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How Does Tea Brewing Actually Work?

August 18, 2017

‘Gongfu’ or ‘gongfu cha’ has come to loosely define a kind of tea brewing methodology, and is often the preferred method for many Chinese/Taiwanese teas. But why? What makes it good?


Let’s investigate what actually happens when tea is steeped, from a viewpoint of physics and chemistry.


Once the tea leaves are immersed in water, a simple process of osmotic diffusion occurs; chemicals and compounds on the surface as well in ruptured cells within the leaf slowly diffuse into the water, moving across the concentration gradient, from high to low. As the water surrounding the tea ‘fills up’ with compounds from the leaves, the rate of diffusion of those compounds slows, and theoretically eventually stops, as it reaches equilibrium between inside and outside the leaf.


How quickly a certain compound infuses into the solvent (water) is determined mostly by their molecular weight, i.e the lighter and/or smaller the compound, the more quickly it will diffuse into solution. There are three rough categories for soluble compounds in the tea leaf:


1) Effectively instantaneously soluble.

Certain volatile chemicals such as aroma compounds (responsible for smell and flavour) fall into this category.

2) Rapidly soluble.

Slightly heavier compounds; perhaps some carbohydrates and the lighter polyphenols/flavanols as well as caffeine, would likely fall into this category.

3) Slowly soluble.

The meatiest compounds; perhaps pigments, heavier polyphenols, flavanols, tannins, etc.


Looking at the above information, it could be determined that most of the chemicals responsible for flavour and smell infuse quite quickly, and there would be diminishing returns on aromatics the longer the tea is steeped. Compounds that contribute to sweetness, mouthfeel and texture infuse more slowly, while most of the astringent and bitter compounds (tannins etc) infuse the slowest.


This is in line with the general experience of tasting a tea as it progressively infuses; first one may notice the smell or aroma, as well as flavours, and as it continues to steep the tea gets stronger/sweeter/thicker/more intense. The tea will then reach a plateau, when it starts to only get more bitter/astringent, with little to no increase in aroma or flavour.


In terms of concentration of dry weight of a tea leaf, volatile aromatics would rank at one of the lowest, with the majority being taken by various polyphenols. As a tea gets steeped multiple times, each steep is removing compounds from within the leaf. Eventually most or all of the instantaneously soluble and rapidly soluble compounds will be removed from the leaf, and only the slower infusing heavier compounds or perhaps some rapidly infusing compounds that are in large concentrations, are left. This again is in line with the general experience of steeping tea multiple times; the first steeps are the most fragrant, the middle steeps get thicker and sweeter, but less fragrant, as the leaf takes on more water and heavier compounds start to represent more of the extraction. The late steeps tend to be proportionally longer to compensate for the drained stores of aroma compounds and carbohydrates, and thus tend to be more bitter and flavourless, with little fragrance and less flavour than earlier steeps. However, a late steep will usually not be as bitter as a long first steep, as during all steeps, the concentration of bitter and astringent compounds has been slowly reducing. Because these compounds make up such a large proportion of the available solutes, they are all that is left. Thus the tea finally ends up as mostly flavourless bitter water.

From these observations, it would at least partially explain why gongfu brewing works as well as it does, as it has the following general characteristics:

  • High ratio of leaf to water reaches desirable aroma, flavour, and strength quickly.
  • Repeat steeps means equilibrium is reset each steep; fresh solvent is repeatedly added and more can be extracted from the leaf.
  • Steeps are generally short which keeps flavour high and bitterness/astringency low. The temperature over a session would likely be hotter on average than the same water steeping for a long time in a pot.

As well as the other benefits:

  • Being able to adjust flavour on the fly. I.e if the tea is too strong you can reduce time and/or reduce leaf amount. If it is too weak you can reduce water input/add leaf and/or increase steep time. If you brew a 500ml batch of tea and it isn’t quite right, it’s a lot of waste.
  • Making a small amount of tea each time; this allows extended brewing sessions over a much longer time period than brewing the equivalent amount of tea all at once, lengthening the enjoyment of drinking the tea.
  • More efficient; it extracts more from the tea in the end because the equilibrium resets, as described above.

 

I hope this article provided some insight and interest about the simple act of brewing tea.
All feedback and comments are encouraged.

Enjoy.

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Sources:

Price, William E., and John C. Spitzer. “The Kinetics of Extraction of Individual Flavanols and Caffeine from a Japanese Green Tea (Sen Cha Uji Tsuyu) as a Function of Temperature.” Food Chemistry, vol. 50, no. 1, 1994, pp. 19–23., doi:10.1016/0308-8146(94)90086-8.

 

Price, William E., and Michael Spiro. “Kinetics and Equilibria of Tea Infusion: Rates of Extraction of Theaflavin, Caffeine and Theobromine from Several Whole Teas and Sieved Fractions.” Journal of the Science of Food and Agriculture, vol. 36, no. 12, 1985, pp. 1309–1314., doi:10.1002/jsfa.2740361216.

 

Spiro, M. “Kinetics and Equilibria of Tea Infusion Part 6: The Effects of Salts and of PH on the Concentrations and Partition Constants of Theaflavins and Caffeine in Kapchorua Pekoe Fannings.” Food Chemistry, vol. 24, no. 1, 1987, pp. 51–61., doi:10.1016/0308-8146(87)90083-5.


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