Chemical Composition of Tea

Tea, an ancient elixir celebrated worldwide for its diverse flavours, holds within its delicate leaves a symphony of thousands of chemical compounds. From the moment tea leaves are plucked until they dance in your cup, an intricate ballet of chemical transformations unfolds continuously, giving rise to the nuanced aromas and flavours we savour. In this article, we embark on a journey through the chemical composition of tea leaves, exploring the key players in this complex orchestra.

Photo: Gwangyang, South Korea. Katrina Wild.

Tea Leaf Processing: The Ballet of Transformation

On the bush, tea leaves contain thousands of chemical compounds, which significantly change and form new compounds during the processing. Tea leaves, comprising mostly water, undergo a transformative journey crucial to production upon plucking when they begin to lose water. Then, withering initiates a cascade of oxidation reactions as cell walls break down and react with oxygen. Through centuries, producers mastered controlled withering and oxidation, crafting teas with desired qualities while balancing science and tradition. Generalising taste based on specific chemicals becomes complex. Thus, important to note that terroirs, varietals and cultivars (i.e., genetics of each plant), processing methods, cultural influences, among other factors play an essential role in what kind of chemical composition we will end up with in our cups. Even brewing methods and water choices will result in a unique infusion with its own tapestry of chemistry. Recent scientific insights unveiled key compounds like polyphenols, amino acids, enzymes, pigments, carbohydrates, minerals, methylxanthines, as well as volatile flavour and aroma compounds.

The Key Players: Polyphenols, Amino Acids, Enzymes, and More

Photo: Wazuka, Japan. Katrina Wild.

Polyphenols: The Guardians of Taste and Health

Polyphenols, the guardians of taste and health in tea, steal the spotlight in the realm of this steeped beverage. Comprising 30-40% of freshly plucked tea leaves, these compounds, categorised as polyphenols due to their many phenolic groups, impart astringency and bitterness to tea liquor. Derived from amino acids under sunlight, polyphenols serve as tea's natural defence against insects, bacteria, fungus, and other pests and illnesses. Therefore, tea grown in the shade has a smaller concentration of polyphenols and a higher concentration of amino acids. The bud and first leaf have the highest concentration of polyphenols and polyphenol levels decrease in each leaf moving down the plant. There are an estimated 30,000 polyphenolic compounds in tea, flavonoids are arguably the most important group of polyphenols in tea and are the source of the many health claims surrounding tea, and specifically tea antioxidants. Other smaller subgroups include flavanols, phenolic acids and tannins.

Flavonoids, including vital catechins, dominate the polyphenolic landscape. The major flavonoids in tea are: catechin (C), epicatechin (EC), epicatechin gallate (ECG), gallocatechin (GC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG). EGCG is the most active of the catechins, and this flavanol is often the subject of studies regarding tea antioxidants. In addition, tea catechins are powerful antioxidants that neutralise the action of free radicals. They contribute to the binding of various toxic substances and their removal from the body.

Flavanols transform into theaflavins and thearubigins during oxidation, lending dark colour and robust flavours to oxidised teas. Green tea, rich in catechins, boasts antioxidant properties and health benefits, while black tea, featuring theaflavins and thearubigins. Catechins in tea contribute to the quality and properties of the infusion, offering benefits ranging from cardiovascular health to immunostimulation.

Tannins, often misunderstood, are a subclass of bitter polyphenols found in tea. Though historically associated with tanning agents in wood to tan animal skins, they contribute to the astringent, dry and bitter flavours of tea.

Indulging in tea, particularly those abundant in polyphenols, can hinder the absorption of iron, potentially resulting in iron deficiency when consumed excessively. So, tea enthusiasts, be mindful and consider taking supplements if you experience the effects of iron deficiency!

Amino Acids: The Sweet Harmony of Umami

Amino acids, the fundamental building blocks of proteins, contribute to the exquisite taste of tea, adding sweetness and umami. Theanine, a prominent amino acid in tea, also known as L-theanine, not only enhances flavour but also induces a unique tea-drinking experience by promoting physical and mental relaxation and mindful alertness when combined with caffeine, increasing the alpha brainwave activity. In fact, L-theanine is quite a rare amino acid in the plant kingdom. It was discovered in green tea as recently as 1949, and to this date it has only been identified in 3 species of plant and fungus: Camellia sinensis, Ilex guayusa tisane and a species of bay bolete mushroom native to North America and Europe. Sunlight drives conversion of polyphenols to amino acids, hence, shaded tea plants result in a more sweet and savoury taste higher in amino acids. Good examples of deliberate shading of tea bushes would be some Japanese green teas: gyokuro, matcha, and kabuse sencha.

