UNSW Sydney researchers have successfully harnessed the power of ultrasonic sound waves to make espresso-strength coffee – cutting energy use by 75 per cent.

What’s more, the beverage was made using room temperature water.

The researchers revealed they had developed a completely new brewing process that uses room temperature water to create the strong coffee with the same rich flavour and caffeine kick.

The process harnesses sound wavs, and by not having to heat the water it reduces energy consumption by around three-quarters. The saving could be especially significant for companies who make coffee-based ready-to-drink products at industrial scale, both in terms of energy use and brewing time.

Dr Francisco Trujillo and his team from UNSW’s School of Chemical Engineering have developed a system that uses ultrasound, high-frequency sound waves that are far above what a human can hear, to help extract the desired flavour, aroma and concentration from coffee grounds.

Their research, published in the Journal of Food Engineering, included blind taste-testing experiments that showed their ultrasonic room-temperature version of espresso was indistinguishable from coffee shots brewed in the traditional way.

“We call it an ultrasonic espresso. It’s a different process, but you can get the same richness and concentration of a normal espresso in under three minutes,” Dr Trujillo said.

“Traditionally, espresso is by forcing hot water through coffee under pressure. But with ultrasound we can use room temperature water instead, reducing energy consumption by up to 75 per cent.

“And when we gave our ultrasonic espresso to 100 regular coffee drinkers in a randomised test, they could not tell it apart from a normal espresso.”

Dr Trujillo had previously developed the patented ultrasound system to create cold-brew coffee, which usually takes 12-24 hours to produce, in as little as three minutes.

However, cold-brew coffee has a distinctively different flavour to espresso, and is often described as more diluted, smooth and mellow while also containing around one-fifth the caffeine concentration.

The UNSW team continued their work to adjust the ultrasound system to create an espresso-strength shot without the need for hot water. The process transformed a traditional filter basket into an ultrasonic reactor to brew the grounded coffee beans. The basket generates high-frequency sound waves that help extract flavour, aroma and body from the coffee grounds.

At the heart of the system is a transducer – a small metal device that generates ultrasound while pressing against the side of the coffee basket holding the ground coffee. The ultrasound causes the basket to vibrate rapidly, transmitting vibrations through both the coffee grounds and the water.  The ultrasound creates a phenomenon called acoustic cavitation, which is a rapid formation and collapse of microscopic bubbles in the liquid. When these tiny bubbles collapse near the coffee particles, they act like microscopic scrubbing brushes or jets of liquid, pitting and fracturing the coffee grounds and accelerating the brewing process.

This helps break open the surface of the coffee grounds and allows flavour compounds, oils, and caffeine to move into the water much faster than they normally would at such low temperatures.  The result is a concentrated, flavourful shot of coffee comparable to espresso made with traditional machines.

“We have been working on a range of parameters to discover how to make the perfect ultrasonic espresso,” Dr Trujillo said.

“The most important was the brew ratio – that is how much water is used per gram of coffee, because this helps ensure the final drink is concentrated and not too diluted. Another important factor is how finely the coffee beans are ground. We found that by grinding finer we could extract the flavour more rapidly.

“We also experimented with how long the sound waves were applied, as this can affect both the concentration and flavour of the coffee. What we found is that between two-and-a-half and three minutes is a sweet spot for producing a balanced cup.”

To test their results, the researchers also carried out a blind sensory evaluation in which participants did not know which coffee they were drinking. Four drinks were tested: traditional espresso, ultrasound-brewed espresso, traditional filter coffee, and ultrasound-brewed filter coffee. All coffees were prepared fresh, cooled to the same temperature, served in identical coded cups, and presented in a random order to avoid bias.

Around 100 regular coffee drinkers took part. They were not trained experts, but everyday consumers who drink coffee at least once a week. Each participant rated the coffees on a simple nine-point scale for aroma, flavour, bitterness and overall liking.

The results were striking. For the espresso shots, there were no significant differences between the traditional and ultrasound versions across any of the taste measures. For filter coffee, however, the ultrasound-brewed version performed even better: participants significantly preferred it overall, particularly rating its bitterness as more pleasant.

“These findings showed that using ultrasound did not harm taste, and in some cases even improved it, despite brewing at room temperature and without the heat normally associated with coffee making,” Dr Trujillo said.