Barista Skills "Professional Barista Manual"(IV) Drip coffee extraction

Chapter III theoretical Science of Filtration and extraction

After research and writing this chapter, the author aims to let baristas understand the dynamics of Espresso filtration. Some readers will find this chapter full and fascinating, while others will make their scalps numb. The author believes that reading and absorbing the content of this chapter will have a positive return, and its most important significance is to provide a theoretical basis for judging most of the problems in filtration and extraction.

Filtration dynamics

The dynamics of Espresso filtration is very complex and not yet fully understood. However, there are already some reliable models that can describe the process that has been learned. After we first discuss and observe the close relationship between the coffee powder, the gas, and the water in the filter when brewing drip-filter coffee, the above models can easily be visualized. We can do this by using a hand-made or any drip filter that can observe the coffee powder during brewing.

Filtration and extraction Kinetics of drip Coffee

The first stage: soaking. Pour water on the powder bed and soak the coffee powder, causing the coffee powder to release carbon dioxide quickly. The released carbon dioxide will have an impact on the water, causing disturbance to the coffee powder, which can simultaneously slow down the speed at which the coffee powder is soaked and the water flows through the powder bed. The existence of disturbance can be proved by the layer of foam that covers the coffee powder after brewing.

Water always chooses the path with the least resistance to flow through the powder bed, so the flow through the powder bed is irregular. While water is extracted from the coffee powder, it is also absorbed by the coffee powder, while the concentration of the liquid that is not absorbed by the coffee powder increases gradually as it continues to flow through the powder bed. At the same time, coffee powder expands and bulges after absorbing water.

The second stage: extraction. The coffee liquid that comes out of the filter first is the thickest and the highest concentration. As the extraction process progresses, the concentration of the liquid will become thinner and thinner, because there will be less and less easily extractable matter left in the powder bed.

There are two stages of extraction. In the first stage, the solid substance is washed out from the surface of the coffee powder. In the second stage, the water diffuses into the coffee particles, extracting the solid matter from the inside, and the "diffusion" is always transferred from the areas with higher concentrations to the areas with lower concentrations.

Diffusion is caused by a series of actions. First of all, water comes into contact with coffee particles and gives off gas. After that, the water enters the pores of the coffee particles, causing the particles to expand and the solid matter inside the particles begins to dissolve. Next, the dissolved solid material will first spread to the surface of the particles and then dissolve into the surrounding solution.

During cooking, water is continuously injected into the top of the powder bed to dilute the mixture of liquid, coffee powder, and gas. These diluted liquids near the top of the powder bed, due to the great difference in concentration (the concentration of the solid substance in the coffee powder, the concentration of the solid substance in the solution, the difference between the two), will quickly diffuse the coffee powder in the upper layer. At the bottom of the powder bed, the extraction rate becomes slower because the concentration of solids in the liquid here is higher, reducing the concentration difference. The end result is that the extraction is uneven, and the coffee powder in the upper layer will extract more solids than the coffee powder in the lower layer (Note 1: when using a tapered filter cup, the extraction rate of the powder bed from top to bottom is more uniform than when using a cylindrical filter cup. See the discussion on the shape of the filter cup later in this chapter).

Filtration and extraction Kinetics of Espresso

The kinetics of Espresso is similar to that of drip coffee, although the extraction of Espresso is mainly accomplished by scouring, and the role of diffusion is rare. The model used to describe Espresso filtering is not perfect, but through practice, it has been proved to be effective in predicting the success of the product. The following combines published research and cutting-edge knowledge from the boutique coffee industry.

The first stage: soaking. In the first stage, water fills the head space of the extraction chamber, soaks the coffee powder and expels the gas. As the coffee powder absorbs water, the water also removes the solid from the coffee powder. The absorbed water expands the coffee particles and reduces the voids in the powder bed.

When the water flows through the powder bed, it erodes the solid substances from the coffee powder and causes them to precipitate to the bottom of the powder bed. This will increase the content of solid matter at the bottom of the powder bed during the wetting stage (Note 2: how much of the precipitated solid matter is added, and how much solid matter flows to the bottom of the powder bed with the extraction liquid, it is not yet possible to measure and record by interrupting the extraction process).

In the wetting stage, the coffee powder bed is extremely vulnerable to the damage of the channel. Such as the lack of cohesion of dry coffee powder, the displacement of coffee powder, the reconstruction of the powder bed caused by expansion, the rapid movement of solid matter, and the unexpected increase in pressure on some machines, will greatly increase the possibility of channel formation at this stage.

By the last moment of the wetting stage, the powder bed has been completely changed: the gap has been eliminated, it has expanded, the heat of the cooking water has been absorbed, the gas has been discharged, and the solid material has been transferred from the upper layer of the powder bed to the lower layer. The priority flow path has been formed, and the channel may have been formed.

