When making tea, you may pay attention to water temperature while always using the same leaf size and waiting time. Yet the rate at which aroma compounds emerge depends on the combination of all three.
A green-tea study published in 2020 found that leaf size made a pronounced difference early in brewing, while water temperature became more influential as time passed. Within the range tested, water mineralization had a smaller effect, although some compounds still showed differences.
- Following changes during brewing, not just the finished cup
- Comparing leaf size, temperature and water using the same green tea
- Leaf size stood out in the first 30 seconds
- Temperature became more important with longer brewing
- Water had a smaller effect, but not no effect
- Different brewing methods can approach similar compound profiles
- Conclusion
Following changes during brewing, not just the finished cup
José A. Sánchez-López and colleagues at institutions including Switzerland’s Zurich University of Applied Sciences investigated how aroma-related compounds are extracted from tea leaves. Rather than comparing only finished infusions, they followed changes during brewing.
They varied water temperature, leaf size and the mineral content of the water. Alongside examining how each influenced compound levels and profiles, they asked whether different brewing conditions could produce similar compound profiles.
Instead of treating one compound as representative of tea aroma, they compared patterns across the detected signals and also examined dimethyl sulfide, a compound associated with green-tea aroma.
Comparing leaf size, temperature and water using the same green tea
The tea was gunpowder green tea from Zhejiang Province, China, a type with tightly rolled leaves. The same tea, harvested in 2016, was used throughout. Intact leaves were compared with leaves broken in a household blender into pieces averaging a few millimeters. This was not a comparison with a fine powder like matcha.
The researchers used 3 g of tea with 150 mL of water, combining the following conditions:
- Water temperatures: 60°C, 70°C and 80°C
- Leaves: intact rolled leaves or pieces a few millimeters across
- Water: two commercial waters with hardness values of 105 mg/L and 252 mg/L, expressed as calcium carbonate equivalents
The waters were Aqua Panna and San Benedetto. They differed not only in calcium and magnesium but also in other minerals, so the experiment did not independently vary hardness alone. The infusion was gently stirred mechanically, and each condition was repeated 3 times.
During the first 5 minutes, 1 mL samples were taken every 30 seconds, filtered and cooled. Before measurement, all samples were held at 37°C for 30 minutes. Volatile compounds entering the space above the liquid were measured using a PTR-ToF-MS mass spectrometer.
In other words, the researchers did not directly compare steam rising from tea at 60°C and 80°C as it brewed. They varied the brewing conditions, then compared compounds released from the liquid at a common measurement temperature. The analysis used 88 mass signals, of which 34 were tentatively assigned to compounds.
Leaf size stood out in the first 30 seconds
An analysis combining many signals to compare sample profiles separated intact and broken leaves into different groups at 30 seconds. Broken leaves allowed compounds to be extracted more readily from this early stage.
At the same 30-second point, there was no clear separation by temperature or water type. Leaf shape and size produced the first conspicuous differences.
Breaking leaves changes the surfaces in contact with water, including cut edges, and the pathways by which compounds escape. Drawing also on previous work on mass transfer from tea leaves, the authors discuss this as an explanation for faster extraction.
When the signal tentatively assigned to dimethyl sulfide was examined separately, significant differences due to leaf size were concentrated early in extraction and were no longer significant after 1.5 minutes. The effect of breaking the leaves did not remain equally strong throughout.
Temperature became more important with longer brewing
As brewing progressed to 2, 3.5 and 5 minutes, temperature-related differences became clearer. Overall, higher temperatures tended to yield larger amounts of the measured volatile compounds. By 5 minutes, temperature was the main factor separating samples with substantially different compound levels.
For example, with intact leaves brewed for 5 minutes in the lower-hardness water, the gas-phase concentration of the compound tentatively identified as dimethyl sulfide was approximately 21 ppb at 60°C and 34 ppb at 80°C. These are instrumental concentrations, not ratios of perceived aroma intensity.
The team also compared how signals rose over time by normalizing each to its value at 5 minutes. Once differences in final amounts were removed, the temperature-based separation disappeared, while differences between intact and broken leaves remained.
They interpreted this to mean that temperature mainly influenced the amount extracted, whereas breaking the leaves affected not only amounts but also the time course of release. Raising the temperature and reducing leaf size did not act on extraction in the same way.
Water had a smaller effect, but not no effect
Of the three brewing factors compared, water mineralization had the smallest influence on the overall volatile profile. Separation by water type appeared only in part of the analysis at the 2-minute point.
However, individual signals tentatively assigned to methanol, acetaldehyde, dimethyl sulfide and pentenal showed significant differences between waters when broken leaves were used. These signals were higher with the harder water.
The authors consider effects of minerals on extraction from leaves and on the transfer of compounds from liquid to gas. They did not establish a single mechanism that explained the results. What the experiment showed was that even though the overall difference between the two waters was small, individual compounds could respond differently.
Different brewing methods can approach similar compound profiles
An analysis that grouped similar samples placed intact leaves brewed at 80°C for 30 seconds, 70°C for 1 minute and 60°C for 1.5 minutes in the same group. This group contained early samples with relatively low extraction levels.
This shows that different combinations of temperature and time can produce similar patterns of compound release. The authors suggest that such comparisons could help develop products in different formats, including tea bags and capsules.
The study did not include tasting tests to assess aroma or liking. Nor did it compare the entire cup, including bitterness, astringency and umami. Conditions grouped together therefore cannot simply be used as brewing instructions guaranteed to produce the same flavor.
The experiment also involved just one green tea, brewed with gentle stirring. Whether the same timings apply to sencha, black tea, an undisturbed teapot or other waters would need separate investigation.
Conclusion
In this study of gunpowder green tea, breaking the leaves made a larger difference at the beginning, while temperature-related differences in compound levels became more apparent with time. Leaf size influenced not only the initial rate of extraction but also how compound levels changed over time. Water mineralization had a comparatively small effect, although some compounds differed when broken leaves were used.
The researchers also identified combinations of temperature and time that produced similar compound profiles. Considering leaf form, waiting time and water together, rather than temperature in isolation, gives a clearer picture of what happens as tea brews.
Funding came from PIMMS ITN, supported by the European Commission’s Seventh Framework Programme under grant 287382. The authors declared no conflicts of interest concerning this study. The tea and commercial waters are identified to specify the experimental conditions, not because the study ranked brands by quality.
References
- Sánchez-López, J. A., Yener, S., Smrke, S., Märk, T. D., Bonn, G., Zimmermann, R., Biasioli, F., & Yeretzian, C. (2020). Extraction kinetics of tea aroma compounds as a function brewing temperature, leaf size and water hardness. Flavour and Fragrance Journal, 35(4), 365–375. DOI and original article / Full-text university repository page.
