Effect — Agitation

In my previous article, I began to tell you about a physicist and coffee enthusiast from over 200 years ago who went about defining coffee quality in his quest to brew the perfect cup. He observed that as the aroma of coffee filled a room, the quality of the brew declined. He concluded that in order to brew the perfect cup of coffee, it was important to minimize aroma loss.

As he gazed into his brewed coffee, he noticed that the aroma was escaping quickly — too quickly for it to be a stationary liquid with no movement. He knew that cooling initiated convection currents within the brew, facilitating aroma loss and, consequently, quality loss. He concluded that to preserve quality and achieve the most perfect cup, these convection currents — in other words, the agitation — had to be minimized throughout the entire extraction process.  

He decided to evaluate each step of the brewing process with an eye to maximizing aroma retention by minimizing or eliminating convection currents. Instead of trying to address thermal convection as a whole, he broke it down into its components: the cause (temperature difference) and the effect (fluid movement, or agitation). He wrote,

“All kinds of agitation must be very detrimental to Coffee, not only when made, but also while it is making ….”

By studying each step of brewing, he reasoned, he could systematically discover the potential causes of aroma loss and provide practical suggestions for reducing the aroma loss caused by agitation.

Brew Method ¹

One factor in making the perfect cup of coffee is deciding which brew method to use. Because he lived before the invention of the espresso machine, our coffee lover had fewer brewing options to choose from. The most traditional method, which he recounts, involved ‘putting the ground Coffee into a coffee-pot, with water, and boiling them together’, resulting in what we would now recognize as Turkish, cezve, or ibrik coffee. Boiling violently agitates the brew,² throwing the grounds every which way and, according to his understanding, enabling aroma to escape. Our scientist concluded that this brew method ‘must be very defective, and must occasion a very great [aroma] loss’. While we now understand that coffee lipids and crema³ contribute to aroma retention in these brew methods, the protagonist’s rationale still holds true: vigorous agitation from boiling water in tandem with coffee encourages aroma loss. This is why, in a modern laboratory setting, researchers traditionally degas⁴ water by boiling it before beginning their experiments.

In a contemporary laboratory, our scientist’s claims could be easily substantiated by combining steam distillation with the immersion brew method.⁵ Steam distillation involves collecting the vapour produced during the boiling process and condensing it into a separate container. This, ideally,⁶ allows for the recovery of any coffee aroma that may have escaped as part of the condensate.⁷ Steam distillation is commonly used in laboratory and industrial-scale extraction processes, so it should come as no surprise that the method has been patented multiple times, in multiple formats. One patent from 1966 discusses using this method in instant coffee production, suggesting that the aroma condensate that was recovered amounts to approximately 0.1–2.0% of the original coffee’s weight.⁸  

Although he did not have a steam distillation apparatus, our protagonist conducted ‘several experiments, which [he] made with great care, in order to ascertain what proportion of the aromatic and volatile particles in the Coffee escape, and are left in this process, [he] found reason to conclude, that it amounts to considerably more than half.’⁹ By methodically testing his hypothesis, he demonstrated dedication to the scientific method and proved himself to be a genuine coffee scientist.

The Scientific Method

When conducting an aroma audit,¹⁰ especially in systems that have the potential to create new aromas, it is important to measure in more than one location in order to understand what is happening within that system. Our protagonist did just that. After smelling the aroma being lost from the brew, he tasted the beverage, describing it as ‘very bitter’. This observation is insightful, as he was breaking flavour down into its volatile (aroma) and nonvolatile (taste) components. He noted a progressive imbalance within the flavour profile as aroma was lost. Like many consumers today, our scientist did not enjoy the taste of the nonvolatile components. He demonstrates this negative bias towards the immersion brew method with his assertion that ‘[b]oiling hot water extracts from Coffee … a considerable quantity of astringent matter’.¹¹  

Dissatisfied with the quality of coffee produced by the immersion method, he proposed a different method,¹² in which the coffee grounds would not be tossed around by the boiling water during the brewing process, driving off the coffee’s precious aroma. Instead, the coffee grounds would be placed ‘in a cylindrical vessel, which has its bottom pierced with many small holes, so as to form a strainer, and a proper quantity of boiling hot water being poured cautiously on this layer of Coffee in powder, the water penetrates it by degrees, and after a certain time begins to filter through it’, resulting in the brew. This method is commonly referred to as percolation, drip, or filter coffee.

In William H. Ukers’s 1922 book All About Coffee and possibly other literary works, this scientist is sometimes portrayed as falsely claiming credit for the invention of percolation. While our protagonist takes credit for the innovations he developed from the original invention, it is also important to recognize that he actively promoted the original product, as illustrated by the following passage:

‘This method of making Coffee, by percolation, has been praised many years, and its usefulness is now universally acknowledged. I do not know who was the first to propose it, but being thoroughly persuaded of the merit of the contrivance, I have been desirous of recommending it; and I conceived that the most effectual way of recommending it, would be to explain the mechanical and chemical principles on which its superiority depends.’