How to make a cup of coffee that keeps workers warm? Use these scientific methods

If you are troubled by the unstable taste of the coffee you make, you may need to understand which factors affect the flavor and make more precise control variables for related parameters. Fortunately, the regulation of coffee flavor is also a topic of concern in the academic community, and many scientific researchers are helping you solve these problems.

Written by Li Cunpu (Professor of College of Chemistry and Chemical Engineering, Chongqing University)

Coffee has become a daily staple for many people in both the West and the East. Unlike the traditional tea culture of soaking tea leaves for drinking, the mainstream coffee preparation process is to grind coffee beans into powder and then use hot water to rinse the coffee powder for extraction. When tasting, there is less of the old cadre's attitude of spitting tea leaves into the cup, and more of the social animal's demeanor of swallowing the remaining coffee powder.

Nowadays, busy workers are constantly looking for more delicious and stable coffee flavors, but the results are only satisfactory. This article will explore the methods and strategies for precise control of coffee flavors based on the flavor, taste, and concentration requirements of coffee, in order to help readers make a more stable and perfect cup of coffee.

1

What are we drinking when we drink coffee?

When I was still more accustomed to pretending, I tried to study the coffee flavor wheel shown in Figure 1. I hoped that when I took a 50mL cup of espresso that had just been extracted with 9 bar pressure ("extraction" in chemistry is mostly used to refer to liquid/liquid dissolution separation, coffee is "solid-liquid" extraction, and "leaching" is mainly used in chemistry to express it.), I would quickly suck it in and come to a conclusion such as "the top notes have the taste of nuts and dark chocolate, and the base notes have a clear citrus flavor."

Figure 1 Common coffee flavor wheels.

But now, I just like to order a cup of iced Americano in advance, which is as bitter as life, and drink it all in one gulp. Putting aside the complex coffee flavor wheel that may not be distinguished by everyone, the taste of coffee itself is indeed complex and tempting, because the coffee liquid contains more than 2,000 molecules. As shown in Figure 2, in addition to the most directly needed caffeine molecules, various aromatic molecules, organic acid molecules, carbohydrates, lipids, etc. are combined to form the flavor of coffee. For example, isobutyraldehyde provides spicy taste, acetaldehyde brings fruity aroma, furanone makes coffee produce sweetness, and pyrazines bring soil taste to coffee, etc. These sour, bitter, astringent, fruity flavors, and the smooth taste provided by some esters together form the flavor of coffee. The differences in coffee bean varieties and origins are naturally self-evident. The processing methods of various coffee beans, such as washing, sun drying, roasting temperature and time, fermentation degree, etc., will affect the various components and their proportions in coffee beans. This is especially important in boutique coffee shops-many special beans are paired with skilled baristas to let us drink a cup of coffee full of surprises.

Figure 2 Representative molecules in coffee[1]. In addition to caffeine, coffee also contains many other organic compounds, including sulfur-containing organic compounds, aldehydes and ketones, phenols, furans, and pyrazines.

2

Control variables and make a good espresso

Many people choose to buy a coffee machine so that they can make coffee at home or in the office at any time. This allows them to better adjust the taste of coffee according to their own preferences, and it is more cost-effective than going to a coffee shop every day. What is frustrating is that the coffee you make with a coffee machine is certainly difficult to compare with the products of boutique coffee shops, and even compared with the products of chain coffee shops using the same coffee beans, there is a big gap. Moreover, using the same machine and the same coffee beans, the taste of the coffee is difficult to be stable. (Unpredictable coffee, coupled with a boss who changes his face faster than Sichuan Opera, makes the life of office workers even worse.)

So, is there a way to make stable and delicious coffee with a home coffee machine? Yes. For most coffee machines, the basic coffee product is espresso. After simple modulation, espresso naturally derives into Americano, latte and other varieties. Therefore, if you master the core technology of preparing espresso, you can be far ahead in all kinds of fancy coffee. The method of espresso is very simple. Grind 15-22g of coffee beans into powder, spread it all over the powder bowl and compact it, then use 9-10bar pressure to inject 92-95℃ hot water into the powder bowl, and collect 30-60g of coffee liquid. It seems that as long as the above-mentioned variables such as pressure, temperature, volume, etc. are controlled, the taste and flavor of coffee should be very stable.

Unfortunately, accurate pressure and temperature control requires complex mechanical structures and expensive sensors. For example, if you want to accurately and stably control the water outlet temperature, you need a boiler with good thermal insulation and stable heating to provide hot water. Therefore, only expensive commercial coffee machines can relatively accurately control the above factors. The actual working state of home coffee machines often has significant deviations due to insufficient performance. But it doesn’t matter. Your own coffee machine is designed to serve you. By controlling variables, we can always make our own coffee machine a powerful tool for making "special" coffee.

In 2020, Christopher H. Hendon of the University of Oregon and Jamie M. Foster of the University of Portsmouth published a paper in Matter magazine, reporting a mathematical model that can systematically optimize espresso. As shown in Figure 3, if the preparation process of espresso is regarded as a chemical extraction process, then the place where the extraction takes place is the powder bowl filled with coffee powder, and the preparation process of coffee is to extract the flavor components from the coffee powder into the coffee liquid. The weight ratio of the substances extracted into the coffee liquid to the dry coffee beans is defined as the extraction yield (EY).

