A must-read for coffee brewers | Coffee brewing from the perspective of water

This article is reproduced from the official account: Roast Magazine Operation

A cup of hand-brewed coffee is more than 98% water, so water quality is critical and complex. Brewing coffee and dissolving flavors into water increases the complexity of water issues. Understanding what happens during the brewing process can be very helpful in choosing water for brewing coffee.

Flavour characteristics of brewed coffee

When analyzing coffee flavor profiles, we should acknowledge a practical problem: each person perceives the same coffee differently. This makes coffee the most difficult beverage to measure, both chemically and from a sensory perspective.

A cup of coffee has to go through the process of "from seed to cup", and brewing is just the last step in this complex process. But if the water used to brew coffee does not meet the requirements, all the efforts mentioned above - the hard work of coffee farmers, the unparalleled special climate environment of the origin, and the superb roasting skills of roasters will be wasted.

Coffee drinks are mostly water, and we should understand the relationship between the flavor-containing substances in coffee beans and water. Extracting flavor substances requires a comprehensive consideration of the solution (water), solvent (coffee), temperature, and other variables and factors in the coffee brewing process.

Coffee as a whole is composed of liquid (including oil), gas, and soluble solids. Some substances float on the surface of the coffee liquid and present a moist aroma , while some solid substances sink to the bottom of the cup. This phenomenon is more obvious when brewing coffee without filter paper.

All flavor substances in brewed coffee have their own individual characteristics. These different flavor substances can complement each other, promote each other, or offset (cover up) each other . (Note: Due to the mixture of various olfactory substances, there is a phenomenon of odor compensation, such as the alleviation of stench and the volatilization of aroma.) The final flavor of brewed coffee is a complex and comprehensive balance result of many independent flavor bodies . If the content of some flavor substances is too high or separated from the overall coffee, its flavor is not easy to be accepted by people.

The olfactory perception of coffee comes from the olfactory lobe in the nasal cavity. In the experiment, the subjects were asked to close their eyes and pinch their noses to taste a mixture of cinnamon powder, sugar, and salt. Generally, the subjects could only taste the sweetness and saltiness but could not judge the cinnamon powder. After releasing their hands and restoring their sense of smell, the subjects could perceive the presence and intensity of the cinnamon powder component and then have an overall judgment and perception of the flavor and taste of the experimental mixture. (Note: In a sense, there is no taste without smell. Smell can produce specific tastes in the nasal cavity, mouth, and brain, which are closely related to what we like and hate to eat.)

The taste of coffee comes from the taste buds on the tongue. The sensitive nerves in the mouth are used to perceive the coffee's thickness, smoothness, temperature, texture, and pressure. The trigeminal nerves perceive the flavor of stimulants such as onions, chili peppers, mint, black pepper, etc. The perception of coffee flavor is not just a simple accumulation of the above discussion, but these contents help us analyze coffee extraction and the impact of water quality on coffee flavor.

aromas

Aromas or aromatic compounds (Aromatics) are perceived through the olfactory lobes in the nasal cavity. The sense of smell is generally produced through two pathways (which can also explain that smell is a dual sense): orthonasal and retronasal. Orthonasal olfaction is understood as inhaling from the nostrils (sniffing in), such as the perfume we smell, something outside, the aroma of food before entering the mouth, etc. Retronasal olfaction is called retronasal because it comes from the back of the mouth (from the mouth to the throat). This is how we perceive smells when we eat. The gas produced by food exhaled (inhaled) through the mouth stimulates the cells in the nasal cavity, and the smell is produced. (Note: This is consistent with the wave theory or vibration theory (R. H. Whight) in the theory of smell, which advocates that the excitation of olfactory cells is the stimulation of molecular vibration energy. Sipping in coffee cupping should be understood as maximizing the vibration of aroma molecules, rather than "gasifying" aroma substances. Another theory of smell is: the microparticle theory believes that olfactory cells absorb odor particles (molecules) to produce chemical reactions, thereby causing excitement. This theory is based on general water-soluble substances, lipid-soluble and volatile strong olfactory substances. Among these olfactory substance molecules, there are olfactory atomic groups or fragrance groups (osmophoris groups) such as H groups, lipid groups, ketone groups, phenyl groups, and nitro groups.

The olfactory lobe (cells) has a large number of cilia that exist in a layer of secreted mucus. Soluble aroma substances dissolve in the mucus. The cilia of the olfactory lobe sense these aroma substances and send signals to the brain, so that people can perceive the stimulation and analyze the type and intensity of the smell.

In brewed coffee, the perceived aroma comes from the aromatic substances in the ground coffee and other aromatic substances that are not from the ground coffee but are produced (dissolved) in the coffee liquid during the brewing process. The aroma of brewed coffee is different from the aroma of ground coffee. Some obvious odors in ground coffee, such as roasted flavor and sulfur flavor, do not dominate the brewed coffee. On the contrary, some aromas that are difficult to detect in ground coffee can be the main flavor of brewed coffee. The volatility and stability of aromatic substances also have a great influence on the aroma of brewed coffee. For example, long-chain aldehyde aromatic substances such as floral and fruity aromas are very fine and easily oxidized and volatilized quickly, while earthy odorants are very stable once extracted.

Different aromas have different intensities, and their content also affects the perception. For example, diacetyl, an important aromatic substance in coffee, has a pleasant buttery aroma at low levels, but too high a content will produce burnt and sulfurous odors. Vanilla aroma exists in coffee to some extent, but it is difficult for us to detect it because of other stronger aromas.

Most of the aroma compounds are found in the oils of brewed coffee. The extent to which these aroma compounds evaporate into the atmosphere depends on the temperature, their characteristics, and the interaction with the gases on the surface of the liquid. For example, acetic acid is very active and can be easily detected in any liquid. Liquid surfaces can release more volatiles than solid surfaces, and these volatiles will move with the gases in the environment. For example, when a cupper breaks the surface a crust of coffee, the carbon dioxide accumulated under the wet powder layer will release aroma compounds into the ambient atmosphere.

More than 1,200 aromatic compounds have been discovered in coffee. Aroma is the most variable component of coffee. Aroma is affected by the origin of coffee, coffee tree species, and the degree of coffee roasting. There are many aromas and we generally classify them into categories such as sweet, earthy, etc. (SCA flavor wheel). Different aroma descriptions constitute the flavor profile of a particular coffee.

Some acids are also necessary components of coffee aroma. If they can be sensed by olfactory cells, they should be classified as an aroma (Note: Some research reports indicate that humans have four basic senses of smell, namely aroma, sourness, sugariness and rancidity). Important acids include lactic acid (lactic acid has a similar perception to sour cream) and acetic acid. If the pH value or total alkalinity of the water used to brew coffee is relatively high, this type of aroma will be affected.

The overall aroma of a cup of brewed coffee is affected by the coffee grind, water quality, brewing temperature, and brewing time. These variables will affect the quality and amount of extracted aroma.