Going sonic to change crystallisation
How ultrasound and a giant hangar in Oxfordshire could redefine industrial processes forever.
Crystallisation is a process in many of the industries our modern lives depend on. But it’s not easy to control or influence, leading to inefficiency and energy wastage. Research led by an internationally renowned expert at the University of Leeds is using ultrasound to rewrite the rulebook.
Key information
- Major funders: Unilever
- Partners and collaborators: Diamond Light Source
- Disciplines: food science, engineering
- Investigators: Professor Megan Povey.
A globally ubiquitous process
Professor Megan Povey knows an awful lot more about crystallisation than the average passerby.
“It’s a critical process in so many industries,” Megan said. “Whether it’s food, fine chemicals, agrochemicals or pharmaceuticals, crystallisation plays a huge role in creating products we rely on every day.”
Yet despite being used so widely in manufacturing, crystallisation – when solids and liquids form into a crystal, typically due to freezing or precipitating from a solution – can be an unpredictable, unreliable and expensively inefficient thing.
So what if we can improve how we achieve crystallisation in laboratories and factories?
How might ultrasound hold the key to modify how it occurs?
And what could that mean for businesses looking to reduce their energy usage, increase productivity and raise quality?
Professor Povey has been on a long mission to address these questions – and some of the answers are now tantalisingly within reach.
Melt, freeze, melt again
They can fix this by melting the chocolate back down and letting it cool in a mould again, which is called reworking, but that heating and stirring all takes up extra resources, of course
While Megan’s research covers a field as relevant to diesel fuel as it is to dairy foodstuffs, chocolate nevertheless provides a representative example of its challenges and opportunities.
“Making chocolate is probably one of the most energy-intensive food processes there is,” she explained.
“The average electricity needed for every kilogram of product is something like 25kWh, which represents an incredible amount when you think of how many bars and boxes we have in every single supermarket.”
In particular, the tempering process of slowly heating and then cooling required to form cocoa butter crystals with the right qualities often leads to energy being wasted.
“It’s an inexact science that can’t be easily controlled, so manufacturers often end up with issues like bloom on chocolate, when you end up with a dull white finish.” Professor Povey said.
“Making chocolate is probably one of the most energy-intensive food processes there is.”
Chocolate conching is a crucial step in chocolate production where the chocolate mass is intensely mixed, kneaded, and aerated at a warm temperature.
Bloom can be a costly and frustrating headache for manufacturers.
When surveyed by scientists at Ghent University, 38.1 per cent of Belgian companies reported that they frequently experienced problems with fat bloom on filled chocolate products, while 57.1 per cent said this was something they ‘sometimes’ had to deal with.
“They can fix this by melting the chocolate and cooling again in a tempering machine, which is called reworking, but that heating and stirring all takes up extra resources, of course,” Megan added.
Making waves
Megan and her colleagues are working on a more permanent solution to the blooming pitfall: sonocrystallisation.
It’s not exactly a new discovery, having been first reported on in 1927.
But the technique is still relatively recent by scientific standards, with Professor Povey’s team experimenting at its forefront.
“It’s basically using much lower-intensity ultrasound than in conventional sonocrystallisation to change the way that freezing occurs, she said.
“The idea is to control crystal nucleation by carefully applying the right ultrasound frequency, power and duration for the material in question, so we can promote or suppress crystallisation.
“Ultrasound had already been used to observe crystallisation and help us understand how it happens, but we wanted to try to actually affect the process as well.”
It’s an idea that dates back some 15 years, when a single realisation offered Megan the chance to investigate an idea at the cutting edge of her interest area.
“There’s a simple fact that any action that includes a compression and an expansion as part of a soundwave irreversibly changes the material surroundings that are subjected to the pressure fluctuation,” she explained.
“I was working with the polymer scientist Ken Lewtas, and we realised that, with a really good understanding of the physics at play, we could turn the power down and increase the amount of time we insonified to get that reaction again and again.
“By multiplying the difference between the compression and expansion 10 million times in a second, you’ve suddenly got an irreversible change in the system you’re interested in.
Back then, in that initial project, the system in question was waxes.