The second stage: pressurization. The pressure difference causes the water to flow from the high pressure area through the powder bed to the low pressure area of the filter hole at the bottom of the powder bowl. According to Darcy's law in fluid mechanics, the amount of water passing through the powder bed increases with the increase of applied pressure. However, there are two obvious contradictions between the experimental evidence in academic publications and Darcy's law:

1. With the increase of pressure in the extraction process, the initial flow rate of water will accelerate, and then slow down after reaching the fastest speed, gradually approaching a constant flow rate.

two。 By sampling the extracted products under different applied pressure, it is found that the flow rate of water is faster when the pressure is higher. However, the pressure cannot be higher than a certain value, and once it is exceeded, the flow rate will not change or even slow down. In short, if you adjust the pump pressure of an espresso machine from 9 to 12 atmospheres, the flow may slow down.

There are several possible reasons why the flow rate slows down during the pressurization process. First, during the pressurization phase, the remaining dry coffee powder is also soaked and expanded, making fewer pores in the powder bed, thus increasing the flow resistance. Second, the increase of pressure not only makes the powder bed more compact, but also increases the flow resistance. Finally, the increased pressure "helps" the movement of coffee particles (that is, the displacement of ultra-fine powder), making the powder bed more compact.

The third stage: extraction. At present, there are still different opinions on the study of the two different extraction forms of scouring and diffusion in the cooking process. After collating the data, a group of researchers concluded that the extracted substance was mainly done by scouring the outer wall of coffee cells. Another group of researchers studied the same data and concluded that 85% to 90% of the extract was done by intracellular diffusion in the first minute (and then 100%). If this is what the latter says, then diffusion is the protagonist of Espresso extraction.

However, according to the results of the study using large filters, diffusion occurs only when the coffee particles meet the following conditions:

1. "the right amount of bound water". Coffee granules can hold bound water equal to 15% of their own weight.

two。 Permeated by a free extracted liquid.

3. All the gas has been discharged.

The standard Espresso extraction time is not long, and it is almost impossible to meet the above three conditions at the same time to allow diffusion to occur. Therefore, just as grease comes from emulsification (Note 3: the emulsification of grease seems to be caused by high pressure during Espresso cooking. It is controversial whether emulsion is one of the most important criteria for Espresso), it is possible that the whole extraction process of Espresso is completed by scouring. Even if there is credit for proliferation, it is only a drop in the bucket.

The color of ↑ coffee powder (represented by a stacked rectangle) is crimson in the first picture, indicating a high concentration of solid coffee. In the next few pictures, the lighter the red, the lower the concentration of the solid.

T =-10 seconds: dry coffee powder before the pressure pump is started. Full of solid matter, the very fine powder is scattered all over the powder bed.

T =-1 second: the powder bed near the end of the pre-immersion. The water has permeated almost the whole powder bed, but the extraction has not yet begun. After the coffee powder absorbs water, the powder bed expands. A channel (represented by a yellow line) is formed in the middle of the powder bed. The solid matter at the top of the powder bed has begun to lose, and the solid matter in the lower layer of the powder bed has increased. The ultra-fine powder began to move towards the bottom of the powder bed.

T = 0 s: the first drop of extraction liquid appears. The first drop of extraction liquid appears at the exit of the channel. Ultra-fine powder and solid substances are increasingly concentrated in the lower layer of the powder bed. The powder bed shrinks with the increase of pressure.

T = 5 seconds: the initial stage of extraction. Solid matter and ultra-fine powder leave the powder bed quickly. After the pump pressure is full, the powder bed is further compressed.

T = 15 seconds: in the middle of extraction. The powder bed shrinks due to mass loss. The extractable solids at the top of the powder bed have almost been lost. Most of the ultra-fine powder and solid matter are concentrated at the bottom of the powder bed.

T = 25 seconds: the end of extraction. The extractable solids at the top of the powder bed have been emptied. The powder bed lost about 20% of its dry weight.

Changes in liquids

When the preparation is well done, the initially extracted liquid should be sticky and dark (Note 4: it is generally believed that the higher the concentration of caramelized solids in the extracted liquid, or the lower the concentration of carbon dioxide, the darker the color will be. However, there may be other factors that also affect the color of the liquid. ). After that, the liquid will become thinner and thinner, and the color will gradually fade and eventually turn yellow. When the liquid turns yellow (or golden), the flow should be cut off to prevent the concentration from being diluted without any benefit to the flavor. This is because by the end of the extraction, the content of coffee flavor substances in the extracted liquid is already very low.

The solid material at the top of the powder bed is extracted rapidly at the initial stage of wetting and extraction, because the high temperature makes it easier for the particles to shift during the wetting stage, and also benefits from a wide concentration difference.