Figure 3 Schematic diagram of the structure of an espresso powder basket and the distribution of coffee powder [2].

According to the recommendations of the Specialty Coffee Association (SCA), espresso should have an extraction rate of 17-23%. When the extraction rate exceeds 23%, the coffee will be bitter; when the extraction rate is less than 17%, the coffee will be sour. Obviously, the grinding of coffee powder, the pressure of hot water for extraction, and the amount of coffee powder used will directly affect the final extraction rate of coffee and the taste of coffee.

Now let's make a 40g cup of espresso.

Grinding and flavor

The first step is to grind the coffee beans. According to Hendon's model, as shown in Figure 4, the finer the particles of the coffee beans ground by a grinder, the greater the extraction rate. This is intuitive because the finer the particles, the larger the total surface area, and the easier it is to extract the flavor substances in the coffee powder (the blue line in Figure 4C). The model is also modified according to actual conditions, because in practice, as the desired particle size set by the grinder becomes smaller and smaller, not all coffee beans can be ground evenly, and there are always some relatively large particles that are not ground to the desired particle size. When brewing with hot water, these large particles interact differently with water and other fine powders, forming a partially blocked fluid. (For partially blocked fluids, the Hendon model is represented by the yellow line in Figure 4C.)

In the experiment, when the grinder was set to a finer particle size, the actual extraction rate (circle) was closer to that of a partially blocked fluid; when the particle size was set to a coarser particle size, the partial blockage phenomenon no longer existed, and the actual extraction rate (circle) was closer to that of a standard fluid.

Figure 4 (AB) Particle size distribution of coffee powder; (C) Relationship between coffee powder grinding coarseness and extraction rate when the extraction pressure is fixed to 6 bar. The blue line is the standard fluid model, that is, the finer the particles, the higher the extraction rate; the yellow line is the partially blocked fluid model, that is, when there are large and small coffee powders, the fluid will not flow evenly. The circles are the actual situation, which is close to the yellow line in the case of fine grinding and close to the blue line in the case of coarse grinding.

Coffee Quantity, Pressure and Flavor

Another key question in making coffee is how much coffee powder should be used? As shown in Figure 5, the larger the amount of coffee powder, the lower the extraction rate of the coffee. This is a bit counterintuitive. After all, the more powder, the stronger the coffee should be. How can the extraction rate be lower? In fact, the more coffee powder, the stronger the espresso will be; but for all the coffee powder, the proportion of extracted substances in the coffee powder decreases. This means that if you put more coffee powder and a relatively coarse grinding degree, you can get a strong (more powder) and sour (low extraction rate) espresso; you can also use relatively less coffee powder to make a lighter (less powder) but bitter coffee (high extraction rate).

As for why the extraction rate affects the flavor, solubility may be one of the key factors. Acidic substances in coffee, such as quinic acid, have a high solubility, so they are easy to dissolve and most of them will be dissolved at a lower extraction rate; while some bitter flavor substances, such as phenylindane and quinides, have a low solubility and can only be dissolved at a higher extraction rate. Therefore, a low extraction rate is more acidic and a high extraction rate is more bitter.

Figure 5 (A) The relationship between the amount of powder and the extraction rate: the more powder, the lower the extraction rate (but the coffee may be stronger); (B) The relationship between pressure and extraction rate: the greater the pressure, the lower the extraction rate (but it may bring out more flavor substances that are not easily soluble in water).

Another interesting thing is the relationship between extraction pressure, extraction rate and flavor. As shown in Figure 5(B), since the total amount of coffee liquid is controlled to 40g, a higher extraction pressure means a faster water flow rate, and the water flows quickly through the coffee powder, so the extraction rate is naturally low. However, for special flavor substances in coffee, some have very low solubility, and they are brought out by the hot water flowing through instead of being dissolved (just like washing hands, water takes away some dirt instead of dissolving it in the water), so in practice, higher extraction pressure often brings a richer taste, and finer coffee powder is needed to obtain a neutral taste.

Customize your flavor

For our own home coffee machine, we have mastered the key to regulating the taste. Increasing the amount of powder can increase the concentration, increasing the pressure can increase the richness of the taste, and controlling the grinding particle size can regulate the extraction rate - when the coffee tastes sour, grind the powder finer and reduce the amount of powder; if the concentration is not bitter enough, increase the amount of powder and use pressure to control the acidity. If you want a particularly comprehensive flavor, you can even make two cups with different parameters and then mix them together. As shown in Figure 6, we can use less coffee powder and coarser grinding particles to get a cup of espresso that is not too strong and has a high bitterness ratio (green dot); and then use more coffee powder and higher grinding particles to get another cup of espresso with higher acidity and concentration (purple dot). After mixing the two, you can get a double espresso with a relatively neutral acidity and bitterness. It's more bitter than your own life, and more sour than looking at other people's year-end bonuses.

Figure 6 The comprehensive flavor can be obtained by mixing two cups of purple and green. Sour and bitter, life is just like this.

