Common sense of coffee powder pressing some information about coffee powder pressing

Purpose:

The loose powder is compacted by pressure so that the pressurized hot water penetrates evenly. There are several problems involved here, and in fact, to put it mildly, it is a vector problem. The direction of the powder pressing, and the strength of the powder pressing force.

The direction of powder pressing: (it may be more complicated than we think)

This is obvious, straight down. If the pressure is tilted, the hot water from the brewing head will seek to penetrate the layer with the least resistance, resulting in uneven extraction. It is also easy to have what we often call channel effects.

I need to explain that. It doesn't take much. However, regarding the direction of pressing powder, it will indirectly involve another problem, that is, the problem of grooming powder.

The biggest reason why we want to distribute powder is naturally the control of powder amount, that is, the powder amount is evenly distributed, and the excess powder amount is scraped off, so the same action every time can ensure the consistent powder amount. Another reason is to distribute the powder evenly. Because a traditional theory is that the water in the brewing head is evenly permeable, the powder should be flat, the powder should be flat, and the extraction should be uniform. (It is also believed that the water coming out of the brewing head will eventually concentrate at one point, so the powder pressing hammer needs to be curved, but after testing and blind testing, there is no essential difference between the two)

OK, this is the traditional idea. Some people think otherwise.

If you notice Stephen Morrissey in 2008, he actually ground it through a grinder, and he didn't dust it. When the powder piles up into a conical hill, press the powder directly. This practice has always bothered me. Even if I discuss it with several friends, there is no exact answer. But from coffeegeek's April release on VST basket, more or less find some answers.

First of all, although our water output is even, pressure points can also control uniform distribution, but a very important hardware limitation is the basket. Looking first at a diagram from coffeegeek, we can see that most basket holes vary in size.

Secondly, we have the opportunity to look at the basket of coffee handles. In fact, the distribution of holes is not uniform, but the center is dense and the edges are sparse. In other words, the chances of water penetration in the middle are much greater than at the edges. This, more or less, explains Stephen's uneven powder distribution during the game. Because when a cone is piled up, most of the powder is concentrated in the center of the basket, which can compensate for the problem of uneven extraction.

It is also because of this discovery that VST has its own basket hole size, and distribution are very uniform. Thus applied for a patent (www.coffeegeek.com/opinions/markprince/04-29-2011) Of course, with this basket, it is not possible to directly press the powder cake without powder. This is also a new basket. When giving coffeegeek an experiment, Mark Prince had to change Stephen's behavior.

The power of powder pressing (probably simpler than we think)

The most basic question is how much strength, how many kg. I've heard it weigh 25kg before, and I've heard that Italians regard coffee as life, and it can be touched lightly.

In explaining this problem, one of the most basic problems needs to be corrected. Force should be described by Newton. It's not a quibble, for reasons explained later.

If we use Newton's explanation, then we first convert it. 15kg is a common description, so converted to Newton units, it is 15kg*9.8N/KG, which is about 150N.

Next, let's see how much force 9 atmospheres of water produces on coffee cake.

Nine atmospheres means nine atmospheres of force per square meter, which is 900KN/M^2.

Our typical 58mm diameter cake has an area of (58/2/1000)^2*3.14, which is about 0.00264 square meters.

Then the resulting pressure is 900*1000*0.00264, or 2380N.

The 150N produced by the hammer is obviously insignificant compared with the 2380N flow at 9 atmospheres.

At the same time, there is also a very important factor to consider. When La Marzocco designed the Swift automatic filling device around 2000, they experimented with forces ranging from 2 pounds to 30 pounds (that is, about 15kg). It was found that no matter how much force there was, the force reached the bottom of the powder cake and was almost unstressed. This is like a car crash experiment. The extrusion and deformation of the car body itself is the result of absorbing energy.

Therefore, the conclusion that these two points give us is very simple, the strength of the powder is not much related, as long as the powder is pressed tightly. At the same time, it is more important to keep the same pressure every time.