Photo: Hadong, South Korea. Katrina Wild.

Enzymes: Architects of Oxidation

Polyphenol oxidase and peroxidase, the key enzymes in tea leaves, play a central role in oxidation, orchestrating the transformation of flavour, colour, and aroma. Breaking down polyphenols when cell walls are breached and exposed to oxygen, these enzymes trigger the browning process, i.e., oxidation, similarly when we bite into an apple it turns from yellow to brown. In green tea production, heat prevents browning by deactivating these enzymes (either steaming or pan-frying, known as kill-green process), keeping leaves green and fresh, while in white tea, moisture deprivation achieves the same effect during long withering. Although these enzymes may not directly impact the tea's taste, they drive the transition to oolong or black tea, converting polyphenols into theaflavins and thearubigans, altering the leaf color from green to brown—an intricate dance in the alchemy of tea processing.

Photo: Chlorophyll is concentrated in structures called chloroplasts seen through a microscope within plant cells. Kristian Peters.

Pigments: Painting the Palette of Colour

Plant pigments are responsible for absorbing light for photosynthesis. Tea's vibrant palette of colours is painted by chlorophylls and carotenoids, the primary pigments in fresh tea leaves. During withering and oxidation, these pigments undergo transformations that influence the hues of the final tea.

Chlorophylls, responsible for the green colour, degrade during oxidation, contributing to the dark appearance of oxidised teas. The first products that chloroplasts make from carbon dioxide and water are called isoprenes, which using oxygen to start forming terpenes and terpenoids, the compounds delivering delicious aromas. In shade, chlorophylls collect much less light energy, so photosynthesis decreases and chloroplasts shift away from making sugars and sugar backbones to degrading chloroplast proteins and thereby increasing amino acid content, phenylalanine in particular, which in sunlight is converted to catechins. When shaded, phenylalanine is converted to aroma compounds like benzaldehyde (stone fruit and almond aroma) and 2-phenylethanol (sweetness of rose and honey).

Carotenoids, including orange (carotenoids) and yellow (xanthophylls) components, play a crucial role in the coloration of finished tea leaves. Carotenoids also contribute to flavour in the cup, with compounds like damascenone providing sweetness. Theaflavins introduce a yellowish-brown-red tint, thearubigins impart a reddish-brown hue, flavonol glycosides contribute to a light yellow shade, while pheophorbide and pheophytin add brown and black tones, respectively.

Carbohydrates: Fuels of Flavour

All plants store energy formed during photosynthesis in starches and sugars, otherwise known as carbohydrates. Carbohydrates serve as the fuel for flavour, storing essential energy for enzymatic reactions during oxidation and are also responsible for the creation of polyphenols in young tea leaves. Constituting approximately 11% of extract solids in steeped tea, carbohydrates, including glucose, fructose, sucrose, maltose, rhamnose, raffinose, and stachyose, play a vital role in shaping the tea's sweetness.

Photo: Doi Mu Puen, Northern Thailand. Katrina Wild.

Methylxanthines: Stimulating the Senses

Caffeine, the primary methylxanthine, defines the stimulating nature of tea and belong to the alkaloids. Together with theobromine and theophylline, forming a natural defence mechanism for the tea plant against insects and pests. On average, methylxanthines in tea leaves make up 2% to 5% of the dry weight of the fresh leaves. While contributing to the bitter taste of the infusion, caffeine levels vary based on factors like tea variety, climate, leaf age, and propagation and processing methods. It is great fun to play with chemical composition altering by simply using different brewing techniques: for instance, cold brewing reduces the caffeine content of a tea and noticeably extracts the sweet compounds over the bitter and astringent (catechins) ones. Although, the longer you infuse it, the more caffeine gets extracted as the low temperatures just slow down the extraction.

Photo: Wistaria Tea House Taipei, Taiwan. Katrina Wild.

Minerals and Vitamins: The Silent Supporters

Tea leaves, a treasure trove of nutritional elements, boast a rich mineral profile comprising 28 elements like fluorine, manganese, arsenic, and potassium. Fluorine, renowned for its role in dental health, highlights the diverse mineral composition within tea. Levels of these minerals fluctuate with each harvest, influencing the taste and health properties of the final brew. Tea stands out among other plants with its elevated concentrations of essential elements like nickel, selenium, iodine, and aluminium. Additionally, tea harbours a spectrum of vital vitamins, including provitamin A-carotene, B vitamins, and ascorbic acid, contributing to eye health, nervous system function, skin and hair improvement, and immune system support. Green tea, in particular, has some vitamin C content also, although depleted and lost when brewed in water above 30°C.