3

Super secrets of grinding

We may have discovered that grinding seems to be the most complex part of coffee making. Christopher H. Hendon et al. from the University of Oregon further discussed the issue of triboelectric charging of coffee powder by grinding in the January 2024 issue of Matter magazine. [3] In fact, during the coffee grinding process, the coffee powder will be charged due to the friction between the coffee beans and the blades, the uneven distribution of charge during the coffee splitting process, and other reasons. It is easy to understand that triboelectric charging with the blades is due to the mutual friction contact between the two substances, and the electrons of one substance are transferred to the other. As a result, the substance that loses electrons is positively charged, and the substance that gains electrons is negatively charged.

It is worth mentioning that the process of coffee beans cracking also generates static electricity, and its principle is very similar to the charging of volcanic ash explosion. As shown in Figure 7, at the small particle level, when coffee particles crack, the electrons/ions in them will be released due to the breaking of covalent bonds[4]. Therefore, the remaining coffee powder will carry positive or negative charges. This is the main reason why coffee powder is charged during the process of grinding into fine particles.

Figure 7 Volcanic ash charging and subsequent condensation mechanism. At the small particle level, the edge electrons are released after the particles are cracked, so that the remaining particles carry positive charge.

The charge of coffee powder will cause the powder to aggregate, affecting its particle size distribution. Researchers have found that the variety of coffee beans, roasting method, and the charge and particle size distribution of the ground coffee powder have little to do with each other, but deep roasted beans are relatively less charged and the coffee powder aggregates less. Maybe this is why Starbucks uses more deep roasted beans?

Figure 8 The horizontal axis is different coffee bean brands, and the vertical axis is the charge/mass ratio of the coffee powder. The gray is the first fine grind, and the black is the second coarse grind. It can be seen that the particle size distribution remains unchanged, but the charge of the coffee powder is significantly improved.

In addition, if you don't mind the trouble, the second grinding can significantly reduce the charge and agglomeration of coffee beans. As shown in Figure 8, the first fine grinding will obtain finer coffee powder, but the coffee powder will be more charged and agglomerated; then use a coarser gear to grind once, which will not make the coffee powder finer, but can reduce the charge and agglomeration of coffee powder.

More importantly, increased humidity can reduce the charge of coffee powder. As shown in Figure 9, no matter what kind of coffee beans, spraying a little water before putting them into the grinder can significantly reduce the charge and agglomeration of coffee powder and improve the quality of coffee. This may be the essential reason for the "ross droplet technique" technology widely circulated on social media (spraying water on coffee beans before grinding can effectively reduce static electricity flying powder).

Figure 9 Spraying a little water on the coffee beans during grinding reduces the aggregation of coffee powder and makes the particle size finer. Therefore, the flow rate of coffee liquid slows down and the concentration of coffee increases

4

Seeking scientific answers

Research on coffee making has always been an interesting area of ​​academic interest. For example, in 2015, Sauro Vittori of the University of Camerino in Italy compared the differences in flavor and chemical composition between espresso and brewed coffee[5]. The caffeine and chlorogenic acid content of brewed coffee is higher than that of espresso, which means that brewed coffee may have a better refreshing effect. If you want to control the preparation of coffee from the source, Magdalena Jeszka-Skowron of the Poznan University of Technology in Poland published an article in 2016 discussing the effects of coffee bean varieties and roasting processes on the flavor components of finished coffee beans[6].

Perhaps in the near future, when we can analyze and easily obtain all the flavor molecules in coffee, the flavor of the coffee we make may be more stable: we select the amount of caffeine, tannic acid, various thiols, and phenols, use program control to quantitatively inject various flavor molecules into water, heat and shake them, and then deliver them to your hands. The taste will always remain constant and can always be customized, but perhaps this has deviated from the original demand of coffee - relaxation rather than refreshment.

Drink this cup of customized coffee that belongs to you alone and embrace life with enthusiasm.

References

[1] https://www.compoundchem.com/2015/02/17/coffee-aroma/

[2] Cameron et al., Matter 2, 631–648. https://doi.org/10.1016/j.matt.2019.12.019

[3] Mendez Harper et al., Matter 7, 266–283. https://doi.org/10.1016/j.matt.2023.11.005

[4] Cimarelli, C., Behnke, S., Genareau, K. et al. Volcanic electrification: recent advances and future perspectives. Bull Volcanol 84, 78 (2022). https://doi.org/10.1007/s00445-022-01591-3

[5] Giovanni Caprioli, Manuela Cortese, Gianni Sagratini & Sauro Vittori (2015) The influence of different types of preparation (espresso and brew) on coffee aroma and main bioactive constituents, International Journal of Food Sciences and Nutrition, 66:5, 505-513, DOI: 10.3109/09637486.2015.1064871

[6] Jeszka-Skowron, M., Sentkowska, A., Pyrzyńska, K. et al. Chlorogenic acids, caffeine content and antioxidant properties of green coffee extracts: influence of green coffee bean preparation. Eur Food Res Technol 242, 1403–1409 (2016). https://doi.org/10.1007/s00217-016-2643-y

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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