How do you get the oil on the surface of espresso beans? Is the oil not fresh?

During the roasting process, a large amount of carbon dioxide is produced in the coffee beans, most of which will be emitted during the cooling process, a few will continue to be kept inside, and the grinding will release these gases, so coffee must be made as soon as possible after grinding.

When hot water hits coffee powder under high pressure, it will emulsify the insoluble oil of coffee powder and dissolve a large amount of carbon dioxide supersaturated, which is much larger than the solubility of hot water under normal pressure. this is why countless small foams immediately appear when the liquid flows from the handle. But this does not fully explain the "Crema", open a can of Coke, we can see countless bubbles coming up, but they simply do not last.

Each cup of espresso has a layer of brown fine oil on the surface, which is called crema. If you look closely, you will find that the crema color on the surface of each espresso is different, and the common crema colors range from light gold, hazelnut, ochre to dark brown. What message does these crema with different colors reveal to us? In fact, the color of crema does tell us a lot of things.

To produce stable bubbles, we need some bubbles and some compounds to "wrap" the bubbles, making the bubble structure 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-- oil and fat, which are solid and liquid, and the presence of oil often destroys the structure of foam. (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 fail. 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, inelastic, and then disappear quickly. 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 the foam on the surface of a cup of coffee, because the liquid is much thicker than the rapidly extracted Espreeso, which is confirmed by our taste.

Can you tell us: 1. Whether the coffee is fresh.

A large number of robusta beans allow us to see a lot of bubbles, so we can't judge the freshness of coffee by watching the speed at which the bubbles disappear. Fresh coffee will have a lot of Crema throughout the production process, that is, when the coffee flows out, it looks like Coke, and only when the extraction is nearing the end, a large amount of water will stop, so that two very obvious stages of the process can be clearly observed. Stale coffee beans often see a thin, separate Crema, and the resulting coffee is often thin and watery, meaning less Crema and faster disappearance.

2. The deeper the coffee is extracted, the more Crema means that more dissolved matter enters the cup. (contains over-extracted coffee impurities)

3. The more stable the uniformity of coffee extraction, the better and stronger the coffee we make, which means that there are fewer gaps in coffee powder (channel effect) and fewer mistakes in the production process.