Kinetics of Steeping Tea

I can’t help being a nerd; it’s in my genes!   I hope you can appreciate this geeky discussion on the science of steeping tea.  Cheers, Margo

by Tony Gebely 3 months ago

kinetucs-moleculesSteeping is the final step in the lives of tea leaves. And in their final act, the they slowly unfold and unravel, creating a beverage that tells the story of where they came from, what the weather was like before they were plucked, and how they were handled, processed, and stored before they reached your cup.
Tea as a beverage is most commonly made by infusing tea leaves in a solvent (water) to make a solution wherein the solute (what’s being dissolved into the water) is made up of the soluble compounds within the tea leaves. What we end up with is a solution that is on average 98% water and 2% compounds from within the tea leaves.

How Steeping Works

When tea leaves are added to water, they absorb some of it and become rehydrated. This absorption of water into the tea leaves allows for the initiation of steeping, the process of extracting the soluble compounds from the tea leaves and dissolving them in the water. The driving force of the steeping process is the difference in concentrations of dissolved compounds in the leaves and in the water. Compounds in the leaves move from an area of high concentration to an area of low concentration until equilibrium is achieved, a process called diffusion.
The type of tea being steeped is the most important determinant when it comes to the types and amounts of soluble compounds within tea leaves. Tea types are defined by the processing steps the leaves go through and thus, the resulting chemical components and amounts of them in the leaves are similar for all styles of finished tea within a type.
Across all tea types, the major chemical components in tea leaves fall into the following categories: polyphenols, amino acids, enzymes, pigments, carbohydrates, alkaloids, minerals and volatiles. Individual chemical components lend themselves to a portion of the cup in the form of taste, color or body.
The chemical composition of a cup of tea depends not only on the chemical compounds found within the leaves, but also the ratio of leaves to water, the surface area of the leaves, the chemical properties of the water, steeping temperature, and the length of time the leaves are in contact with water.

Surface Area of Tea Leaves

The surface area of tea leaves refers to the total area of the exterior surface of the leaves. For example, let’s take 5 grams of two finished teas: Sample A is a broken-leaf black tea, Sample B is a full-leaf, unbroken black tea. Even though we have 5 grams of both Sample A and Sample B, the surface area of Sample A is greater than that of Sample B because it has more exposed leaf parts due to it’s broken nature. When steeped, Sample A will have a higher area in contact with water allowing for more soluble compounds to dissolve into the water at the same time. This means that when steeping a tea that has a high surface area (such as Sample A), you will achieve a beverage of desired strength faster than you would with a tea with lower surface area (Sample B).
Ratio of Tea Leaves to Water

The amount of tea leaves used during steeping affects steeping in the same way that surface area does because adding more leaves to the same amount of water increases the overall surface area and area of interface between leaves and water, literally making more tea available for dissolution. So keeping the amount of water the same, increasing the amount of tea used will allow you to achieve a beverage of desired strength faster. Using less leaves, you can still reach a beverage of desired strength, but it will take more time.
Water Temperature

Temperature is related to kinetic energy and increasing the water temperature increases the kinetic energy among the water molecules allowing them to more effectively dissolve solute molecules. Thus, an increase in water temperature increases the rate of dissolution of soluble compounds in tea leaves. The solubility of individual tea compounds peak at different temperatures, so the chemical composition of a cup of tea steeped at different temperatures will vary, sometimes greatly, and so will it’s taste. In the case of solids, solubility increases and the rate of dissolution increases as the water temperature increases.

Atmospheric Pressure and Temperature

Atmospheric pressure at sea level is around 14.7 psi, the boiling point there is 100°C. Below sea level, where atmospheric pressure is higher, the boiling point of water rises above 100°C. Above sea level, where the atmospheric pressure is lower, the boiling point of water dips below 100°C.
Steeping Vessel

The kinetics of steeping work irrespective of the vessel that the steeping occurs in, however, when choosing a vessel, there are two things that you must take into consideration:
How much heat will the vessel absorb away from your water?

Do you have a way to quickly separate the leaves from the water to halt the steeping process?

Pouring water that you so carefully heated to the degree into a steeping vessel at room temperature can decrease the temperature of the water by up to 15 degrees after 30 seconds depending on the mass of the vessel and the thermal conductivity of it’s material. However, by preheating the vessel this effect can be greatly reduced. To preheat your vessel, simply fill it with water at or slightly above the desired steep temperature and letting it sit for 30 seconds, then decant the water. Preheating reduces the decrease of temperature after 30 seconds to about half.
Second, it is important that you are able to quickly separate the leaves from the water to halt the steeping process. The most simple way to halt the steeping process is to decant the vessel’s contents through a strainer. Some steeping vessels are equipped with a built-in strainer that can be removed once steeping is complete.
Steep Time

Steep time refers to the amount of time the leaves are in contact with water and thus, the amount of time that dissolution can occur. If given enough time, some compounds in the tea leaves would reach equilibrium, meaning that the concentration of these compounds in the leaves would be the same as the concentration of the same compounds in the water. This wouldn’t necessarily make for a palatable infusion, so we adjust the steep time to control the concentration of the compounds in our cup.
Relationship Between Time and Temperature

When steeping tea, the results of water temperature and steep time on our beverage are inversely correlated. This means that if you have a preferred steep time and water temperature that you use to prepare an infusion of a particular strength, slightly increasing the steep time, and decreasing the water temperature will yield a similar result. Likewise, decreasing the steep time, and increasing the water temperature will also yield a similar result. I have observed that for each 20 degree rise in steep temperature, you can halve infusion time and likewise, for each 20 degree decrease in steep temperature, you can double infusion time. Remember though that changing the temperature will alter the solubility of the chemical compounds in the leaves with water and will result in cups of tea with differing chemical compositions, so while adjusting time can increase or decrease overall strength of the beverage, adjusting temperature will slightly affect taste.

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