The Chemistry Behind Tea Flavor

This blog post was written by guest Jimmy Burridge, PhD in Plant Science and tea aficionado, with a burgeoning interest in the intersection of tea agronomy, chemistry and terroir (you can blame him for the confusing science parts!). 

 

Different types of tea can have very different flavor profiles. A sencha green tea, for example, may have a sweetness and a thickness on the tongue that you’ll never find in a black tea. A withered green tea may have a simplicity and clarity that are both refreshing and stimulating. Tea cultivars typically used for black tea become more aromatic during oxidisation, but Japanese black teas, typically made from cultivars very different from Indian or African teas, tend to be more delicate, with more floral overtones. Kamaricha and tamaryokucha are green teas produced with different processing methods from sencha and offer different taste and aroma-scapes to explore.

ABOVE: A first flush wakocha (black tea) from Ogura Tea Garden in Ashigara, Kanagawa contrasts with five spring sencha (from Yunomi’s Tea Dojo). BELOW: Tasting 5 different black teas from Kajihara Tea Garden.

 

Phytochemicals & Flavor

Phytochemicals have been a part of human life since time immemorial. Many of these phytochemicals are the basis of traditional and modern medicines, such as aspirin. The active ingredient in this common pain reliever comes from acetylsalicylic acid, a form of which was first derived from willow tree bark by the ancient Egyptians (Desborough and Keeling, 2017).

Other phytochemicals are fundamental aspects of flavors in food, such as the citric acid in citrus fruits. Around 400 volatile chemicals have been identified in tomatoes, with around 12 being particularly important, and then of course there’s lycopene, a type of carotenoid, that gives tomatoes their characteristic color (Petro‐Turza, 1986; Cheng et al., 2020; Tomatosphere, 2022). Carotenoids, present in particularly high quantities in carrots, are accessory pigments of chlorophyll. Both carotenoids and chlorophyll are very important components of tea aroma and taste. Transformation of carotenoids to other chemicals is one of the principal functions of oxidation, which helps to distinguish black teas.

Growth conditions, shading, pest pressure, fertilization, elevation, temperature dynamics, soil type, processing, aging, etc. can all effect phytochemical profiles and thus the taste and cup experience (Ahmed et al., 2013; Zhang et al., 2020; see previous article on climate change). Thousands of chemical compounds can be present in tea, and they can be transformed through the tea making process. When the finished tea is finally steeped, some of the more volatile phytochemicals are released in the aroma, while the non-volatile ones are contained in the tea liquor.

Chemical structures of three important phytochemicals present in tea. L-theanine is the amino acid primarily responsible for the umami flavor in tea. Bitter-tasting caffeine is a stimulant from the methylxanthine class, which actually functions by blocking a specific receptor in the brain, leading to enhanced production of a neurotransmitter. Damscenone is one many aromatic chemicals in black tea and is derived from the transformation of carotenoids. It belongs to the rose ketone chemical family, can exist in a variety of isoforms, contributes flavors ranging from fruity to floral to woody and is also found in bourbon.

Altering the Chemical Composition of Tea Leaves

Farmers and tea drinkers have known for centuries that certain growing conditions and harvest times produce tea with different flavor. Among the first to make the connection may have been the farmers in the Uji region who learned that shading the leaves produced a tea with more umami (previous article on shading).

Similarly, tea farmers and drinkers have also noticed that the first spring flush of tea often tastes particularly smooth with more umami compared to shibumi (astringency). Both of these phenomena are linked to less bitter-tasting catechin and tannin content and greater amino acid content, and specifically of the unique amino acid L-theanine.

Shaded tea production using traditional reed mats at Kuma Tea Gardens in Yame. Shading for a few days, or up to a few weeks for gyokuro, increases chlorophyll and amino acid content, reduces tannins and gives a sweet, mellow tea with more umami.

Amino acids are the basic building blocks proteins. More than 35 different amino acids have been identified in tea and particular amino acids have been linked to specific flavors, such as umami, as well as floral aromas and wine-like aromas (Li et al., 2022). L-theanine is about 50% of total amino acid content in tea leaves. L-theanine, an amino acid unique to tea and a specific mushroom, is responsible for tea’s calming sensation by increasing alpha brain waves according to some researchers and is suspected to have other health benefits (Juneja et al., 1999; Vuong et al., 2011).

