The secret to making every cup of coffee taste good: brewing coffee with science

Coffee comes in many varieties and is rich, mellow and smooth, making it a favorite of many people. According to statistics from the National Coffee Association, the average American drinks more than three cups of coffee a day, and the total economic impact of the U.S. coffee industry was $225.2 billion in 2015. Although coffee is extremely popular, regular coffee drinkers may have encountered this situation: even in the same coffee shop, with the same barista and the same batch of coffee beans, sometimes the coffee tastes great yesterday but tastes burnt and bitter today. So, what determines the taste of a cup of coffee?

Espresso

Borrowing the formula of solid-liquid extraction diffusion principle (physical extraction) (those who have a headache when they see the formula can skip this paragraph), G represents the amount of diffused solute. The taste of a cup of coffee depends on the concentration of dissolved coffee substances. Therefore, the taste is closely related to the extraction rate (we call it water-powder ratio), extraction temperature, pressure, extraction time, grinding coarseness, etc.

G, the amount of diffusing solute; D, diffusion coefficient - corresponding to the determining factors "water temperature" and "pressure"; F, diffusion area - corresponding to the determining factor "grinding coarseness"; ΔC, interface concentration difference; ΔX, diffusion distance; t, extraction time.

About 30% of a coffee bean is soluble, but the more these substances dissolve, the better it tastes. Dr. Christopher H. Hendon of the Massachusetts Institute of Technology (MIT) (currently an assistant professor at the University of Oregon) has conducted a series of in-depth studies on coffee. He found that for most people, a cup of coffee tastes good when it contains 1.2%-1.5% coffee ingredients and 8%-10% coffee ingredients. The former happens to be the extraction degree of most filter coffee, while the latter corresponds to the extraction degree of espresso.

There are very few methods that can extract 8%-10% of coffee ingredients, and the espresso machine is the most common one. However, there are many methods that can achieve an extraction ratio of 1.2%-1.5%, and the common ones are hand pouring, French press, syphon pot, Aeropress, American drip coffee, etc. The equipment for these methods has one thing in common: they are all cheaper than espresso machines.

Coffee extraction method

Of course, an espresso machine can also make coffee with a ratio of 1.2%-1.5% - diluting a cup of espresso with hot water gives us what we often call "Americano". So the question is, when the coffee content in coffee drinks is similar, why do they taste so different?

Coffee berries on the tree

This brings us to the extraction order of coffee. The first to be extracted are some organic acids with fruity sourness and Maillard reaction products that occur during the roasting process; as the extraction rate increases, the bitter substances in the coffee will continue to be extracted; and the last thing to be extracted will be some unpleasant flavors such as burnt taste and astringency. The Specialty Coffee Association of America (SCAA) once drew a coffee "flavor wheel" to help people analyze the different flavors of coffee.

SCAA’s Coffee “Flavor Wheel”

Therefore, the right water temperature helps to extract a coffee liquid with a balanced acidity and bitterness, creating a "sweet" taste, while too high a water temperature will cause many bad-tasting compounds to flow into the water, causing the coffee to taste burnt and bitter, which we call "over-extraction."

The grinding size of coffee is also very important. The finer the coffee powder, the larger the specific surface area, the more complete the extraction, but the speed of water flowing through will also be slower, resulting in over-extraction. Studies have found that the type of coffee beans does not have much effect on the grinding size, but different processing temperatures of coffee beans will lead to obvious differences in size. When the coffee powder is uneven in size, it is easy for the coarse coffee powder to be under-extracted, while the fine coffee powder is over-extracted, and this cup of coffee must be a cup of unpalatable coffee.

Particle size distribution is independent of bean type. Image source: Sci. Rep.

When choosing a coffee grinder, do not choose a propeller blade, because the coffee powder it grinds is uneven in thickness and is nicknamed a "bean chopper". If conditions permit, it is best to choose a parallel blade or a star-chasing blade, which can grind coffee powder of relatively uniform thickness.

Propeller cutterhead (left) vs. parallel cutterhead (right)

The freshness of the coffee beans also affects the taste of a cup of coffee. Freshly roasted coffee beans contain carbon dioxide and other volatile substances that are slowly released from the coffee beans. Over time, these volatile substances will gradually decrease, which will detract from the taste of the coffee. For this reason, most cafes will not use coffee beans that have been roasted for more than a month for hand brewing. Lower temperatures will reduce the rate of evaporation, which explains why storing coffee in the refrigerator can extend its shelf life.

Steaming steps during hand brewing, the escape of gas from coffee powder

In addition, water quality can also affect the taste of coffee. Compared with "soft" water, using "hard" water with higher magnesium and calcium ion content will make the coffee have a stronger taste, because compounds such as caffeine will "stick" to the magnesium ions during the brewing process. Hard water also contains higher levels of bicarbonate, which will make the coffee more bitter.

“For whatever reason, science has been away from coffee for a long time,” said Christopher Hendon, who is nicknamed “Dr. Coffee.” “Therefore, there is a lot of wealth to be found in it. I look at coffee as an object for experimentation. Remove the human factor and the sensory influence, and you get the coffee that is truly pure.”

Latte. Image source: Xiaoxi

To sum up, the secret to making every cup of coffee taste good is: use a grinder with parallel blades or star-chasing blades to grind the freshly roasted coffee beans into uniform coffee powder, then use slightly hard water, under the appropriate water-to-powder ratio and extraction temperature, pressure, and time conditions, to achieve an extraction degree of 1.2%-1.5% or 8%-10%.

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Person A: Have you heard that “Starbucks coffee causes cancer”?

Xiaoxi: Go away...

Person A: The American judge said it!

Xiaoxi: You should be more careful. If coffee causes cancer, all Americans would have died long ago. Why would we open Starbucks?

A: Coffee contains a carcinogenic chemical - acrylamide!

Xiaoxi: Oh~~ You know about acrylamide. I’m too lazy to tell you.