Tuesday, March 27, 2007

The Role of Stress in Tea Growth and Manufacture


Stresses & Their Relation to Flavour

Let us have a look at some of the factors observed by tea planters that give more flavorful tea.

• High-elevation tea bushes give more flavour
• Non-fertilised tea bushes give more flavour
• Tea bushes on rocky soils give more flavour
• A slow-growth period gives more flavour than a free-flush period
• Frost-damaged tea leaves give more flavour. Tea bushes affected by frost injury, have metabolites such as pyruvate, acetaldehyde, and ethanol accumulated in the leaves of tea manufactured from frost injury leaves gives better flavour.
• Insect-damaged flush in general gives more flavour
• Droughty conditions give more flavour

Stress Exerted on the Tea Leaf

My professional training in engineering led me to surmise that the common factor in all the situations listed above is stress. The stress exerted on a tea leaf can be of various types, as follows:

• Low-temperature stress (as found in high-elevation teas)
• Nutrition stress (as in the case of underfertilised teas, or teas grown on bad soils like rocky soils.
• Ultraviolet radiation stress: UV radiation is more concentrated in high elevation and on clear sky days. In India, it is also highest in south-facing sections of Darjeeling, and least in north-facing sections. Due to this factor only, Darjeeling’s sequence of initial or first of First Flushes follows the acronym “NEWS”: i.e. north-facing sections will flush first (since north-facing sections initially receive the least sunlight, and thus also the least UV rays), followed secondly by east-facing sections, followed thirdly by west-facing sections. Fourth and last to flush are the south-facing sections, due to the UV rays of the sun during the sunshine hours.
• Moisture-related stress, as in case of droughty conditions
• Mechanical-damage stress, as in case of frost-damaged leaf or insect-damaged leaf
• (And so on).

But the following fact must be kept in mind: Any time the tea plant faces stresses -- biotic and/or abiotic -- it tries to adjust itself to these: outwardly, by slowing the growth of the tea flush; and inwardly, in the quality and quantity of the biochemicals present in the tea leaf. If the stresses continue, the growing apical bud stops growing and thus becomes what in India is called a Banjhi Bud. (This term is derived from the Hindi word Banjh, which means a woman who can not have children: a non-fertile woman.)

I knew that the stress was the agent that causes the increase in a tea's flavour, but I did not know exactly how this occurred.

Long ago I had discussion on this topic with a Japanese scientist, Professor Kanzo Sakata of Kyoto University. It has now been proven by Professor Sakata and his team that stresses during processing, while the leaf is alive, induce the expression of various new genes in the tea-plant. These induced genes result in the enhancement of the aroma formation of oolong teas. (ref: Proceedings of the 2005 International Symposium on Innovation in Tea Science and Sustainable Development in Tea Industry, pp. 541-545)

Moisture Stress on the Plucked Leaf

When the moisture content in the tea-leaf is reduced to around 70%, the moisture stress developed in the leaf produces reactive species of oxygen such as super-oxides, peroxides, and other free radicals. If these do damage to the cell membranes, the cell structure will begin to deteriorate.

There is a minimum critical moisture level below which the cells of the plucked tea shoot cannot survive. The best telltale sign of the dead tea-leaf cell is that it not able to take atmospheric oxygen for respiration. Very hard withers will lower the moisture-level in tea-leaf cells to the point that some or all of them will die.



corax said...

and thanks to you, michael, for visiting CHA DAO, and for your [characteristically] thoughtful comments. from what i have seen of sakata's research [just summaries, actually], you are spot-on about the leaf-hoppers and 'oriental beauty.' it seems that the 'insect attacks' [as i recall sakata terming them] cause genetic -- i.e. mutational -- response in the plant, which is rather stunning. i wonder what these chemical reactions 'do' for the plant from an adaptive standpoint. do they produce a flavor that repels the leaf-hopper, thereby halting wholesale destruction of the leaf?

and a separate question: would other types of tea -- hong cha for example -- made from these nibbled leaves not taste nearly as good to us?

like you, i am happily awaiting SMC's next instalment.

anodyne said...

