyll will be constantly converted to pheophytin, lose its bright green color and turn into dark brown, so after a long storage of green tea, its green color will gradually disappear, and it will become dark brown and lose luster.
In addition, the tea polyphenols in green tea will produce Browning after automatic oxidation. Some oxidation products in Black tea will also conduct further oxidation polymerization, produce some dark color of the high polymer, the black tea color and infusion color deepen, the taste lost freshness and irritation.
The ascorbic acid in tea, namely vitamin C, is also the same as tea polyphenols, which can be oxidized during storage. The oxidized ascorbic acid produced after oxidation is yellowish brown in color. The lipids in tea leaves, after automatic oxidation during storage, will produce some stale taste and odor substances, and will also produce sunshine smell ater exposure to the sun. Amino acids in tea will also be gradually reduced during storage, which will gradually weaken the freshness of tea.
In a word, a series of changes occur in many components of tea in the process of storage, but the degree of such changes is closely related to the storage conditions. It has been proved that the main factors for tea deterioration are temperature, humidity, oxygen, light and odor.