We now know that the mechanism behind shading producing umami rich tea is that shading slows the transformation of amino acids to polyphenols. Slowing the transformation of amino acids results in tea leaves with higher amino acid content and thus more umami. Modern tools have similarly confirmed what the farmers have always known, that the highest concentration of L-theanine is typically in the first bud and leaf of a season (Li et al., 2022).

Oxidation is another example of how controlling natural chemical reactions leads to desirable tea qualities. As plant cell walls break down, either through withering or kneading, chemicals that had previously been protected by the cell wall come into contact with the atmosphere. These molecules then react with oxygen, in a progress called oxidation. Furthermore, living leaves, and even freshly harvested leaves have substantial enzymatic activity, which when left unchecked transforms much of the polyphenols, chlorophylls and carotenoids, darkening the leaves and changing the flavors.

Damascenone, present in many black teas but rarely in green teas is an interesting lesson in chemistry. Leaves with high carotenoid content can be carefully oxidized to transform the carotenoids to damascenone, contributing a sweet taste and smooth mouthfeel. Heating the leaves by steaming or pan-firing (pan-roasting) stops these oxidative and enzymatic processes, preserving the intense green color of the leaves. These different types of chemical reactions contribute to the sometimes dramatic difference in color, taste and mouthfeel among types of tea.

LEFT: A new planting of the less common Koshun cultivar at the Kaneroku Matusmoto Tea Garden, who specializes in black and smoked teas using the chagusaba method (World Heritage recognized agricultural system of traditional grass mulching). RIGHT: Handpicking Zairai cultivars at Kajihara Tea Garden. Zairai cultivars come from cross pollination rather than rooted cuttings and contribute interesting flavors to the finished tea, in this case, a kamaricha (a withered and pan fried green tea. Zairai discussed in this previous blog).

Leaves also have a certain amount of carbohydrates, in a variety of forms. These carbohydrates are used as an energy source by enzymes, another reason why stopping enzymatic activity, usually by steaming, is related to a sweeter finished tea.

Caffeine is a type of methylxanthine. Theobromine and theophylline are similar stimulant compounds also present in tea. They can contribute a bitter taste. The amount of each varies widely with cultivar, age of leaf and environment. A range of minerals are found in tea leaves and environment influences their relative abundance. Processing and drying of the tea can effect mineral bioavailability and influence flavor, aroma and mouthfeel.

Less than 0.1% of a tea leaf

Volatile chemicals constitute less than 0.1% of the weight of dried tea leaves, but they are largely responsible for the aroma and flavor. There are thousands of chemicals that interact with each other and change over time to form the complex aroma that we enjoy as tea enthusiasts.

With the aid of state-of-the-art instruments, scientists are increasingly able to quantify specific molecules in tea leaves that give different teas their characteristic flavors. Some of these measuring devices, including liquid and gas chromatography, can quantify the amount of a huge range of non-volatile (taste) and volatile (aroma) chemicals.

Other tools include spectroscopic and hyperspectral reflectance, which identify different chemicals by color differences (Yamashita et al., 2021). Mass spectrometry is another tool that works by measuring tiny differences in weight among the many molecules in a sample and is frequently applied to amino acid detection in tea research. Interestingly, these tools are sometimes employed in authenticating the origin or cultivar of a particular tea product (Engelhardt, 2020).

Soil and climate are major factors that determine which cultivars grow well in a particular area and what flavor the tea will have. LEFT: Kurihara Tea Garden in the mountains of Yame, on Kyushu, Japan’s southernmost island covered in snow during winter. RIGHT: A view of Furuichi Seicha’s lower elevation tea fields on the same island, but farther south in Kagoshima.

The Human Tongue

Modern scientific tools give a very detailed perspective on what is in the tea leaves and even what comes out in the aroma and tea liquor. However, the tools can’t quantify how particular combinations of myriad taste and aroma elements interact. It is this array of aroma, taste, mouthfeel and aftertaste that give the multi-faceted experience of enjoying a quality cup of tea.