The solid content in the lower layer of the powder bed began to increase from the wetting stage, and then stabilized at the initial stage of extraction, because the lower coffee powder not only lost a small amount of soluble solids, but also increased the precipitated ultra-fine powder. The final result is that the proportion of solids contributed by the upper layer of the powder bed is much higher than that of the lower layer of the powder bed.

Very fine powder

The displacement of ultra-fine powder or ultra-fine cell wall fragments is an "unknown factor" in Espresso extraction. Although I do not know any direct measurement method to quantify the ultra-fine powder displacement, there is a lot of indirect evidence to prove its existence in the published research report. On the assumption that the ultrafine powder will shift and form a dense layer at the bottom of the powder bed, its existence can also be inferred from the mathematical model (Note 5: there are some mathematical models that can be used to simulate Espresso extraction and allow the input of different variables, such as the proportion of the powder bed to be wetted during pre-soaking, the amount of solids left in the upper and lower layers of the powder bed after extraction, and the flow rate during extraction. Practice has proved the accuracy of the results speculated by these models based on the above variables.

If an obvious dense layer is formed, the filter hole at the bottom of the powder bowl will be blocked and the uniformity of extraction will be destroyed. The formation of dense layer will destroy the quality of Espresso from the following aspects.

1. Unexpectedly slow down the flow rate. Every barista who has witnessed the slowing down of the flow rate during the extraction process should be the witness of the increase in flow resistance after the formation of the dense layer.

two。 Uneven extraction and channels.

3. Reduce Body if too much fine powder replaces useful solids (both soluble and insoluble) into the cup.

Effect of ultra-fine powder on the quality of Espresso

Putting aside the dense layer, the ultra-fine powder has both good and bad effects on the quality of Espresso. In order to deeply understand the influence of ultra-fine powder, the author uses a 90-micron powder sieve to remove a large amount of ultra-fine powder before filling it. (note 6: the author did not remember the weight of the removed fine powder, but only shook the powder sieve for about a minute. Until no ultra-fine powder continues to pass through the sieve). The most significant effect after the removal of ultra-fine powder is the acceleration of flow velocity, which indicates that ultra-fine powder will increase the resistance of water flow. After adjusting the grinding degree to return to the normal flow rate, continue to produce a few cups with the sifted powder. Compared with "normal" products, products that sift out very fine powder have less Body and less bitterness.

Since ultra-fine powder has both advantages (more Body) and disadvantages (more bitterness) for Espresso, only by finding the best proportion of ultra-fine powder for a given amount of powder can you make the best Espresso. However, this proportion should be able to limit the displacement of very fine powder and prevent the formation of dense layer. There is no practical method to measure the generation and displacement of ultra-fine powder, but at least there are many ways to reduce the production and displacement of ultra-fine powder.

Limit the production of ultra-fine powder

Due to the low hardness of coffee beans, very fine powder will inevitably be produced during grinding. For a given grinding setting, there are four ways to reduce the production of ultra-fine powder: using a sharper cutterhead; using a shallower baking degree; using a slower speed grinder; and using coffee beans with higher water content.

Limit the displacement of ultra-fine powder

Baristas can monitor the displacement of the ultra-fine powder indirectly in two ways: using a bottomless bowl to observe whether the flow rate and color of the extracted liquid is uniform (the color differences between different regions should not be too great); or after knocking out the used pressed powder, check the filter holes of the powder bowl (the holes cannot be clogged). Through the above observation, the barista can know whether the very fine powder displacement exists.

The simplest and most efficient way to reduce the displacement of ultra-fine powder is to use low-pressure pre-soaking. The use of finer grinding can also reduce the displacement of ultra-fine powder, which reduces the space between coffee powders, thus compressing the displacement path and making the powder bed more compact. Of course, only fine grinding will slow down the flow rate, but the original flow rate can be maintained by adjusting the grinding degree while reducing the amount of powder filling, or using a larger powder bowl.

Shape and extraction of Powder Bowl

A standard single bowl is shaped like a cone with the top cut off, while a standard double bowl is (or close to) a cylinder. Will the shape of the powder bowl affect the quality of the extraction? The answer is not entirely yes.

As mentioned earlier in this chapter, during the extraction process, more solid substances are extracted from the upper layer of the powder bed than from the lower layer of the powder bed. This kind of uneven extraction is harmful to the flavor and concentration of the product: the upper layer will extract too much, resulting in bitter and astringent taste, while the lower layer will lack extraction, resulting in lower sweetness, lower concentration, and possibly underdeveloped flavor.

The use of cylindrical powder bowls will aggravate this uneven extraction, while the use of truncated conical (trapezoidal) powder bowls can make some compensation for uneven extraction. To help understand, we assume that the preliminary work of the extraction is well done, and then use a cylindrical powder bowl and a tapered powder bowl to complete the extraction. It is assumed that in the two extraction processes, the fine powder has no displacement, and there is no obvious channel formation. Now imagine that you can observe the inside of the powder bed during extraction and cut the powder bed into thin slices in your mind, that is, the cross section (can be imagined as a stack of discs).

