In our fast-paced world, cold brew tea is surging in popularity, prized for its refreshingly smooth taste that’s free from bitterness. Unlike traditional hot-brewed tea, the cold brew method involves steeping tea leaves in cold water for several hours, allowing the flavor compounds to release slowly and gently.
Recently, a team from Anhui Agricultural University published a study in Food Chemistry titled “Metabolomics analysis reveals the chemical basis of aroma, taste and color formation in cold-brewed green tea infusion.” Their research discovered that cold-brewed (4°C) green tea develops its unique quality characteristics after just one hour of steeping.
Research Findings
Low Temperatures Cultivate Floral Aromas
For this study, researchers selected Anji Bai Cha, a type of Chinese green tea, and used a consistent tea-to-water ratio (1:50 g/mL). They prepared infusions under both cold brew (4°C) and hot brew (80°C) conditions, testing them at intervals of 2, 4, and 8 minutes, and 1, 2, and 3 hours.
Figure 1: Changes in the content of various volatile compounds over steeping time.
The results showed that in the initial phase of cold brewing (2-8 minutes), a small amount of low-boiling point alcohols (like 3-methyl-1-butanol) were released, lending a faint grassy note, but the floral aroma was not yet apparent.
When the cold brew time was extended to 1 hour, key aroma compounds began to shine. The concentration of alcohols like linalool and geraniol—known for their lily and rose scents—increased significantly, giving the tea a distinct floral profile.
After 3 hours of cold brewing, these alcohols reached their peak concentration. The aroma transformed from a subtle hint into a rich, lasting fragrance, as if the essence of a whole flower had been infused into the water.
In contrast, with hot brewing (80°C), the alcohols were released rapidly within 8 minutes. However, the high temperature also accelerated their volatilization. After 1 hour, the alcohol concentration in the hot brew was actually lower than in the cold brew, causing the floral aroma to fade.
Figure 2: Changes in the aroma characteristics of cold and hot-brewed tea infusions over time.
Furthermore, within the first hour of cold brewing, the release of aldehydes (such as hexanal and pentanal) increased linearly, infusing the tea with fresh notes of grass and citrus.
After 1 hour, the concentration of other aldehydes like (E)-2-heptenal rose further, making the “fresh aroma” even more prominent. Meanwhile, ketones like 4-methyl-3-penten-2-one continuously accumulated between 1 and 3 hours of cold brewing, adding a unique fruity undertone to the floral bouquet.
Hot brewing showed the opposite trend. Aldehydes were released in large quantities within 4 minutes, with (E,E)-2,4-heptadienal contributing a strong fatty aroma, but their concentration dropped quickly after 8 minutes due to volatilization. For ketones, trans-β-ionone peaked at 1 hour in the hot brew, imparting a rich floral scent, but its extraction level in the cold brew remained consistently lower.
Figure 3: Changes in different volatile compounds over steeping time.
A Fresher, Sweeter, and Smoother Taste
The study found that in the initial cold brew phase (2-8 minutes), only a minimal amount of caffeine was extracted, resulting in virtually no bitterness.
After 1 hour, the caffeine concentration reached 23.20 μg/mL, which is about 57% of the level in a 4-minute hot brew (40.90 μg/mL), keeping the bitterness mild.
If cold brewing continued for up to 3 hours, the rate of caffeine extraction slowed, preventing any significant increase in bitterness.
Figure 4: Changes in the taste characteristics of cold and hot-brewed tea infusions over time.
At the same time, the release of EGCG, the compound responsible for astringency, was even more restrained. After 1 hour of cold brewing, the EGCG content was 45.6 μg/mL, a mere 47% of that in a 2-minute hot brew (97.56 μg/mL).
Even after 3 hours, the EGCG concentration only slowly climbed to 71.82 μg/mL, meaning the astringency remained barely perceptible. The low temperature acts like a “slow-motion button” for bitter and astringent compounds, preventing the intense, rapid release that occurs in hot water.
Figure 5: Comparison of catechin, theanine, and caffeine extraction in cold and hot brews over time.
Regarding the umami flavor, after 1 hour of cold brewing, the extraction of L-theanine (the compound responsible for umami) was 9.2% higher than in a 4-minute hot brew, giving the tea a distinctively fresh and savory taste.
When extended to 3 hours, the L-theanine concentration continued to rise, transforming the flavor from “lightly savory” to “full-bodied,” evoking the fresh sweetness of spring buds.
The dissolution of soluble sugars (glucose, sucrose) followed a simple rule: the longer the steep, the sweeter the taste. After 1 hour of cold brewing, the sugar content was at 85% of the hot brew equivalent.
After 3 hours, its concentration reached nearly 95% of the hot brew, working in synergy with L-theanine to elevate the taste from “faintly sweet” to a “clear, sweet finish.”
The Secret to a Greener Brew
The vibrant green color of green tea comes from chlorophyll. During cold brewing, the low temperature inhibits the activity of chlorophyll-degrading enzymes, preserving high levels of chlorophyll a and b.
Data shows that after 3 hours of cold brewing, the infusion’s a* value (a measure of greenness) dropped from -2.64 to -4.68 (a more negative value indicates a more intense green). In contrast, the a* value of the hot brew rose from -4.05 to -2.30 over the same period, indicating a loss of green color. The low temperature acts as a “protective shield” for chlorophyll, maintaining the tea’s fresh, vibrant green hue.
Additionally, the high temperature of hot brewing accelerates the extraction of flavonoids. These compounds not only contribute to astringency but also give the infusion a yellowish tint.
Cold brewing significantly reduces the extraction of these flavonoids while also inhibiting the conversion of chlorophyll into brownish degradation products (like pheophytin). This dual action ensures that the color of cold brew tea remains much closer to the emerald green of fresh tea leaves.
Figure 7: Changes in the color of cold and hot-brewed tea infusions over time.
Conclusion
Cold brewing is more than just steeping tea in cold water; it’s a method of “precision extraction.” By controlling the rate of molecular movement and the stability of chemical compounds, it selectively extracts desirable flavors—retaining umami, sweetness, and floral aromas while suppressing bitterness and astringency, all while locking in the natural green color.
The magic of cold brew lies in trading time for flavor. As the minutes turn to hours, the aromatic compounds in the tea blossom in layers, and the taste evolves from light and fresh to rich and complex, with the low temperature consistently preserving that unique, verdant quality. The next time you make a cold brew, try tasting it at different stages: the refreshing crispness at 1 hour, the deep richness at 3 hours. Experience for yourself the wondrous transformation of tea over time.