Most of us think of espresso as a hot, high-pressure ritual. Finely ground coffee goes into a machine, boiling water is forced through it, and in about 30 seconds we get a concentrated shot with crema, aroma, bitterness, body and caffeine.
As someone from Colombia, I like to think coffee is in my blood – and I’m proud to come from a country known for producing some of the best coffee beans in the world.
So perhaps that’s why I have spent a lot of time in my laboratory with my team asking a simple question: does espresso really need hot water?
Our new research suggests the answer may be no.
Low energy, full strength
We have developed what we call an ultrasonic espresso: a room-temperature brewing process that uses high-frequency sound waves to extract the flavour, oils, aroma and caffeine from coffee grounds. The result is an espresso-strength coffee made in under three minutes, but needing far less energy than the conventional method.
Saving up to 75% of energy by not heating the water is a minor benefit for home users or small coffee shops. But for companies making ready-to-drink coffee products at industrial scale, it could be very significant indeed.
A concentrated room-temperature coffee could be used directly in bottled drinks, milk-based beverages or cold coffee products. It can also be shipped as a concentrate and diluted later. This would reduce not only energy use, but potentially processing time as well.
Ultrasound replaces heat
The key to the new process is ultrasound. These are sound waves above the range of human hearing.
In our system, a small metal device called a transducer presses against the side of a traditional espresso basket and makes it vibrate rapidly. Those vibrations move through the water and coffee grounds.
This creates a phenomenon known as acoustic cavitation. Tiny bubbles form and collapse in the liquid.
Naliyadhara et al. / Journal of Food Engineering, CC BY
When these bubbles collapse near coffee particles, they produce microscopic jets and forces that act a little like scrubbing brushes. They pit and fracture the surface of the coffee grounds, helping flavour compounds, oils and caffeine move into the water much faster than they normally would at room temperature.
In other words, ultrasound helps us replace heat with mechanical energy.
Water, grind and time
This is not the same as cold brew. Cold brew is usually made by steeping coffee in cold water for 12 to 24 hours. It tends to be smooth, mellow and much less concentrated than espresso. In earlier work, we used ultrasound to speed up cold brew dramatically.
But the challenge in this project was different: could we produce something with the strength, body and intensity of espresso, without heating the water?
Richard Freeman / UNSW
To do that, we adjusted several variables. Brew ratio was one of the most important: how much water we used for each gram of coffee. Too much water and the drink becomes diluted; too little and extraction becomes difficult.
Grind size also mattered. Finer grounds allowed us to extract flavour more rapidly. Finally, we tested how long the ultrasound should be applied. We found the sweet spot was about two-and-a-half to three minutes.
The taste test
Of course, making a concentrated coffee in the laboratory is one thing. The real test is whether people want to drink it.
So we ran a blind evaluation with around 100 regular coffee drinkers. They were not trained judges; they were everyday consumers who drink coffee at least once a week.
We served them four coffees in identical cups: traditional espresso, ultrasound-brewed espresso, traditional filter coffee and ultrasound-brewed filter coffee. All were freshly prepared, cooled to the same temperature and presented in random order.
For the espresso samples, participants could not reliably tell the traditional and ultrasonic versions apart. There were no significant differences in aroma, flavour, bitterness or overall liking. For filter coffee, the ultrasound version was actually preferred overall, with participants rating its bitterness more pleasantly.
Those results show espresso may not need to begin with hot water after all. By using sound waves to shake the coffee grounds, we were able to create the same richness, body and intensity, but with far less energy.
