Crema in-depth Analysis of espresso flow rate sharing

Many years ago, David Schomer, a Seattle barista, wrote Espresso Coffee, a best-selling book in the coffee industry, which quickly swept through Espresso production and made all coffee lovers known as "Father of Italian Coffee", "Father of Seattle Coffee" and "Coffee Godfather". I also tasted this "Espresso Coffee". When I think about it, I benefit a lot.

The following is an excerpt from a chapter of the book:

Espresso principle

Establish a knowledge architecture

Just a bunch of facts, just a bunch of facts. Theory is the skeleton of fact. The experience of teaching others to cook espresso taught me one thing: if I told newcomers about the basic theory of espresso at the beginning, it would be easier for them to cook a good espresso after the course. Because in order to cook a good espresso, you have to face complicated practical problems. With a proper theoretical basis, their ability to understand problems can be enhanced.

Why is it so hard to cook espresso?

The problems with making a great espresso are mostly due to two factors: "uneven flow of water" and "the aroma of coffee is very volatile".

The problem of current

Modern espresso machines can produce water pressures of 8 to 9bar, while coffee cakes are less than half an inch thick. The big problem with espresso comes from espresso's feature: using pressurized hot water to pass through a thin coffee cake.

Water has one characteristic:

He will always find the shortcut with the least resistance in the coffee powder, instead of trying to extract the soluble coffee oil. So a large part of this book is about how to make coffee powder evenly resistant to high-pressure water.

As long as the coffee filter cake has a little uneven density or the surface collapses, the high-pressure water will rush up and break through your "cover door". In the coffee filter cake, the low density part will be over-extracted after the delicious ingredients flow out. On the other hand, the parts with higher density are not extracted enough.

When this espresso flows out, the water flows in a spiral shape. The surface tension is relatively high and it is tightly attached to the outlet. In addition to the delicious reddish-brown liquid, there are white lines and small cups of coffee.

To make matters worse, it happens when the beans are ground too thick or the resistance of the filter cake is uneven. As soon as the high-pressure water is leaking, the light brown liquid is pouring down.

It looks like a toilet in color and taste.

To cook a perfect espresso like this

Grind the correct amount of coffee powder, carefully flatten it and disperse it, and then rotate and grind it out after being evenly compacted with a filler. After being done, there should be some gaps between the surface of the coffee filter cake and the shunt net of the machine outlet. Turn on the filter head switch and the water will flow down and wet the coffee powder for about a second before the bubble head is really activated. This is called "prepreg", which helps seal the surface of the filter cake (Italian engineers are well aware of the benefits of "prepreg"), and the delicious ingredients in the powder do not stick so tightly when they come into contact with hot water. It's easier to extract when high-pressure water comes over.

Because the cooking temperature is as high as 200 degrees Fahrenheit and the pressure is as high as 120 pounds. If the resistance of the filter cake is uneven, it will rush out of the hole or pore diameter, resulting in uneven extraction. The part of insufficient extraction will cause the yellowish-brown liquid of water in the outlet head, and the part of over-extraction will dissolve other substances and cause white stripes.

In fact, what is said above is just an idealized overview. There are many factors that seriously affect the ability of coffee powder to withstand water pressure, such as: filling, dosage, freshness of beans, thickness of grinding, blade form of bean grinder, and air humidity. Only by controlling these many factors and understanding their relationship with each other can the matter be resolved.

When all is done, for the next 25 seconds, you can see high-pressure water squeeze through the coffee powder and inject a perfect cup of espresso.

Photo caption: Lao Qin Photography

Teacher Qin Zhibin believes that 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, and 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 "Crema".

Photo caption: Lao Qin Photography

In order to produce stable foam, caema needs 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. Another type of grease is needed, and the presence of grease often destroys the structure of the foam.

Photo caption: Lao Qin Photography

Will the oil in the coffee cause crema to disappear quickly in a few minutes? 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 speed 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 it is quickly flushed. This is also confirmed by our taste.

What does the color of Crema tell us?

"Photo caption: Lao Qin Photography" extracted insufficient color of espresso Crema

"Photo caption: Lao Qin Photography" extracted the color of excessive espresso Crema

I often say: the color of crema should be light reddish brown, but what does yellow-white or very black almost charred brown represent? 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. This also explains why excessive quenching can lead to the formation of crema that is as dark as charred.

"Photo caption: Lao Qin Photography" espresso Crema Tiger markings and Leopard markings

Conclusion: coffee is no matter the personality of Brazil, or the bitter smell of mocha, or free and unrestrained espresso. Have their reasons for being in the world. There is always a coffee that suits you, meeting you from thousands of miles and making it difficult for you to fall asleep.