Favorites│Coffee soluble matter, extraction rate and TDS encyclopedia: the secret of coffee beans

What do we extract from coffee beans?

Humans have been studying this issue for thousands of years.

Nature has locked up thousands of delicious flavors in the tiny coffee bean, and all we have to do is release them into our cups.

We grind the coffee into powder and then pour hot water into it, turning each coffee bean into delicious coffee.

Why is it that sometimes the taste of coffee is intoxicating and other times just so-so?

Today, we will explain the principles of coffee extraction from a microscopic perspective and explore what happens when hot water meets coffee powder.

By analyzing the coffee extraction process at the single-cell level, we will have a clearer understanding of the importance of water-to-powder ratio, brewing time and grinding degree, so as to better understand advanced concepts such as coffee extraction and total soluble matter ratio (TDS).

As long as you have a thorough understanding of the basics of coffee extraction, you will not be stumped by any advanced technical terms! Without further ado, let’s start with the cell structure of coffee beans.

The Secret of Coffee Beans

Let's start with Arabica coffee beans. If we think of coffee beans as a "prison," then this prison holds more than 4.5 million "prisoners" (cells)! If you split a coffee bean in half, the general situation should be like this:

The picture above shows a roasted coffee bean being cut open. Notice how the coffee bean is full of holes where the coffee cells were hidden when the bean was still in the fruit stage.

Let's zoom in a little more. The picture above shows the internal structure of a coffee bean at 750 times magnification, from which we can clearly see the individual coffee cells. The width of each cell is equivalent to half the diameter of a human hair (about 50-70 microns).

During the roasting process, these cells produce a lot of carbon dioxide, which causes the cells to swell and contain a lot of soluble substances. "Soluble substances" is a general term for substances in coffee beans that are soluble in water. The purpose of brewing coffee is to liberate these soluble substances from the cells.

Soluble substances come in all shapes and sizes. The chart above summarizes the 4 most common types of soluble substances and the flavor characteristics each substance may bring to coffee.

Fruit acids and caffeine are the most soluble substances and they give coffee a bright taste and fruity flavor.

Cremes are the fats and oils that naturally occur in coffee. These substances are incompatible with water but are released during the brewing process.

Brewing devices that use metal filters, such as French presses and espresso machines, will retain the oils in the coffee, so the coffee brewed with these two devices tastes mellower and smoother.

Because the holes in the filter paper are too fine, the brewing equipment using the filter paper will filter out the oil. Compared with the brewing equipment using the metal filter, the oil content of the coffee made by the drip coffee equipment, such as the hand-brewed pot, is only 1/10 of the original.

During the roasting process, the coffee beans undergo a "Maillard reaction" and produce some melanoidins, which turn the color of the coffee into dark brown.

Carbohydrates make up nearly 50% of the total weight of dried green coffee beans, but only a small portion of them are soluble in water. Carbohydrates contribute to the sweetness and earthiness of coffee.

Let’s go back to the coffee beans. Look, all these colorful soluble substances are locked up!

Don’t be afraid, our “hero” water is here to save them! Once water enters the coffee cells, it will begin to dissolve the soluble substances in them. After they are fully dissolved, the solution formed by water and these soluble substances is the coffee we are familiar with, as shown below:

Once water enters the coffee cells, it begins to dissolve the soluble substances in them. After sufficient dissolution, the solution formed by water and these soluble substances is the coffee we are familiar with, but not all soluble substances are good.

Of the total weight of coffee beans, only 30% is soluble in water , and the remaining 70% is insoluble fiber and carbohydrates.

Of these 30%, only 20% are good soluble substances (i.e. "ideal soluble substances"), and the remaining 10% are bad soluble substances (i.e. " excess soluble substances "). These excess substances make the coffee unpalatable. In order to make coffee more delicious, we need to extract as much "ideal soluble substances" as possible from the coffee and try to avoid extracting "excess soluble substances".

The good news is that “excess soluble matter” usually dissolves more slowly, so controlling brewing time is crucial to coffee quality.

The longer the water is in contact with the cells, the more soluble substances are extracted.

In the coffee industry, " extraction yield " is used to describe the ratio between the total amount of soluble substances locked up in the coffee bean and the total amount of soluble substances that are extracted.

In principle, the longer the water is in contact with the coffee cells, the more soluble substances will be extracted.

