Brewing with ultrasound
UNSW/Cecilia Duong
A high-tech method for producing cold brew coffee that uses ultrasonic waves to extract flavour could cut the time required from 24 hours to just a few minutes.
Cold brew coffee, which is made by steeping coffee grounds in cold water, is gaining popularity because it results in a less bitter drink than traditional methods using hot water. But the technique is also a headache for coffee shops as they need refrigerator space and must allow up to 24 hours to make a brew.
Now Francisco Trujillo at the University of New South Wales in Sydney and his colleagues have found a unique way to quickly extract a cold shot of coffee – by bombarding the grounds with ultrasonic waves. The resulting drink can be ready in less than 3 minutes.
Trujillo says the initial idea for using ultrasound, which smashes up the grounds in a process called acoustic cavitation, was that it might allow the extraction of more antioxidants. This turned out not to be the case, but their initial set up, requiring around £15,000 of ultrasonic equipment, produced a surprisingly good coffee.
“There’s nothing like it,” says Trujillo. “The flavour is nice, the aroma is nice and the mouth feel is more viscous and there’s less bitterness than a regular espresso shot. And it has a level of acidity that people seem to like. It’s now my favourite way to drink coffee.”
The researchers have refined their process to work with existing espresso machines at a fraction of the cost of their initial equipment. They say their preferred technique for a cold espresso is 60 seconds of ultrasound, pumping a small amount of water every 12 seconds, but they also experimented with a 3-minute brew.
Trujillo says there are some important differences that make the ultrasonic cold espresso unique. Firstly, unlike a traditional hot espresso shot, the coffee grounds aren’t tamped down. The cold ultrasonic espresso shot also isn’t transparent and is a different colour when compared with a traditional cold brew, since the sound waves emulsify the oils in the coffee.
Samples of the new cold brew were tested by the Queensland Alliance for Agriculture and Food Innovation at the University of Queensland, where it was assessed for aroma, texture, flavour and aftertaste.
Trujillo says the ultrasonic 1-minute shot had generally similar ratings to a 24-hour cold brew but scored lower in aroma intensity, suggesting the cold shot may be under-extracted. On the other hand, he says, the 3-minute shot scored the same on aroma intensity as a 24-hour cold brew but with a slightly greater bitterness, indicating that the 3-minute brew was over-extracted. Somewhere between 1 and 3 minutes, and tweaking other parameters, will make a perfect cold brew, he says.
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