World Coffee Day, have you had breakfast today?

Produced by: Science Popularization China

Author: Zhang Tao (Kunming Institute of Botany, Chinese Academy of Sciences)

Producer: China Science Expo

Today is not only Mid-Autumn Festival, but also World Coffee Day. Are you going to celebrate with a cup of coffee?

In history, some people used the roots of European chicory to make coffee, and now some people use soybeans to make coffee, but the taste of these two kinds of "coffee" is hard to describe, and they are not the same taste at all. So what kind of magical substance makes coffee stand out from many plants and attracts coffee lovers from different countries and different times?

Coffee is delicious: the chemical treasure box in the little beans

This question may have troubled people in the past for a long time, but modern scientific methods have long since unveiled the mystery of coffee beans.

Coffee beans are like a box with a delicate structure. When we open it, the first thing that surprises us is that such a small space is filled with a wide variety of things. According to modern technical analysis, there are about 200 different compounds in unroasted coffee beans, and after roasting, the number of compounds rises to more than 1,000 .

Chemical composition of coffee beans (Image source: barista)

Therefore, the title of "nature's chemical warehouse" is fully deserved by coffee beans.

The wonderful flavor of coffee is of course the result of the combined effects of these compounds. But some of the most important compounds and chemical processes deserve to be listed separately, which are very helpful for us to understand the flavor of coffee, or just to understand more clearly what kind of plant coffee is.

Caffeine: No flavor, but it does get you going

The primary reason why many people like coffee is that it can refresh you, and this function is brought by caffeine.

How important is caffeine to humans? There is a saying that the history of human pursuit of beverages is almost the history of the discovery and use of caffeine.

Chinese tea contains caffeine, cola contains caffeine, and the national drink of South America, yerba mate, which Messi loves, still contains caffeine. In an age when people didn't know what caffeine was, people all over the world seemed to choose caffeinated drinks.

Chemical structure of caffeine molecule (Image source: Veer Library)

If we look at it from the perspective of plants, it is probably difficult to understand this "weird" behavior of humans. Any substance that plants consume energy to produce has its function. In actual application, caffeine is almost a "pesticide". Very young insects will experience endocrine disorders or nerve paralysis after eating leaves or fruits containing caffeine. This is also one of the ways plants protect themselves from harm.

Caffeine is toxic to insects, but just right for humans. The reason we feel sleepy and tired is because the adenosine secreted by the brain binds to the corresponding adenosine receptors. Just like turning off the lights and entering a dark environment, we feel that we should go to sleep. But caffeine can hinder the binding of adenosine and receptors , so the lights that should have been turned off are still on, and we don't feel sleepy.

Now that we have found the reason why coffee can make people stay awake, what exactly affects the flavor of coffee?

Bitterness, the most unique aesthetic characteristic of coffee

Modern coffee quality control uses a coffee cupping system. There are 10 items in the system. Cup testers score each item and finally judge the quality based on the score.

Coffee flavor wheel (Image source: Veer Library)

Surprisingly, bitterness was not among the 10 items.

Coffee is naturally bitter, or it can be said that coffee provides a rare opportunity to taste bitterness. So it is somewhat surprising to learn that this system does not include bitterness evaluation. A more or less self-consistent explanation is that bitterness comes from roasting. Just like roasted meat, it tastes bitter when it is burnt, but the meat itself does not.

Indeed, the brown pigment produced during roasting is the main source of coffee's bitterness, so the darker the roasted coffee, the darker the color and the more bitter it tastes.

Left: Dark roasted coffee beans; Right: Medium roasted coffee beans

(Photo source: Veer Gallery)

Another important substance is trigonelline.

During the roasting of coffee, trigonelline will decompose into niacin and pyridine, thus producing bitterness. However, the main function of trigonelline is not to produce bitterness. As the roasting deepens, it creates more complex taste sensations and produces some bitterness by the way .

In fact, bitterness is one of the reasons why coffee tastes richer.

Sweet, but I don't know where the sweetness comes from

Some people may not be able to taste the sweetness of coffee, but this does not mean that there is something wrong with their taste buds.

Sugar cubes and coffee (Photo source: Veer Gallery)

Coffee berries are full of sugar. If you go to a coffee producing area, you can taste the red fresh berries and they are indeed sweet. This sweetness indicator is also very important for coffee harvesting.

So does harvesting the sweetest coffee cherries necessarily mean that the coffee will become sweeter?

