Scientists examine how roasting affects the perfect brew

A new study in published in Scientific Reports suggests that the perfect cup of coffee is influenced by a complex blend of variables such as bean processing method, brewing time, and grind size, not just the roast level.

Caffeine content and extraction yield are two of the most vital variables for coffee enthusiasts, especially those who approach it with precision.

Extraction yield is a measure of the amount of soluble material from the coffee grounds that gets dissolved in the brewed coffee. It essentially reflects the efficiency of the brewing process in extracting compounds from the coffee grounds.

Led by Dr. Zachary R. Lindsey, Assistant Professor of Physics at Berry College, U.S., the study focuses on how the degree of roast affects these two variables. Phys.org spoke to Dr. Lindsey, a self-proclaimed coffee nerd about the study.

“Over 20 years ago, I heard a barista claim that dark roasts have more caffeine, but a decade later, I was exposed to the contrasting idea that light roasts were the king of caffeine. Yet, I couldn’t find any convincing data.”

“It wasn’t until I picked up coffee roasting as a hobby in 2022 that I started to see the missing pieces of the puzzle. Luckily, two passionate undergraduate students on my research team were also intrigued by this mystery, and we got to work,” said Dr. Lindsey.

Choice of coffee, roast, and brew method

The researchers chose Ethiopian coffee to conduct their study. Ethiopia has a long tradition of producing coffee dating back centuries as it is the country where Coffee arabica, aka the coffee plant, originates.

In this, they are investigating natural and washed processed coffee.

In the natural method, the coffee cherries are dried with the seeds still inside. The seeds are separated after drying, resulting in fruity and complex flavors in the coffee beans. On the other hand, in the washed method, the seeds are separated from the coffee cherries and then dried, leading to a cleaner and brighter flavor profile.

The researchers then used five different degrees of roasts for the green coffee beans, choosing a brewing time of one, two, and ten minutes.

The researchers chose the AeroPress brewing method with a 15:1 water-to-coffee ratio. The AeroPress is a pressure-based brewing method, similar to an espresso machine, but on a smaller scale. The AeroPress steeps the coffee and uses pressure to extract the brew through a paper filter.

Dr. Lindsey explained the choice behind the AeroPress, saying, “When selecting a brew method, the main goal was to implement a procedure that could consistently produce brews within a wide range of extraction yields by only varying the brew time.”

“The AeroPress stood out as a means to achieve these desired outcomes with minimal variation across all roast batches.”

Overall, the researchers had 30 unique combinations of brewed coffee to study.

Analyzing the coffee

The researchers used three analysis techniques to analyze caffeine content and extraction yield.

To measure compounds like caffeine, chlorogenic acids, and other soluble compounds in the brewed coffee, they used high-performance liquid chromatography (HPLC).

This method separates different compounds in the coffee based on their interactions with a standard material, quantifying individual concentrations.

Next, they used refractometry. This method measures the bending of light through the brewed coffee, indicating the extraction yield, i.e., how much soluble material is dissolved from the coffee grounds.

Finally, they used scanning electron microscopy (SEM) to observe the surface of the coffee beans and grounds. This helped them to examine the grain size and porosity. SEM provides information about the impact of roasting on the physical features of the coffee beans.

“SEM allows for a straightforward characterization approach that provides two-dimensional information about the structure of the roasted coffee. The evolving porosity of the roasting coffee plays a pivotal role in compound mobility during roasting and brewing,” explained Dr. Lindsey.

Porosity, caffeine, and extraction

The researchers found that caffeine content in the brewed coffee depended on the roasting process and the extraction yield.

“During roasting, the volume and porosity of the coffee seeds increase as the roast progresses, which makes it easier for compounds to move in or out of the system,” explained Dr. Lindsey.

A greater porosity implies more of the inner surface area of the coffee grounds is exposed, making it easier for water to penetrate and dissolve compounds like caffeine and flavors. This has an impact on the entire extraction process that occurs during brewing.

For the caffeine content, the researchers found that when using identical brewing setups, light and medium roasts had a higher caffeine content than darker roasts. This is due to the caffeine loss during roasting, resulting in typically lower extraction yields for darker roasts.

Conversely, they found that the darker roast’s caffeine content was higher when the extraction yield was kept consistent for all the roasts.

“However, darker roasts consistently exhibited lower extraction yields than light and medium roasts, so it was not always possible to achieve a common extraction yield for all degrees of roast,” added Dr. Lindsey.

New insights

The competing mechanisms of increased porosity improving extraction efficiency and darker roasts losing extractable compounds revealed a unique insight contradicting previous assumptions.

Caffeine sublimation—the process of caffeine transitioning directly from a solid to a gas—occurs at higher temperatures than previously thought.

“Although the interplay between roast degree and caffeine content has been addressed over 20 times in the literature, the prevailing theory is that caffeine remains stable during the roasting process.”

“However, we establish a clear relationship between roast degree, caffeine content, and extraction yield,” said Dr. Lindsey.

The researchers plan to extend this work to study the relationship between roast degree and extraction yield for decaffeinated coffees. They also aim to test it with percolation-based brewing methods to see if they yield similar results.

The bottom line is, if you want a cup of coffee with the maximum caffeine content choose a medium roast, says Dr. Lindsey.

© 2024 Science X Network

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