What follows is a heaping dose of conjecture. It is the product of a deductive line of reasoning from certain know facts and, before any firm conclusions can be offered, the scientific research must be completed. Nonetheless, I believe this is how it operates.
The “it” I am talking about is pectin and its role in the structure of brewed tea. The richness of a tea’s “mouth feel” is, in large part, a function of the level of pectin in the tea; the more pectin the richer and more viscous the tea.
What is pectin?
Pectin is a polysaccharide that is found in in most fruits and is especially abundant in apples and citrus fruits. But pectin is also a substance that is found in the leaves of all terrestrial plants. It is important in providing the strength and also the flexibility to the cellular walls in leaves of these plants. The principal role of a cell wall is to give strength to the plant and to act as a barrier to the outside environment. The main function of pectin is to participate in these two tasks together with other cell wall polymers.
Pectin is something that most of us have eaten in our jams and jellies and, if you are as old as I, it is something that you remember having added to the mixture to make grape jam (or strawberry jam etc.) I remember the hours spent pressing grapes through a cheesecloth to extract the juice and then adding pectin so that a fine jam resulted. I was, and remain, a loyal devotee of grape jam.
But why is pectin so important an addition to fruit jams that otherwise would contain so little? Simply, it is a thickening agent and a stabilizer that is responsible for the gelling of jams that would otherwise remain as juices. I am suggesting that pectin has a similar effect in tea creating the “thickness” that we feel in the mouth when we consume fine teas. It follows, then, that, up to a certain unspecified point, the more pectin in the tea broth, the richer the mouthfeel of the tea.
Oh, don’t be thick
Why should the level of pectin in tea leaves vary? I submit that this variance is primarily a function of the thickness of the tea leaf; the thicker the leaf the longer the cell walls of that leaf hence the more pectin in the leaf.
Keeping your cool!
Leaf thickness is a function of light intensity and temperature and both of these vary with elevation. The higher the elevation, the cooler the microclimate of the growing area. Tea plants grown at higher elevation do have thicker leaves than those grown at lower elevations, they have higher levels of pectin and they tend to have a much richer mouth feel in consequence. The slight loss in floral aromatics is more than compensated for by the richer and more complex structure of the brewed tea.
We notice a similar phenomenon at lower elevations when the weather has been cooler. The leaves are thicker and the level of pectin higher.
Seeing the light
A like pattern attends to the effects of light intensity. We know that light intensity, particularly UV light, is greater at higher elevations. We also know that the thickness of leaves in general, and tea leaves in particular, varies with the intensity of the sunlight they receive. It follows then, that we would expect to find thicker leaves at higher elevations and, indeed, we do. These thicker leaves, as already noted, contain more pectin and this results in a richer character to the tea.
The conclusion to be drawn is that the level of pectin, which accounts for the rich mouth feel of teas, varies according to the elevation at which tea plants are grown and this is why these higher grown teas tend to have a richer, more complex structure than lower grown teas. This variance in structure and complexity also leads to variances in tea pricing with high grown teas being generally more expensive than low grown tea.
Is all of this just so? Perhaps not. It is perfectly possible that my line of reasoning is completely off base. But I don’t think so and until such time as my hypotheses are disproven, I’m going with what I’ve got.
That’s my take. What’s your view?
 The literature on pectin in tea is sparse. See Ele-Ekouna, Jean-Pierre et.al “chemical Characterization of Pectin from Green Tea (Camellia Sinensis)” Carbohydrate Polymers, vol. 83 no. 3 January 2011 pp. 1232 – 1239; Xia, T. et. al., “Impact of ultrasonic-assisted extraction on the chemical and sensory quality of tea infusion” Journal of Food Engineering, v.74, no. 4 2006 pp. 557 – 560 among a very few others. Most studies of pectin in tea examine the effect that pectin has on the anti-oxidant properties of tea (it doesn’t.)
 This line of reasoning was first suggested to me by Chen Kuan-Lin, 2018, private correspondence.
 Harholt, J. et.al. “Biosynthesis of Pectin” Plant Physiology 2010 vol. 153 pp. 384 – 395
 Coneva, V and D. Chitwood “Genetic and Developmental Basis for Leaf Thickness in Arabidopsis Cvi Ecotype” Frontiers in Plant Science, 2018 v. 14
 Fuchinoue, H. “On the Effects of Shading upon the Characters of Tea Leaf” 1961 working paper, Saitama Tea Experiment Station
 This is clear to anyone who has noticed how easily it is to get sunburned at high elevation.
 Fuchinoue, op. cit.