How to understand the extraction of hand-brewed coffee the core principle of hand-brewed coffee is dissolution and diffusion

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During the roasting process, chemical reactions take place within the coffee cells to produce a variety of components that emit the aroma and flavor of the coffee. As a by-product of these chemical reactions, the gas slowly formed inside the coffee causes the cells to expand. These gases and water are expelled through very small pores inside the cell. These countless pores contain the ingredients that dominate the flavor of coffee formed during baking. To dissolve these ingredients, you must first crush the coffee beans and expose as many pores inside the coffee bean cells to the surface as possible. Then slowly inject water, you can dissolve the ingredients of the coffee, this is the dissolution.

If you look at the roasted coffee beans under a microscope, you will find that there are many small pores. Because of this porous structure, the coffee powder will float on the surface when injected with water. A close look at the coffee powder floating on the water shows that it has been divided into two layers. The bottom layer because the coffee powder is in full contact with water, resulting in the active dissolution and diffusion of coffee ingredients, but the upper layer is mixed with the gas discharged from the coffee, so the coffee is not in full contact with water. So even if you put in enough coffee powder

Some of the coffee ingredients cannot be completely dissolved. In order to fully form dissolution and diffusion, the coffee is filled with water and sometimes stirred in the filter cup with a spoon. Stir with a spoon and mix the layered parts again so that the coffee and water can be fully contacted. If you inject water more carefully, you can stir it without a spoon, and the coffee powder can be in full contact with the water.

In order to dissolve smoothly, we can consider grinding the coffee powder to the finest so that all the ingredients in the cell can be dissolved. However, if you grind it too fine, it is easy to block the filter paper, so in actual extraction, the coffee powder is only ground to the size of sesame grains. In this way, some cells containing coffee are exposed to the surface, while others are not exposed to the surface. There are also some cells that contain coffee ingredients that cannot be broken by crushing. These cells cannot be extracted by dissolution, and the extraction can only be formed by diffusion.

Put the coffee into a filter cup, fill it with a small amount of water until it is soaked, and the coffee begins to expand. This is because hot water soaks into the coffee cells along the capillaries between the coffee cells formed during baking, pushing the gas out of the cells, causing the coffee to expand. The water that enters the coffee cell begins to dissolve the coffee ingredients and form a thick coffee solution. Stand still for a while and then inject water into the filter cup again, when there is a concentration difference between the solution formed in the coffee cell and the newly injected water, and the thick coffee solution begins to push the coffee ingredients into the newly injected water. This process is called diffusion.

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