How does tea brewing work?

Inspiration

I brew tea almost every day, yet I don’t understand the mechanism through which tea polyphenols, flavonoids and caffeine leave the leaves (haha) and enter the water. Why does the water have to be hot? I know there are cold brew methods, so what’s the difference?

My guess before digging in

It’s probably related to diffusion.

Etymology

• Osmosis: Latinized form of obsolete osmose, extracted from endosmose, exosmose, from French, equivalent to end- end-, ex- ex- 2 + Greek ōsm(ós) “a push, thrust” + French -ose -osis
• Turgid: Latin turgidus, equivalent to turg ( ēre ) to swell + -idus -id

Findings

This question immediately led to more questions, so the answers are in the Follow-up questions section below.

Follow-up questions

What’s the difference between ‘steeping’ and ‘brewing’?

• Steeping seems to be a process in the act of brewing. To brew tea, you must steep the leaves in hot water. I guess this is similar to steering being a process of driving a car, but you can say that your steering or driving.
• According to this article (opens in a new tab), “Chemically speaking, steeping refers to the act of infusing tea leaves in a solvent (water) to make a solution that is on average 98% water and 2% compounds from within the tea leaves.”
• “steeping, the process of extracting the soluble compounds from the tea leaves and dissolving them in water”

What‘s the difference between diffusion and osmosis?

• Osmosis requires a semipermeable membrane and involves the movement of solvent particles from a dilute solution into a concentrated solution. Seeks to equalise the solution on both sides of the membrane.
• Diffusion involves the movement of solvents from higher concentration to lower in an effort to equalise the concentration of a solvent in a medium.
• The system seeks balance or equilibrium. If the solute particles can’t cross a barrier, the only way to equalize concentration on both sides of the membrane is for the solvent particles to move in.
• “The driving force of the steeping process is the difference in concentrations of dissolved compounds in the leaves and 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.”

What is a semipermeable membrane?

• A selectively permeable membrane is one that allows only specific ions and molecules to pass through, while it obstructs the movement of others.

Then is osmosis just a type of diffusion?

Yes. Diffusion has the following types:

• Simple diffusion: A process in which the substance moves through a semipermeable membrane or in a solution without any help from transport proteins.
• Facilitated diffusion: A passive movement of molecules across the cell membrane from the region of higher concentration to the region of lower concentration by means of a carrier molecule
  • Dialysis: It is the diffusion of solutes across a selectively permeable membrane.
  • Osmosis: It is the movement of solvent molecules from the region of lower concentration to the region of higher concentration through a semipermeable membrane. Since water is solvent in every living being, biologists define osmosis as the diffusion of water across a selectively permeable membrane. For example, plants take water and minerals from roots with the help of osmosis.
    • Endosmosis: When a substance is placed in a hypotonic solution, the solvent molecules move inside the cell and the cell becomes turgid or undergoes deplasmolysis.
    • Exosmosis: When a substance is placed in a hypertonic solution, the solvent molecules move outside the cell and the cell becomes flaccid or undergoes plasmolysis.
    • Source: What Is Osmosis? - Definition, Types, Osmotic Pressure (opens in a new tab) and Endosmosis And Exosmosis - The Major Differences (opens in a new tab)

What does the “-ium” suffix mean? In ‘sodium’ for example

• Used for naming some elements
• “Temperature is related to kinetic energy. 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.”

Source: What Is Diffusion? - Definition, Types & Examples Of Diffusion (opens in a new tab)