In a cylindrical double bowl, the amount of liquid flowing through each layer is equal (let's ignore the water absorption of coffee powder for the time being). This complicates the calculation of the amount of liquid flowing through each layer, but does not affect the fact that the liquid flow per unit area and the extraction rate at the bottom of the truncated cone bowl are higher than those of the cylindrical bowl). And the area of each floor is exactly the same. Therefore, the amount of liquid per unit area flowing through any layer is equal.

In a tapered powder bowl, the amount of liquid flowing through each layer is also equal. However, the larger the area of the upper layer, the smaller the area of the layer through which the liquid flows down. Therefore, from top to bottom, the amount of liquid per unit area of each floor tends to increase (just as two lanes merge into one, and the traffic flow before and after the parallel road is the same. However, after merging, the traffic flow in a single lane is doubled.

The lower layer in the truncated cone-shaped powder bowl, the greater the flow per unit area, so that more material is extracted from the lower layer. Therefore, in this hypothetical extraction experiment, the truncated tapered powder bowl provides a more uniform extraction.

This principle is also applicable to drip coffee, and the use of a tapered filter can also improve the extraction uniformity of the powder bed. It is not difficult to use a tapered filter to make drip coffee, but as far as I know, no commercial trickling filter is equipped with a cone basket. Commercial machines usually offer a variety of compatible filter baskets, preferably the one that is closest to a cone in shape.

Proportion and standard of Espresso cooking

What is Ristretto? What is Normale? What is Lungo?

Although there is a so-called Italian standard, the rest of the world still uses a variety of powder fillers and cups to make Espresso. Therefore, the three names of subheadings have different meanings for different baristas.

Of course, in the same cafe, Normale must refer to the standard product; Ristretto uses the same amount of powder, but less; Lungo uses the same amount of powder, but more. So these three names also represent the proportion of Espresso brewing. (note 8: the term "brewing ratio" was originally used for drip coffee, referring to the ratio of dry coffee powder to the water used for brewing. When making Espresso, because the water absorption of coffee powder is changeable, it is difficult to measure how much water is used in brewing. Therefore, from a practical point of view, we slightly misinterpret the original meaning and define the cooking ratio of Espresso as the ratio of the weight of dry coffee powder to the weight of coffee liquid produced.

According to tradition, baristas use capacity to measure the size of a product, such as 1 oz or 30 ml is the standard Italian Normale. This brings a problem: the amount of Crema varies greatly from product to product, and the amount of liquid from product to product varies greatly. Any barista who puts several products at the same time for a few minutes will find that the amount of liquid left after the Crema has dissipated is very inconsistent.

Using fresh coffee beans, freshly ground and boiled beans, adding robusta beans, using bottomless handles, and other means can all increase the amount of Cerma.

The correct way to compare the proportion of Espresso and the "size" of the product is to weigh the dry coffee powder and the coffee liquid produced. It is not realistic to weigh the products in the course of coffee shop business, and I do not recommend that baristas weigh all the products, but I think baristas should weigh some products intermittently to help improve the consistency of products. Weighing the product also allows baristas to describe more vividly when discussing the amount of powder, the amount of cups, and the proportion of Espresso.

The idea of replacing the production capacity with the weight of the product to calculate the Espresso brewing ratio comes from my friend Andy Schecter, a talented amateur coffee scientist from Rochester, New York. (note 9: a specific discussion of the concept of Andy, as well as the original version of the table shown below, see this page: http://www.home-barista.com/forums/brewing-ratios-for-espresso-beverages-t2402.html).

↑ in this table, AndySchecter defines Ristretto, Normale, and Lungo by Espresso cooking ratio. Not all baristas can agree with the definition of Andy, but the standard he proposes is a common Italian practice, and it is simple to explain and easy to remember. Let's take a look. Andy defines standard Ristretto as the weight of coffee liquid and dry coffee powder; the weight of standard Normale, that is, standard Espresso is twice the weight of dry coffee powder; and the weight of standard Lungo is three times the weight of dry coffee powder. Cafe Crema just stretches the extraction time of Espresso for a long time.

An interesting thing is that the products made by baristas using the quantitative buttons set by the coffee machine are much more consistent than the products that baristas end manually by observing. The amount of Crema produced by using program buttons will vary, but the weight can be guaranteed to be basically the same.

How can baristas use this information about product weight and Espresso cooking ratio? First, the author believes that baristas should weigh some products every day, which helps to maintain the consistency of products. Second, when discussing the extraction rate, bakers and baristas should not only consider the amount of powder and water temperature, but also consider the weight of coffee liquid. Third, baristas should try to use the program's ration button, but should not relax their vigilance on the flow rate and passage.

(chapter 3 ends)