Not necessarily. During the coffee roasting process, the large amount of sucrose contained in the coffee beans themselves will be degraded and turned into raw materials for forming aromatic and acidic substances.

Aromatic coffee beans (Photo credit: Veer Gallery)

The requirement for sugar content in green coffee beans is actually to ensure that more sugar can ensure that the roasting process creates richer aromas , thus making the taste richer.

Although we feel sweetness when we drink coffee, the actual situation is that the sugar has disappeared . This is just like feeling that chicken soup or some vegetable soup is sweet, but there is no sugar in it.

Therefore, the sweetness of coffee is not the sweetness of sugar, but a kind of sweet aroma . As for how this sweetness is formed and what substances it is composed of, there seems to be no very convincing research to prove it.

Acidity emerges: a new experience of coffee in the new era

In Beethoven's time, the sourness of coffee should have been rarely mentioned, because the sourness of coffee is the result of the continuous change of coffee aesthetics.

Nowadays, coffee is mainly served with a light roasting process. The high-quality sour taste is loved by more people, and the substances that produce sour taste are also particularly valued.

Coffee roasting (Image source: Veer Gallery)

In the coffee cupping course, there is an organic acid identification project. Students are required to identify solutions containing citric acid, malic acid, acetic acid and phosphoric acid, and judge their strength, which is very difficult.

We should not forget that although the coffee bean is brown after roasting, it is indeed the fruit of the plant. In fact, the fresh coffee fruit is very similar to cherries, so it is normal for it to be rich in acidity.

Fresh coffee cherries (Photo credit: Veer Photo Gallery)

These four acids are indeed the common sources of sourness in coffee. However, the content of a certain acid depends on the particularity of different growing environments. In a high-altitude growing environment, coffee beans are more likely to produce more citric acid , while coffee beans in a low-altitude environment show more malic acid characteristics, but of course it is not absolute. The acidity of coffee in Kenya is unique. Scientific analysis shows that the soil there contains phosphorus , so the proportion of phosphoric acid in coffee is very high.

At the same time, the acidity also comes from another very important compound - chlorogenic acid.

During the roasting process of coffee, citric acid or malic acid will gradually degrade, and when it reaches the dark roasting stage, not much will remain. However, chlorogenic acid will decompose into quinic acid , which becomes an important source of sourness in medium-dark roasted coffee .

Molecular structure of chlorogenic acid (Image source: PubChem official website)

Interestingly, coffee has a higher chlorogenic acid content when growing in a relatively harsh environment, so plants also need to produce chlorogenic acid to improve their own immunity.

800 kinds of aromas: drinking and smelling are both indispensable

Aroma is probably the most complex phenomenon. Even with the most advanced scientific equipment, it is probably impossible to accurately replicate the aroma and changes produced when roasting beef.

Roasted coffee contains about 800 types of aromatic compounds, most of which have small molecular weights and are very ethereal and volatile.

Some disappear the moment baking is completed, some are only released when they come into contact with hot water, and some turn into different flavors over time.

The aroma of dark roasted coffee beans (Image source: Veer Gallery)

The aroma created by the roasting process itself is not constant . Different temperatures and roasting times will lead to changes in the aroma of coffee. It is impossible to describe all of them. However, there are two important processes that produce coffee aroma that are worth understanding.

The Maillard reaction occurs in almost every cooking process. It is a browning phenomenon that occurs when carbohydrates and amino acids are heated . The entire process of coffee roasting is almost a battlefield for the Maillard reaction. It is difficult for us to fully understand what is happening in the microscopic world, but judging from the magnitude of the taste change, it must be extremely intense.

Caramelization occurs mainly in the later stages of coffee roasting, mainly the dehydration of monosaccharides . The various nutty chocolate aromas we feel in coffee are directly due to this.

Conclusion

Small beans can make countless people in the world intoxicated and obsessed. There is a vast world of flavor chemistry hidden in coffee, bitter, sour, sweet, fragrant... Just a sip, these brown beans can stimulate all your senses.

The process of tasting coffee is not only a process of letting your taste buds embark on a flavor journey, but also an excellent opportunity to deeply understand the flavor science behind coffee.

References:

1. William Harrison, All About Coffee, 1922.

2. Antony Wild, Coffee: A Dark History, 2005.

3. Shi Xie Zhiguang, You Don’t Understand Coffee, 2014.

4. Scott Rao, The Coffee Roaster's Companion, 2014.