Photo: Chin Xin Gan Zhi Pre-Qing Ming Taiwanese White Tea, Illuseum Tea Room, Riga, Latvia. Katrina Wild.

Volatiles: Aromatic Alchemists

Volatile compounds, making up a mere 0.01% of dry tea leaves, are the aromatic architects behind tea's unique bouquet. Originating from both fresh leaves and processing, hundreds or even thousands of flavour and aroma compounds, such as linalool, geraniol, and benzaldehyde among many others, form the aroma complex that defines the sensory tapestry of each tea. Many of these aromatic compounds do not exist in fresh tea leaves and are derived during processing. While only a small fraction of the tea's weight, these compounds play a significant role in crafting the flavour and aromatic symphony of the drink, reaching our olfactory system as vapour. Sometimes the aroma complexes are divided in primary aroma (from fresh tea leaves) and secondary aroma (products of processing). In 1990 Owuor and colleagues classified the volatile flavour compounds in tea into other two groups: Group I volatiles were derived primarily from cell membranes and included the hex's and aldehydes with higher carbon numbers such as heptanal and nonanal. Group II were compounds with a floral aroma, such as linalool and nerolidol, and higher concentration of these would mean higher quality tea.

Compounds such as linalool (citrus and floral) and linalool oxide (floral and earthy) are responsible for floral notes and sweetness; geraniol (rose, geranium) and phenylacetaldehyde (hyacinth, lilac) are responsible for floral aromas; nerolidol (fresh bark), benzaldehyde (almond), and methyl salicylate (root beer, mint, wintergreen) are responsible for fruity flavours; pentanal represents almond, nutty, cacao, and malty notes; a-linolenic is the starting point of jasmine aroma; butyrates and butanoates stand for apple aroma; coumarin for sweet new-mown hay flavours; b-damascenone for rose, plum, berry, and tobacco; strawberry furanone for pineapple and strawberry; farnesene for lavender and green citrus; safranal for saffron notes; theaspirone/theaspirane for tea-like, woody, cool, herbal, tobacco, minty quality; geranyl acetone for cooling green rose with a waxy quality and tropical fruit; and trans-2-hexenal (apple, peach, red fruits, plum and tomato), n-hexanal (freshly cut grass and green peas), cis-3-hexenol (herbaceous and grassy), and b-ionone (violets) are responsible for a tea’s fresh flavour.

Ambient temperature, altitude, harvest time and soil quality are factors among many others that impact the quality of aroma and flavour of tea. For instance, slower growth yields less leaves in higher altitudes accompanied with fog and temperature contrasts, and yet produces higher quality tea abundant with aroma chemical precursors with sweet, floral, and honey-like notes. Tea from plants grown in acidic soils with high iron content, for example Wuyi mountains in China, may have a mineral character, which comes from compounds like kaempferol and quercetin. If you have heard the poetic Chinese term of Yán Yùn, or 岩韵, it means the aroma of "cliff melody/essence/rhyme" that describes yancha. Well, kaempferol might be behind the Yán Yùn effect of Wuyi rock tea. In terms of seasonality, first flushes and pre-Qingming have higher levels of theanine and aroma compounds. You might have heard of Dong Fang Mei Ren's (Oriental Beauty) production entailing intentional insect bites. The leaves respond to the attack by making two compounds, hotrienol and 2,6-dimethyl-3,7-octadiene-2,6-diol (DOD) as a bug repellant. Both of those compounds have a muscatel aroma and increase when processing the leaf. Tea's fluffy hairs called trichomes have small globule of essential oils at their base, which provides a number of compounds with floral and somewhat spicy flavours. The list of describing magic of aroma in tea could go indefinitely! Ongoing research into tea volatiles promises further insights into our olfaction system and the intricate world of aromatic alchemy within tea.

The Concluding Notes

In the alchemy of tea, the journey from the bush to the cup reveals a chemical tapestry woven by nature's hand and human craftsmanship. The complexity of tea chemistry, a fascinating mosaic of compounds, awaits further exploration. As tea gains popularity, the scientific community endeavours to unveil the still mysterious aspects of tea leaves, promising a clearer understanding of this wonderful beverage. The next time you savour the richness of your favourite tea, remember that you're not just sipping a beverage but partaking in the harmony of a chemical masterpiece. Cheers to the chemical wonders steeped in every cup of tea!

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