What Chemical Changes Occur During Tea Storage?
I. Main Chemical Components of Tea
1. Polyphenols
Polyphenols and catechins are the main components constituting the taste of tea. They coordinate with amino acids, sugars, and other components to create a rich, fresh, and astringent tea soup. However, polyphenols, especially catechins (which account for about 70% of total tea polyphenols), are prone to auto-oxidation during storage. Although this oxidation occurs slowly and is not as intense or rapid as enzymatic oxidation, it becomes significant over long storage periods, especially under conditions of high tea moisture content, high storage temperature, and high air humidity.
The transformation pathway of catechins in polyphenols begins with dehydrogenation to form quinones, which further oxidize and polymerize into brown substances. Catechins and their intermediate oxidation products also combine with amino acids and proteins to form dark high-molecular polymers, disrupting the harmonious structure of tea soup taste. This results in a bland tea soup lacking astringency and freshness, with a darker color shifting from its original hue to orange-yellow, red, or brown.
According to experiments on green tea stored for 12 months, polyphenols decreased significantly from 23.71% to 21.12%, a reduction of 10.92%. Catechins, the main component of polyphenols, decreased from 102.71 mg/g to 61.64 mg/g, a reduction of 39.99%. The extent of polyphenol reduction and oxidation rate during storage are also related to the tea's moisture content and air humidity. After 12 months of storage, theabrownin in black tea increased from 6.6% to 10.10%, a rise of 53.03%. Theabrownin, being brown, is a key component causing tea soup darkening; high levels result in a bland taste and dark soup, affecting black tea quality.
2. Amino Acids
Amino acids are important taste substances in tea, crucial for tea quality. They undergo significant changes during storage. Amino acids can combine with quinones, the auto-oxidation products of tea polyphenols, to form dark polymers, affecting the color of green tea and the brightness of tea soup. In black tea, amino acids also interact with theaflavins and thearubigins to form dark polymers. Additionally, amino acids can decompose and transform under certain temperature and humidity conditions. For example, theanine, which plays a key role in the freshness of tea soup, easily hydrolyzes to form ethylamine and glutamic acid, thereby reducing the content of free amino acids.
Theanine is the main free amino acid and is particularly significant for the taste quality of tea soup. It decreases linearly during storage, dropping from 770.18 mg/100g to 404 mg/100g, a reduction of 47.54%. Other important amino acids like glutamic acid, aspartic acid, and arginine are also heavily oxidized, further impacting tea quality.
3. Aromatic Compounds
In an oxygen-rich environment, as storage time prolongs, tea aroma gradually weakens, the pleasant taste diminishes, and stale flavors emerge until the tea completely deteriorates and loses its drinking value. Tea contains substantial lipids, especially some free unsaturated fatty acids, which are important bases for tea aroma but are highly unstable. Under high temperature and oxygen, lipids hydrolyze to form free fatty acids, which further oxidize and decompose, producing volatile low-molecular aldehydes, ketones, and alcohols with undesirable odors. This is a key reason for changes in tea aroma during storage. During storage, changes in free fatty acids like linoleic acid and linolenic acid are similar to those of lipids.
It is widely believed that components contributing to the "new tea aroma" of green tea include dimethyl sulfide, nonanal, cis-3-hexenyl acetate, and other unknown compounds. In the presence of oxygen, these components decrease significantly during storage. After 2 months of storage, dimethyl sulfide decreases by 43%, nonanal by 80%, and cis-3-hexenyl acetate by 39%. Conversely, new compounds such as 1-penten-3-ol, cis-2-penten-1-ol, 2,4-heptadienal, 3,5-octadien-2-one, and propanal are generated during storage. These substances gradually increase with storage time. These compounds, not found in new tea, have grassy and oily odors. Due to their low thresholds, even minute quantities can impart an unpleasant stale flavor.
Additionally, carotenoids are easily oxidized. During green tea storage, carotenoid oxidation products like 2,6,6-trimethyl-2-hydroxycyclohexanone, β-cyclocitral, α-ionone, and 5,6-epoxyionone also increase. These compounds are closely related to the stale flavor of green tea.