This brings to mind the "noble rot" (Botrytis cinerea)a fungus that affects wine grapes. From what I understand, ripe grapes are susceptible to this under moist conditions. With too much prolonged moisture, it's a disaster. But under the right drying conditions, the grapes become raisiny and those picked at just the right time produce those concentrated intense sweeter wines. There are different versions of how it was discovered, but it may have been an accidental discovery.

anodyne said...

And I also wonder then if the Nilgiri "Frost Teas" (Chamraj, Havukal, Kairbetta, and Burnside Estates) I have posted on at length elsewhere fit into this category of stress in tea manufacture? As the story goes, "to capture the flavor for which this clone is renowned, [the] manufacture of these buds, after withering, was done under extreme wintry conditions."

More info at:



corax said...

anodyne, i think you are spot-on here regarding these forms of stress. i have long thought that the 'frost teas' were analogous to what the germans call eiswein; but i had never thought about the 'noble rot 'connection until you mentioned it here. thanks!

Gabi Greve said...

Hi all,
in Europe we have some grapes that grow tastier whith mold on them ...
and the frost wines, from frozen grapes ...

stress for the plants ... quite a thought

GABI, a medical doctor

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Anonymous said...

Thank you SMC for your interesting and thought-provoking article. I'm curious for the continuation...
anodyne, could you send a link to your post on Nilgiri frost teas. It is quite an elusive kind of tea. Many highly reputable tea vendors have never heard of them (answers as: "Frost and tea are mutually exclusive" are quite common) and I'm trying to find out more about them.

anodyne said...

If you go to the link below and search under Bespoke Teas it will bring up my July 24, 2002 tasting notes for the Nilgiri Frost Teas when I first encountered them:


And here is the 2003 tasting notes of the "Frost Teas":

Drinking the 2003 Havukal Breakfast BOP Nilgiri (www.teasociety.org) which, just as when I taste the 02, still strikes me as the most "puckery" of the Bespoke teas, even perhaps slightly more than the Kairbetta. It has that outstanding aroma, reminiscent of the Kairbetta with honey sweet and rich floral-green pungency. Slightly deeper green-nutty notes. Like the Kairbetta BOP, you have to cut back on leaf amount and brewing times to avoid an overly astringent cup, but if you hit the happy medium, the puckery green quality is quite enticing. Like the Kairbetta Breakfast BOP, this is one I do enjoy with milk and sweetener which serves to bring out that honeyed note. Those who dislike a tea with distinct pungency may not go for this one, esp if you overbrew it or use too much leaf.

The larger leaf Havukal brews up to a tea with lighter liquor but still has the puckery quality, not quite as aggressively as the BOP version, but distinctly a part of the tea's profile. The website tasting notes refer to a hint of "artichoke" and I guess that's what I experience as green-nutty. For some reason, I've not eaten enough plain artichoke to have this well situated in my memory banks. This one I like to drink straight up. Liquor would be too light for milk, though the BOP version is well suited to that.

The Chamraj Estate Nilgiri 2003 Frost Tea falls out rather where I remember the 2002 one. It has that enticing aroma that reminds me more of the Kairbetta and Havukal Estate than the Burnside--a very intense aroma that makes me think floral-green-honey, only with a deeper underlying layer of aroma that strikes me as rather fruity-musky today. Very, very aromatic. This tea retains something of the puckery character of the Kairbetta and Havukal, but it comes across less aggressively (even up to a four minute brewing here). As I noted in 2002, I really like the match of aromas/flavor in this one. I had forgotten that the aroma was this pervasive and had somehow "softened" it more in my memory than it really is. But if one likes the aromas of the Kairbetta and Havukal but wants a slightly less puckery experience, this is the one (though it is not as mellow as the Burnside). I had favored the Kaibetta BOP for the tea to drink with milk and sweetener, but for my drinking "straight up" experience, this one is definitely the winner for my own tastes. If one favors a very fragrant Darjeeling with some toasty depths, I'd think this Nilgiri tea might be quite appealing as well. I didn't quite remember it having this level of depth in taste/aroma. I am pleasantly surprised.

