Liquid tea gets its (wonderful) taste and aroma by infusing tea leaves in water, mostly hot but cold infusions are possible, for a set amount of time. Basically, substances are dissolved in water to elicit aromas, flavors and texture to create that beverage that captivates and inebriates.
The finished tea leaf has literally hundreds of compounds in it that are extracted by the act of brewing tea, or more accurately, infusing or steeping the leaves in water.
The Taste of Tea- Chemical Composition
Group Compound Taste
Tea polyphenols
Tea polyphenols, Catechins, Flavones Bitter & astringentEster type Catechins More pronounced bitterness & astringency
Gallocatechin Astringency
Epicatechin Mildly sweet aftertaste
Anthocyans Bitter
Gallic acid Sour & astringent
Amino acid
L-theanine Brisk & sweetGlutamic acid, aspartic acid, glutamine, asparagine Brisk, sweet & sour
Glycine, Alanine, Serine Sweet
Arginine Sweet with bitter aftertaste
Others
Dissolvable Carbohydrates SweetPectin Tasteless but gives body
Caffeine Bitter
These are just a handful of the hundreds of compounds in tea and their corresponding taste.
Apart from taste and aftertaste, aroma also plays an integral part in our enjoyment of tea. There are more than 500 identified volatile aroma compounds in tea.
Type Composition (Weight)
Raw leaf 0.03-0.05%
Green tea leaf 0.005-0.01%
Black tea leaf 0.01-0.03%
Some of these aroma compounds are naturally occurring, others form during processing, such as the ‘zuo qing’ stage in production of oolong tea (see chapter 3), others are reduced during processing- such as cis-3-Hexen-1-ol which gives rise to that grassy smell, is evaporated during processing, notably ‘shaqing’.
For example, the cinnamon (rougui) fragrance in certain types of oolong tea is given by the cinnamic aldehyde compound which is formed during ‘zuo qing’.
Therefore, the act of infusing tea is to dissolve taste- including sensations and aftertaste- and fragrance contributing compounds to form a balance that pleases the drinker.
For example I may not like bitterness (caffeine and polyphenols) on its own, but when balanced with sweetness (amino acids and tea polysaccharides), it forms a bittersweet taste that is intriguing and delightful. Too much of sweetness though, is unsatisfying.
The art of brewing tea therefore can be viewed as akin to extracting these compounds, much like a chef mixes condiments and sauces to blend the perfect taste.
Let us look at how these compounds are dissolved:
Temperature
It does not take a chemistry major to tell you that stuff dissolves faster in hot water, whether it is instant coffee (oh the sacrilege :p), cocoa, sugar or just about every soluble item, even to our naked eye, there is a discernible difference in how fast in dissolves when comparing boiling water to water at room temperature.
To be more specific, we will look at the effect of water temperature on the rate of solution of amino acid and polyphenol compounds:
Impact of Water Temperature of Solubility of Compounds[1]
Compound
100⁰C/212⁰F
80⁰C/176⁰F
60⁰C/140⁰F
Quantity
%
Quantity
%
Quantity
%
Dissolvable compounds
16.66
100
13.43
80.61
7.49
44.96
Free Amino Acids
1.81
100
1.53
87.29
1.21
66.85
Polyphenol Compounds
9.33
100
6.70
71.81
4.31
46.20
As seen from the table above, free amino acid is more soluble at lower temperature as compared to most dissolvable compounds with 87.29% of what was dissolved at boiling temperature, dissolved in water at 80⁰C as compared with 80.61% for all dissolvable compounds. In contrast, only 71.81% of the total quantity of polyphenols that was dissolved at boiling temperature was dissolved at the same temperature.
Let us hold this thought for a moment.
Infusion Time
We can perform a simple experiment- if you drop a lump of sugar into your coffee and do nothing, over time it gets smaller, or in other words it dissolves. This illustrates a simple fact- the length of time an item is infused in water is proportional to the extent of solution.
Impact of infusion time on rate of solution of selected compounds in tea[2]
Compounds
3 minutes
5 minutes
10 minutes
Quantity
%
Quantity
%
Quantity
%
Dissolvable compounds
15.07
74.60
17.15
85.39
20.20
100
Free Amino Acids
1.53
77.66
1.74
88.32
1.97
100
Polyphenol Compounds
7.54
70.07
8.98
83.46
10.76
100
From the table, we can infer that all else being equal, free amino acids dissolve ‘faster than average’ while the reverse is true for polyphenol compounds.
Putting it together
Recall that polyphenols (among other compounds) contribute the bitterness and astringency in tea while amino acid provides the brisk and sweetness to counter that.
The ratio of polyphenol to amino acid is fundamental in producing a drink that tastes ‘good’- under the presumption that ‘good’ means as it does to most people, sweet and briskness that is more apparent than bitterness and astringency.
If the tea tastes overly bitter and astringent, there are two main options available:
i) Reduce the water temperature
ii) Reduce the infusion time
As amino acid dissolves faster and at lower temperatures than polyphenols compounds, for teas with higher content of polyphenols such as green tea, reducing the temperature is an excellent way of reducing the bitterness and astringency.
As with most cases, there is a trade-off but first let us look at other variables in place.
See more articles related to brewing tea
[1] Table extracted and translated from 茶与健康 edited by屠幼英 page 253
[2] Table extracted and translated from 茶与健康 edited by屠幼英 page 254
Author’s Note
Above is an extract from an (as yet) unfinished draft of an upcoming book on Oolong Tea where we would cover more than 30 varieties of oolong tea, history (not legends, history), production, brewing and appreciation, the 4 main geographical areas, and a whole lot more.
If you are an interested agent/publisher, feel free to get in touch.