When brewing coffee, the ground coffee is placed in a filter cup and carefully filled with water. The first injection of water should be a small amount, and let stand until the coffee powder is soaked, and then divided into 3 to 4 injections of water (there is also a one-time injection of water). Why these steps? This is because the principles of dissolution and diffusion are used in caffeine extraction.
When extracting coffee, if you look closely at the filter cup, you can see the changing process of coffee, which serves as a reference for extracting coffee. However, with the filter paper in the filter cup, you cannot directly see the changes inside, so prepare a set of transparent utensils similar to the filter cup. So how do dissolution and diffusion work during extraction?
The core principle of hand-brewed coffee is dissolution and diffusion
During the roasting process, chemical reactions take place inside coffee cells to generate the various components that give off the aroma and taste of coffee. As a by-product of these chemical reactions, gases slowly form inside the coffee, causing the cells to begin to swell. These gases and water are expelled through very small pores inside the cells. These countless pores contain the ingredients that form during the roasting process that dominate the flavor of the coffee. In order to dissolve these components, the coffee beans must be crushed first, and the pores inside the coffee bean cells should be exposed to the surface as much as possible. Then slowly pour water to dissolve the ingredients of the coffee, which is called dissolution.
In order to dissolve smoothly, we can consider grinding the coffee powder to the smallest point, so that all the components in the cells are dissolved. But grinding too finely can easily block the filter paper, so the coffee powder is only ground to the size of sesame grains during the actual extraction. In this way, some cells containing coffee components are exposed to the surface, and some cells are not exposed to the surface. There are also some cells containing coffee components that cannot be broken through crushing, and this kind of cells cannot be extracted by dissolution. At this time, extraction can only be formed by diffusion.
Put the coffee into the filter cup, first pour a small amount of water until the coffee is soaked, and the coffee begins to expand. This is because the hot water is immersed in the coffee cells along the capillaries between the coffee cells formed during the roasting process, and at the same time pushes out the gas in the cells, causing the coffee to expand.
The water entering the coffee cells begins to dissolve the coffee components and form a thick coffee solution. Let it stand for a while and then inject water into the filter cup. At this time, there is a difference in concentration between the solution formed in the coffee cells and the newly injected water, and the strong coffee solution begins to push the coffee ingredients into the newly injected water. This process is diffusion.
In this way, in order to dissolve more smoothly and fully form the diffusion caused by the difference in concentration, the process of standing for 30 to 40 seconds after the first injection of water into the coffee cells before the formal extraction is called pre-extraction (also called smouldering). ). During this process, the coffee is fully prepared for extraction.
The secret lies in the even contact between coffee and water
When you look at the roasted coffee beans under a microscope, you will find that there are many small holes. Because of this porous structure, the coffee powder will float on the surface of the water when pouring water.
If you look closely at the coffee powder floating on the water, you can see that it is divided into two layers. The bottom layer is fully in contact with water, so that the coffee components are actively dissolved and diffused, but the upper layer is mixed with the gas discharged from the coffee, so the coffee cannot fully contact the water. Therefore, even if enough coffee powder is put in, some coffee components cannot be completely dissolved.
In order to fully dissolve and spread, after pouring water into the coffee, it is sometimes stirred in the filter cup with a spoon. By stirring with a spoon, the layered parts are mixed again so that the coffee and water are in good contact. If you are careful when pouring the water, you can stir without a spoon, and the coffee grounds can be fully contacted with the water.
It is often heard that in order to avoid the collapse of the coffee layer when pouring water, it is necessary to carefully control the water flow for pouring. However, as you can see in the picture, even if the coffee powder floating on the water is injected with a fine water flow, the falling water will still have an impact on the surface coffee layer, making the coffee sink to the bottom layer with the water flow and float again. to the water. It is because of this phenomenon that the coffee and the water are in sufficient and uniform contact.
The water injection itself is to fully mix the water and coffee, so as long as the water and coffee can be fully mixed, no matter how the water injection method is, it will not have much effect on the taste of the coffee.