Not to be confused with the aforementioned "Bespoke" Frost Teas (that include the estates Kairbetta, Chamraj, Havukal, and Burnside in the Nilgiri region--info at www.teasociety.org), this Simpson and Vail Nilgiri "Frost Tea" is listed as the Welbeck Estate, located, according to the S and V website (www.svtea.com), "in the lofty mountains of the famous Highlands of the Nilgiris, in the southernmost region of India, at altitudes of about 6500 feet." While I don't know for sure, I am assuming that other Nilgiri estates might be using the same type of processing with their teas as those specific ones listed as "Bespoke Teas."

At least in the case of the Bespoke Teas, the teas were produced in January of the year. And I am assuming that the term "Frost Tea" reflects this. For example: This particular tea (referring to the Bespoke Kairbetta Estate) is from "five-day-old buds" which were harvested on January 12, 02 on a "clear bright day with temperatures ranging from 48-53F." From what I understand in the brochure, the flavor was enhanced by having been processed under "wintry conditions."

I had a chance to taste the Welbeck Frost tea thanks to a fellow tea drinker who had a sample sent to me.

Like the Bespoke Burnside Estate Frost Tea (from www.teasociety.org), the leaf of the Welbeck Estate Nilgiri (www.svtea.com) is bold, large and twisted. The liquor of the tea is an amber color. The aroma strikes me as being rather floral-malty with spicy notes. It remains quite mild and mellow in the cup. In its aroma/flavor profile, it is decidedly more like the Bespoke Burnside Estate tea than, for instance, the more pungent green floral and honey Kairbetta or Havukal estate teas or the softer floral Chamraj Estate from the Bespoke teas.

I had just purchased a sample of the 2003 Burnside Estate Nilgiri (one of the Bespoke Teas from www.teasociety.org), and it also has the bold twisted leaf. Drinking the two cup-to-cup here both made with 3.2 grams of leaf for comparison purpose: The Burnside has a slightly more pronounced spicy-toasty aroma than the Welbeck. The Welbeck is floral-spicy, but the Burnside has a more honey and spice character in aroma with floral/toasty underlying that. They do have a similar character, but the Welbeck, while still quite aromatic, comes across as slightly more muted (both aroma/flavors) and less honey'd than the Burnside, without quite the level of "toasty" I find in the Burnside. As I noted when I tasted the 2002 Burnside, it has a light malty character to the cup against the floral-honey-spice aroma.

The Burnside was the least pungent green of all the Bespoke Nilgiri Frost Teas, especially as compared to the Kairbetta and Havukal. The Chamraj fell in-between with a softer floral range, not quite as pungent, but not with the malty notes of the Burnside (which are also reflected in the Simpson and Vail Welbeck Estate). Both the Burnside and the Welbeck are mellower than the others, and I think this made some folks favor this particular style of Nilgiri Frost tea. I myself preferred the ones that were more flamboyantly and aggressively green/pungent/floral/honey, especially the Kairbetta Estate ones.

Interesting that when the water first hits the leaf of the Burnside and Welbeck Nilgiri teas, the two teas smell rather similar. As they steep, they change slightly, and more so after they are poured out, though the retain a very similar character. Incidentally, I tasted both teas "blind" so that I only knew which was which after making notes. Both are quite aromatic teas, the Burnside giving itself a touch more honey-toasty quality. Seems like the Burnside just fills in a honey'd note in the chord that isn't quite as there in the Welbeck, giving it a touch more depth and complexity. The Welbeck does develop more in the aroma as it cools slightly, edging toward that toasty note. Seems as though the Burnside has a touch more malty quality in the cup that lends a bit of depth to the taste--you notice that especially as they cool down a bit. If you like one, you would assuredly like the other.