It’s no secret that shading tea plants has a drastic impact on the flavour of tea. After all, it’s what gives matcha and gyokuro their characteristic aromas, powerful umami, and vibrant green colour. But how does simply blocking sunlight, something that plants need to thrive, create such extreme (and delicious) changes?
In Part 1, we explored the origins of artificial shading and its current forms, but to understand exactly how and why tea plants react to shade like this, we’ll have to dive deep into the biology and biochemistry of the tea plant: Camellia Sinensis. While this subject can be immensely complex, I have simplified what I can and focussed primarily on the aspects that relate to tea’s final flavour, aroma, and appearance.
Tea and Sunlight
Before we learn how tea responds to intense shade, we must first see how the tea plant reacts to sunlight. Like all plants, Camellia Sinensis converts electromagnetic (light) energy into chemical energy through photosynthesis. In simple terms, this is the process of using the energy of sunlight to create sugars and starches by combining water from the ground and carbon dioxide from the air, with oxygen being produced as a byproduct. This process takes place in specialised cell organelles called chloroplasts which primarily use two types of molecules to collect solar energy:
- Chlorophylls a and b
- Carotenes and Carotenoids
Chlorophylls, especially chlorophyll a which is the most abundant form in the tea plant, are responsible for absorbing visible light in the red-orange spectrum and reflecting green light, giving tea (and most plants) their signature colour.
Carotenes and carotenoids (such as β-Carotene, found in abundance in carrots, but is also present in tea) capture blue light and UV light from the sun, and transfers this energy to the chlorophylls, boosting photosynthesis. When broken down in processing, carotenoids also produce some of tea’s nicest aromas.
Response to Excess Sunlight
While tea plants need sun to grow, too much sun can not only slow growth, but can even damage or kill the plant by breaking down chloroplasts and halting photosynthesis. In full sunlight, the plant already receives more sunlight than the chloroplasts can handle and has two primary ways of protecting itself from this excess energy:
- Decrease the amount of light harvested
- Convert excess light energy to heat
Decreasing The Amount Of Light Harvested
To decrease the amount of light hitting the leaves, the plant can hold its leaves upright, thereby decreasing the angles at which sunlight can hit the leaves. This is partially why many tea plants grown in full sun for sencha have upright shoots, and why some shaded tea plants have leaves that grow horizontally.
The plant can also decrease the amount of light harvested by moving the chloroplasts within the cell, facing them away from the sun. In tandem, the plant will also open stomata (small holes on the bottom of the leaf) to dissipate heat through transpiration, which has the effect of drying out the leaf.
Converting Excess Light Energy To Heat
The aforementioned carotenoids already capture a lot of the high energy UV rays from the sun and disperse it as heat, but tea plants produce another set of UV-absorbing chemicals: catechins. The building block for these catechins is phenylaniline, whose carbon ring can absorb UV light and dissipate the energy as heat, allowing these to act as a sort of sunscreen for the leaves. They are found in the highest concentrations in young buds and leaves at the top of the plant, as they receive the most sunlight¹.
Catechins are the main form of antioxidants found in green tea, most commonly in the forms of epicatechin (EC) and epigallocatechin (ECGC). Despite being good for your health, they are some of the primary bitter-tasting compounds in tea.
When tea leaves are oxidised, say to produce black tea, these simple catechins link together to form larger polyphenols, also called tannins, which give black teas their distinctive aromas, flavours and reddish colour.
Tea in the Shade
Now that you know how all of these chemicals interact with sunlight, you can understand what happens when we drastically limit the amount of light that hits the leaves, often by up to 95%.
Chlorophyll and Carotenes/Carotenoids
With less sunlight, photosynthesis becomes less efficient and the plant is forced to produce heaps of extra chloroplasts and chlorophyll to harvest more sunlight, up to three times more than is present in unshaded leaves. This super-concentration of chlorophyll gives shaded tea leaves their dark, vibrant green colour.
Additionally, more carotenes and carotenoids are produced in order to capture what little UV light passes through, and help the chlorophyll capture more energy. Earlier I mentioned that these carotenoids produce pleasant aromatic compounds. In green teas, some of the major carotenoid derivatives are:
- β-ionone: floral aroma that only about 50% of people can detect
- Nerolidol: fresh, green aroma
- Safranal: like its name suggests, a saffron aroma
- β-damascenone: a sweet rosy/fruity aroma
As the chloroplasts run low on glucose to break down for energy, they begin to break down some of the cell proteins into their component amino acids, thus increasing the concentration of amino acids, especially L-theanine, in the leaves. These amino acids are responsible for the sweet and savoury/umami flavours of green teas.
Another amino acid produced is methionine, which is the precursor to dimethyl sulphide, which imparts a ‘marine’ or ‘nori’ aroma.
As catechins are used as a sunscreen, they are not needed in shade. Instead, they too are broken down into their components, primarily the amino acid phenylaniline, which then is used to produce various other aromatic compounds that also contribute to the unique ‘shaded aroma’ or ooika (覆い香) of shaded green teas. These include:
- Benzaldehyde: almond/fruit aroma
- Benzyl alcohol: rosy/stonefruit aroma
- 2-phenylethanol: rosy aroma, but when combine with β-damascenone creates a honeyed aroma
The relative lack of catechins in shaded leaves is why you don't see shaded black or oolong teas. Without catechins to form polyphenols/tannins, the resulting teas would not have the characteristics desired in these tea styles.
Chemical Composition of Matcha, Gyokuro and Sencha
The effects of shading, while obvious in taste and aroma, can also be seen through chemical analysis of the leaves. Here examples of two shaded teas, matcha and gyokuro, are compared to a sample of sencha which is unshaded¹:
While these numbers are roughly representative of these types of tea, there is still a lot of variation that results from cultivars, terroir, fertiliser, and individual producers' processing preferences. For example, some of the highest grades of matcha have amino acids of over 10g/100g of tea.
If you haven't read Part 1, where I talk about shading's history and techniques, you can learn more here.
1. Goto, T., Yoshida, Y., Amano, I., Horie, H. (1996) Chemical composition of commercially available Japanese green tea. Foods & Food Ingred. J. Jpn. 170:46-52