The tongue can detect 5 taste qualities (sweet, sour, salty, bitter, umami) (Gravina et al., 2013), maybe 8 if you count fatty, spicy and fresh-minty as tastes. And then, of course, there are gradients and combinations. The human nose is far more sensitive, being able to differentiate 1 trillion different scents (Bushdid et al., 2016). Since the nose has a direct connection to the brain, it is particularly well suited to respond to faint scents wafting up from a freshly poured cup of tea.

Perhaps that direct connection is why smells can elicit such rapid, powerful and emotional responses. Just smelling something can take us back to some distant memory or give us a sense of calm and belonging. But maybe the L-theanine we just drank is also helping!

In conclusion, we’ve seen that cultivar, location, altitude, weather and harvest time can influence phytochemical profiles and the resulting taste and aroma. Master tea makers use drying and processing skills to further refine the tea’s flavor and aroma profiles and make more beautiful experiences possible. Enjoy!

 

References

• Ahmed, S., Peters, C. M., Chunlin, L., Meyer, R., Unachukwu, U., Litt, A., et al. (2013). Biodiversity and phytochemical quality in indigenous and state-supported tea management systems of Yunnan, China. Conserv. Lett. 6, 28–36. doi:10.1111/j.1755- 263X.2012.00269.x.
• Bushdid, C., Magnasco, M., Vosshall, L., and Keller, A. (2016). Humans can discriminate more than 1 trillion olfactory stimuli. Science (80-. ). 343, 1370–1372. doi:10.1126/science.124916.
• Cheng, G., Chang, P., Shen, Y., Wu, L., El-Sappah, A. H., Zhang, F., et al. (2020). Comparing the Flavor Characteristics of 71 Tomato (Solanum lycopersicum) Accessions in Central Shaanxi. Front. Plant Sci. 11. doi:10.3389/fpls.2020.586834.
• Desborough, M. J. R., and Keeling, D. M. (2017). The aspirin story – from willow to wonder drug. Br. J. Haematol. 177, 674–683. doi:10.1111/bjh.14520.
• Engelhardt, U. H. (2020). Tea chemistry – What do and what don’t we know? – A micro review. Food Res. Int. 132. doi:10.1016/j.foodres.2020.109120.
• Gravina, S. A., Yep, G. L., and Khan, M. (2013). Human biology of taste. Ann. Saudi Med. 33, 217–222. doi:10.5144/0256-4947.2013.217.
• Juneja, L. R., Chu, D. C., Okubo, T., Nagato, Y., and Yokogoshi, H. (1999). L-theanine - A unique amino acid of green tea and its relaxation effect in humans. Trends Food Sci. Technol. 10, 199–204. doi:10.1016/S0924-2244(99)00044-8.
• Li, M. Y., Liu, H. Y., Wu, D. T., Kenaan, A., Geng, F., Li, H. Bin, et al. (2022). L-Theanine: A Unique Functional Amino Acid in Tea (Camellia sinensis L.) With Multiple Health Benefits and Food Applications. Front. Nutr. 9, 1–12. doi:10.3389/fnut.2022.853846.
• Petro‐Turza, M. (1986). Flavor of tomato and tomato products. Food Rev. Int. 2, 309–351. doi:10.1080/87559128609540802.
• Plant pigments, Tomatosphere (2022). Lets Talk Sci. Available at: http://tomatosphere.letstalkscience.ca/Resources/library/ArticleId/4661/plant- pigments.aspx#:~:text=The red colour of tomato,to a carotenoid called lycopene. [Accessed July 31, 2022].
• Vuong, Q. V., Bowyer, M. C., and Roach, P. D. (2011). L-Theanine: Properties, synthesis and isolation from tea. J. Sci. Food Agric. 91, 1931–1939. doi:10.1002/jsfa.4373.
• Yamashita, H., Sonobe, R., Hirono, Y., Morita, A., and Ikka, T. (2021). Potential of spectroscopic analyses for non-destructive estimation of tea quality-related metabolites in fresh new leaves. Sci. Rep. 11, 1–11. doi:10.1038/s41598-021-83847-0.
• Zhang, L., Cao, Q. Q., Granato, D., Xu, Y. Q., and Ho, C. T. (2020). Association between chemistry and taste of tea: A review. Trends Food Sci. Technol. 101, 139–149. doi:10.1016/j.tifs.2020.05.015.