“We were interested in trying to prevent the crystallisation of wax from solution, which can disrupt the flow of diesel fuels – especially as modern diesel engines use very high pressures, so wax can form at higher temperatures,” Megan said.
That’s the practical side of things in a nutshell, but Professor Povey is at pains to point out that everything she does is underpinned by the theoretical.
“I’ve never seen life as a division between theory and practice,” she said. “I’m an experimental scientist, a theoretician, a mathematician; I need all those skills to come together.”
Industry support and cutting-edge tech
The facility allows us to follow the structural changes that take place at all scales, from just a few angstroms (tenths of a nanometre) right up several hundred nanometres
Yet while Megan’s team continues to develop mathematical-physical models to understand how low-power ultrasound can influence nucleating systems, it wasn’t always certain her interest would get off the ground.
“Because my PhD had significant military potential, I was offered jobs by multiple US defence contractors – but turned them down as I am anti-war and pro civil rights,” she recalled.
“I knew nothing about food science back then, although my father was a milling engineer and I used to visit flour mills with him as a child, but I joined the School of Food Science and Nutrition at Leeds, and thus my investigations into the physics of food began.”
Indeed, a team of visiting Unilever scientists soon heard about Professor Povey’s work, providing encouragement and funding to help develop her ideas.
Megan and colleagues have made extensive use of Diamond Light Source, the UK’s national synchrotron research facility in Oxfordshire, since it went into operation in 2007.
The synchrotron produces intense beams of light 10,000 times brighter than the sun at wavelengths from X-rays to infrared, and is housed in a building larger than six football pitches.
Other research at Diamond Light Source has helped determine the atomic structure of the virus behind COVID-19 and enabled the development of a new generation of antihistamine drugs, among other discoveries.
“The facility allows us to follow the structural changes that take place at all scales, from just a few angstroms (tenths of a nanometre) right up to several hundred nanometres,” she said.
“In layperson’s terms, we can observe how our use of low-intensity ultrasound affects the nucleation and crystallisation at every stage of these processes.”
Applying the theory in the world of business
Megan believes the potential significance of insonification in many different industrial contexts cannot be understated.
It’s why she and Ken Lewtas have filed an international patent to protect their intellectual property around it.
But Professor Povey’s hope is that businesses will continue to partner with the renowned School of Food Science and Nutrition at Leeds and work with her team to reap the rewards together.
“There are all sorts of possible benefits and applications of this science, especially within a food context,” she said.
“We’re looking at how we can use ultrasound to analyse the fat composition, aeration protocols and bubble-size distribution to help commercial partners reduce the calorific value of their products and help tackle obesity.
“We’re also investigating how we might use the technique to develop custom-made food for older people, many of whom can no longer salivate properly and therefore stop enjoying what they eat.”
Away from food, Megan is optimistic that being able to more effectively control the polymorph when developing pharmaceutical ‘actives’ will help drug discovery organisations avoid repeats of disasters such as the thalidomide scandal.
“Our technique can provide faster nucleation and uniform nucleation throughout the sonicated volume, as well as the generation of smaller, purer and more uniform crystals.”
There’s also another opportunity that could prove particularly lucrative for any companies wishing to join forces with the team.
“We’ve been in discussions with partners about injection moulding, which is a high-energy, somewhat hit-and-miss process,” she said.
Typical production processing plastic bottle caps.
“We believe our insonification could be a real game-changer there, reducing waste and cost while improving versatility and sustainability.”
For Megan, it’s still early days in the story of sonocrystallisation.
But with industry collaboration hotting up as more private sector organisations look to tap into the science, progress seems set to accelerate at pace.
Professor Megan Povey
Megan is Professor of Food Physics who has developed novel ultrasound/acoustic methods for food characterisation and processing over the last three decades.
With colleague Jianshe Chen, she has highlighted the importance of sound in the human appreciation of food.
Megan and Phillip Nelson designed several instruments for analysing food and its stability, including the Cygnus UVM ultrasound velocity meter and the Acoustiscan scanner. She is also the co-inventor of the Ultracane blind aid.
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The views expressed in this article are those of the author and may not reflect the views of the University of Leeds.