The changes in aroma during black tea storage are more complex. As lipids hydrolyze and auto-oxidize, besides an increase in stale compounds, many floral and fruity aroma substances in black tea, such as phenethyl alcohol and nerolidol, as well as quality-favorable compounds like isobutyraldehyde, isoamyl alcohol, and linalool, decrease significantly. These changes not only cause loss of aroma but also introduce stale and rancid flavors.
4. Vitamin C
Vitamin C is one of the nutrients required by the human body and is abundant in green tea. During storage in an oxygen-rich environment, vitamin C decreases due to hydrolysis, browning, and other chemical changes, reducing the nutritional value of tea. The 2,3-diketogulonic acid produced from vitamin C oxidation easily undergoes the Maillard reaction with amino acids. Meanwhile, dehydration and decarboxylation of 2,3-diketogulonic acid produce brown hydroxymethylfurfural polymers, causing green tea to turn brown, deepen in color, and lose freshness, thereby reducing tea quality.
Research shows that after 4 months of storage in an oxygen-rich environment, vitamin C in green tea decreases from 306.75 mg/100g to 191.59 mg/100g, a reduction of 37.54%. After 8 months, it drops to 173.57 mg/100g (42.32% reduction), and after 12 months, to 166.10 mg/100g (45.85% reduction). Studies indicate that when the retention of vitamin C during storage is above 80%, quality changes are minor; when it falls below 60%, significant deterioration is perceptible.
5. Chlorophyll and Other Chemical Components
Chlorophyll is the main component of green tea's color, and its changes greatly affect the appearance. Chlorophyll is highly unstable and prone to demagnesium reactions under the influence of water, light, and temperature during storage. After 12 months of storage, the total chlorophyll in green tea decreases by 12.76%. Clearly, prolonged storage not only fades the vibrant green color but also darkens and browns the tea, largely due to chlorophyll demagnesium. Research suggests that when the conversion rate of chlorophyll to pheophytin is about 40%, the tea color remains green; when it reaches about 70%, the color significantly browns.
II. Impact of Tea Storage Environment
From the above understanding of tea's main components, we can see that the primary cause of tea aging is chemical changes in certain components under specific storage conditions. Although many factors influence these chemical changes, the main ones are oxygen, temperature, humidity, light, and packaging.
Auto-oxidation refers to the slow oxidation of substances by molecular oxygen without enzymatic catalysis. Besides moisture, temperature, and light, oxidation is the main cause of tea deterioration during storage. Oxygen in the air, comprising about 21%, is highly reactive and oxidizes polyphenols, chlorophyll, lipids, vitamins, ketones, aldehydes, etc., in tea. The resulting compounds are mostly detrimental to quality. Oxygen also promotes microbial growth, leading to moldiness in tea.
Studies show that under 1% oxygen, green tea stored for 4 months shows almost no change in soup color, whereas with over 5% oxygen, significant changes occur after 4 months. This indicates that higher oxygen levels accelerate tea auto-oxidation under certain conditions.
To prevent auto-oxidation during tea storage, tea must be isolated from oxygen. Common methods include vacuum packaging, nitrogen or carbon dioxide flushing to reduce oxygen in packaging containers. However, oxygen can slowly permeate through packaging materials over time, eventually reaching levels close to atmospheric oxygen. Currently, the most effective method is using deoxidizers for preservation. This approach chemically removes oxygen and adjusts air conditions (e.g., dehumidifying), keeping tea in a relatively oxygen-free and dry environment during its shelf life. Deoxidizer preservation is simple, cost-effective, and highly effective. Additionally, like other organisms, microorganisms in tea require aerobic respiration for metabolism. Most microorganisms on dry foods are aerobic; oxygen supports their growth, while a sealed, oxygen-free environment causes them to suffocate and die. Thus, vacuum sealing with deoxidizers for oxygen-free storage not only prevents mold but also delays tea aging, reducing the loss of key components like polyphenols, amino acids, vitamins, chlorophyll, and aromatic compounds, thereby maintaining tea quality over longer periods.