The ideal extraction rate given by the Specialty Coffee Association of America (SCAA) is 18-22% , which means that the total weight of soluble substances dissolved in water should be 18-22% of the original weight of coffee beans. If the value is lower or higher than this value, the taste of the coffee will be difficult to achieve the ideal, as shown in the following figure:

Water will only extract the soluble substances it comes into contact with

Now that we have understood the coffee extraction process from a microscopic perspective, let’s look at the role of water in the coffee extraction process from a macroscopic perspective. For ease of explanation, let’s consider the coffee bean as a two-dimensional plane (in reality, the coffee bean is a three-dimensional solid, as shown above).

The picture above shows a coffee bean immersed in water. The water only touches the surface cells of the coffee bean, which are highlighted in bright blue .

Our goal is to allow the water to fully contact all the cells of the coffee beans, so we must allow the water to enter the inside of the coffee beans.

Grind the coffee so that the water comes into contact with more coffee cells

By grinding the coffee beans into smaller particles, we are able to get the water in contact with more coffee cells. The smaller the particles, the more coffee cells the water will come into contact with.

The longer the time, the deeper the water penetrates into the individual particles.

Let's imagine a single coffee particle as a box with 90 cells (30×30, as shown above). In the leftmost box, the water is in contact with the cells for 30 seconds, during which time the water only penetrates the two outermost layers of cells in the box.

When the time reaches 120 seconds, the water has penetrated to the 15th layer. If you stop brewing at this point, the cells in the center will still remain in their original state, and the soluble substances in them will still be locked in the cells. This situation should be considered " insufficient extraction ".

When the time reaches 240 seconds , water has completely penetrated into all cells and all soluble substances have been fully extracted .

Particle size determines extraction rate

What happens if you change the grind size (i.e. the size of the individual coffee particles)? First, let's assume that the brewing time and total number of cells for different grind sizes are the same (as shown in the figure above).

Due to the different particle sizes, it is possible to completely extract all the soluble substances in the coffee in 30 seconds.

Particle size does not determine the types of substances that are extracted, but it does determine the time it takes for water to penetrate all the coffee cells.

Now, do you understand the influence of brewing time and grinding degree on coffee extraction? The two complement each other.

Extraction is all about finding the balance between time and grind

What happens if we extract coffee of different grinds for the same amount of time (e.g. 120 seconds)?

We just said that if the water and coffee are in contact for too long, it is possible to over-extract and release some "excess soluble substances". As shown in the box on the far left of the picture above, the cells shown in orange are those "excess soluble substances".

In the middle box, we have successfully extracted all of the "ideal solubles", which is 20% of the total weight of the coffee. At this point, the time and grind level have reached a perfect balance, all of the "ideal solubles" are fully extracted, and all of the "excess solubles" are still locked in the cells.

In the box on the right, because the water and coffee powder did not have enough contact time, the water was unable to extract all of the "ideal soluble substances", and some substances were still locked in the cells.

From the above figure, we can draw another conclusion, that is, the uniformity of the size of the coffee powder particles has a great influence on the degree of coffee extraction . If the size of the coffee powder particles is uneven, some particles will be over-extracted while others will not be fully extracted.

Here are the grind levels to choose for different brewing equipment:

Espresso:

Hand-drip coffee:

French Press Coffee:

Concentration is the ratio of soluble substance to water

After understanding how to completely extract all the "ideal soluble substances", let's talk about "concentration", that is, the soluble substance ratio (TDS).

TDS refers to the ratio of soluble matter to water in a cup of coffee. Due to individual differences and taste preferences, everyone's understanding of TDS is different. In general, 1.15%-1.35% is a more ideal soluble matter ratio .

In other words, 1 part of soluble substance needs to be mixed with 99 parts of water for the coffee to taste good.

The extracted soluble substances determine the flavor of coffee, and TDS can show the intensity of these flavors . If the flavor is compared to "music", TDS is like "volume".

Increasing the TDS will make the coffee taste stronger, but if the concentration of certain flavors is too high, it will cover other more subtle and delicate flavors. The following figure shows the taste characteristics of coffee at different TDS:

Extraction rate and total soluble matter ratio (TDS) are the two most critical factors in measuring coffee quality. I hope that the content of this article can give you a preliminary understanding of the coffee extraction process, as well as the concepts and meanings of extraction rate and TDS.

English original address:

https://handground.com/grind/an-intuitive-guide-to-coffee-solubles-extraction-and-tds

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