Tikalon Blog by Dev Gualtieri

Cold Brew Coffee

July 15, 2024

In the late 1970s, I was visiting some of the laboratories of Bell Labs. This was more than a decade before its slow decline from research leadership after the 1984 breakup of the Bell System into the separate entities of AT&T and the Bell operating companies ("Baby Bells"). During that visit, I was invited to have coffee at a laboratory coffeemaker. In those days, before single-serving coffee makers, the coffee from automatic drip coffeemakers in laboratories and offices was quite horrible, since heat is the enemy of good coffee taste, and the coffee that you're drinking likely has been on a hot plate for an extended period.

It turned out that the coffee was actually quite good, the reason being a scientific innovation by the laboratory staff. They surmised that the principal reason why coffee tastes bad after extended heating was oxidation of the mixture of chemicals that's coffee. They had arranged a continuous nitrogen gas purge of the air space of the coffee pot to exclude oxygen. Nitrogen gas is available in most laboratories via spigots, since it's nearly a free resource. Large laboratories have a liquid nitrogen storage tank, and the natural boil-off of the liquid is routed to the laboratories. Later, for safety reasons, my corporate research laboratory prohibited laboratory coffeemakers, and the same coffee stations as our non-scientist colleagues were deployed in designated coffee rooms of the research buildings.

A cup of coffee with the caffeine structural diagram.

While employees are thankful of their employer's largesse in providing free coffee, any cursory cost-benefit analysis shows that they get back more than they spend.

Coffee's caffeine content boosts a person's focus and attention, and it reduces mental fatigue, all of these leading to increased productivity.

In a research environment, conversations in coffee areas have lead to some innovations; and, as mathematician, Alfréd Rényi (1921-1970) so famously stated, "A mathematician is a device for turning coffee into theorems."

(Coffee cup, via Wikimedia Commons, as is the caffeine structural diagram.)

Long before the introduction of modern automatic drip coffeemakers, vacuum coffeemakers were a semi-automated method for production of drip coffee. In these siphon coffeemakers, the steam from heated water in one chamber forces water at its boiling point through a filter into an another chamber where it mixes with ground coffee. After a desired brewing time, the coffeemaker is removed from heat, and the resulting vacuum formed in the formerly heated chamber draws the brewed coffee through the filter from one chamber to the other.

Figs. 2 and 3 from US Patent no. 54,933, 'Making Coffee,' by Joe V. Meigs, May 22, 1866.

Figures 2 and 3 from US Patent no. 54,933, 'Making Coffee,' by Joe V. Meigs, May 22, 1866. This is possibly the earliest US Patent on a vacuum coffeemaker. The patent text includes the following - "It is the object of my invention to provide an apparatus for making coffee upon philosophical principles, of the best quality to be procured from the berry, and yet so simple and certain in its operation that it can be successfully used by any person of ordinary intelligence, and, in fact, can scarcely be made to fail..." (Image via Google Patents. Click for larger image.)

In all these cases, coffee extraction is at a high temperature, about 100 °C (373 K), and heat modifies the coffee chemicals. Coffee extraction, like any other chemical process, has a temperature dependent rate, and the purpose of using heated water for this extraction in a coffeemaker is to get a rapid extraction. When the rate of a process depends on temperature, we immediately think of the Arrhenius equation,

in which E_a is an activation energy, R is the gas constant, T is the temperature, A is a constant, and k is the rate. The activation energy would vary among the coffee chemicals. However, creation of cold brew coffee, for which the coffee is extracted at room temperature, or lower, takes 24 hours or more. The temperature modified extraction rate for each compound gives cold-brew coffee a different flavor, and cold brew coffee typically has a higher caffeine concentration than standard drip coffee. Cold brew coffee is less acidic, and it tastes less bitter than regular drip coffee.[3]

In today's grab-and-go culture, twenty four hours is a long time to wait for a cup of coffee, and cold brewing demands considerable refrigerated space for the many ounces of coffee needed in a coffee shop. Scientists from the University of New South Wales (Sydney, Australia), the University of Queensland (Brisbane, Australia), and the University of Sydney (Sydney, Australia) have invented a process that reduces cold brew time to the same several minutes of brewing time required for drip coffee.[2-3] Their cold brew process, published in the journal, Ultrasonics Sonochemistry, uses an ultrasonic reactor (see figure).[2-3]

A schematic diagram of the method for coupling of ultrasound into a coffee basket for the University of Queensland experiments on cold brewing of coffee.

(Fig. 1(b) of ref. 2, published under a Creative Commons License. Click for larger image.)

For their experiments, a commercial espresso machine was modified to connect an ultrasonic transducer to the brewing basket through a metallic coupling horn. The ultrasonic waves reflected around the walls of the brewing basket to create an ultrasonic reactor.[2-3] Multiple regions of acoustic cavitation were created inside the reactor at the induced 38.8 kHz frequency.[2-3] This acoustic cavitation of collapsing bubbles generates enough force to pit and fracture the coffee grounds, and this intensified the coffee extraction.[3]

Not surprisingly, decreasing the amount of coffee in the brewing basket increased the extraction yield.[2] The ultrasonic process doubled the extraction yield and caffeine concentration as compared with brewing without sonification.[3] Sonification at 100 watts doubled the extraction yield at a basket coffee fill of 33% from 15.05% to 33.44%, and caffeine concentration, from 15.05% to 33.44%. Fatty acids were increased eightfold, from 1.16 mg/mL to 9.20 mg/mL at 33% coffee fill.[2]

Radar plot of coffee attributes, as determined by a panel of eleven tasters. Significant differences between samples indicated by * for (p < 0.05), ** for (p < 0.01), and *** for (p < 0.001). The scale is 0-100). (Fig. 6 of ref. 2, published under a Creative Commons License. Click for larger image.)

As can be seen in the figure, the ultrasonic process produced coffee that had sensory qualities similar to a 24-hour cold brew.[2] A sensory analysis to compare aroma, flavor, aftertaste, texture, visual appearance, and other qualities between ultrasonic process coffee and regular cold brewed coffee showed nearly indistinguishable properties.[2] This technology might also speed tea brewing.[3] Using the ultrasonic process would allow restaurants and coffee shops to produce cold brews on demand, eliminating the need for extensive refrigeration space.[2-3]

Cold Brew Coffee

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