What is espresso and what is Crema? how to judge the color of grease?

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In the process of roasting coffee beans, a large amount of carbon dioxide is produced in the coffee beans, most of which will be released during cooling, and a few will continue to be preserved inside the beans, and these gases will be released when the coffee powder is ground, so coffee must be made as soon as possible after grinding.

When hot water hits coffee powder under high pressure, it emulsifies the insoluble oil in coffee powder and saturates a large amount of carbon dioxide, which is much larger than the solubility of hot water under normal pressure. this is why countless small foams appear immediately when the liquid flows out of the handle. But this does not fully explain "Crema" (Klima), you usually open a can of Coke, we can see countless bubbles coming up, but they simply do not last, what is the principle of this?

To produce stable bubbles, we need some bubbles and some compounds to "wrap" the bubbles so that the bubble structure remains stable and elastic. The process of this chemical reaction can be thought of as the role of a surfactant. Unlike milk bubbles, which do this through egg whites, coffee uses a substance called protein melanin. It is produced by a chemical reaction of a group of mixtures during baking. In fact, scientists do not know much about the process. Neither protein nor melanin is hydrophilic, so when hot water strikes, they are naturally distributed on the surface of bubbles, so they can come into contact with more air, resulting in countless small bubbles, so we have-- bubbles. There are other things-grease (the author means "oil" and "fat", fat is solid, oil is liquid), and the existence of oil is disadvantageous. Oil often destroys the structure of foam and leads to failure. Think about it when we make a cake, we have to remove the yolk, otherwise the fat in the yolk will cause the egg whites to be whipped, that is, to stir the egg whites quickly and beat them into milk bubbles.

So will the oil in the coffee cause crema to disappear quickly in a few minutes? The answer is both right and wrong. The surfactant is dissolved in water, and due to the action of gravity, the surfactant of the bubble surface will be pulled off the surface of the foam together with the water, making the foam fragile, robbing elasticity, and then quickly disappearing. The speed at which the foam disappears is related to the rate at which the water is dragged away, but the foam on the surface of a cup of coffee lasts much longer than that of a cup of fast-flushing coffee, because the liquid is much thicker than the rapid brewing.

The color of crema should be light reddish brown, but what does yellow-white or very black almost charred brown mean? The answer comes from drinks, because crema is actually carbon dioxide bubbles wrapped in brewed coffee, so the darker the foam means the stronger the coffee becomes a natural assumption. But it's actually very difficult to tell the color of coffee unless you dilute two cups of coffee or look at it under a microscope.

Another factor also plays an important role: the reflection of light from the foam means that the color of the coffee is much lighter than it actually is through the foam. And the smaller the foam, the greater the effect, so even if the espresso itself is very black, it may look very light (which explains why black beer has very white foam at the top).

A 15-second espresso usually has a whiter crema because it is lighter. Coffee powder is quenched by water for a relatively short time, and the viscosity of coffee is lower than that washed out in 25 seconds. For the same reason, coffee made from a lower water temperature will be lighter because it does not have enough energy to dissolve the substance in the coffee (it should be low solubility at low water temperature, not so-called energy). This also explains why excessive quenching can lead to the formation of crema that is as dark as charred.