Ultrafast defrosting with ultrasound

A new process has been attracting attention in the food industry. Developed by the Bremerhaven Institute for Food Technology and Bioprocess Engineering, the new method enables deep frozen fish to be defrosted in record time. And the fish tastes as fresh as the day it was caught.

Fog is a common phenomenon on the German North Sea coast. But when Dennis Fehner is shrouded in fog it is usually of his own making and he uses it in an unexpected way: to defrost fish. The Bremerhaven Institute for Food Technology and Bioprocess Engineering (BILB) has developed a new energy-saving method that can be used to defrost frozen food in record time.

“You’d think it was straight from the sea”

“The most remarkable thing about the process is that we can defrost products like frozen fish extremely quickly without impairing the quality. Quite the opposite in fact. After being defrosted, the fish is so fresh you’d think it was straight from the sea,” enthuses the 35-year-old food technologist. It may sound like alchemy but it is based on sound scientific principles. During the special defrosting process, the flesh of the fish absorbs tiny water droplets. The food technologists got the idea from the baking industry where a fine mist is used during the proving process, or to cool bread rolls. The BILB works on applied food research on behalf of the Bremen government and the Bremerhaven University of Applied Sciences, and is largely funded by the research assignments and projects it carries out for industry. The researchers have been working on the fish-thawing process for a good two years now.

The secret ingredient: ultrasound

Dennis Fehner reaches for a trolley. On it stands an open metal framework about two metres tall. “The trays of frozen fish go in here,” he explains, and slides a tray of Alaska pollock into one of the slots in the middle. Then he opens the door of a large test chamber. As if on command, a dense mist pours out over the sill and flows in all directions across the grey tiled floor. “In contrast to other methods, we don’t use heat or anything like that to thaw things here. The chamber is maintained at a temperature of just 16 degrees.” The secret ingredient: ultrasound.

Making the process up to 30 per cent quicker

The Bremerhaven researchers are making use of a special property of water. “We start the water molecules oscillating. This creates water droplets with a specific diameter. The higher the oscillation frequency, the smaller the droplets. And mist is nothing more or less than an accumulation of extremely small water droplets,” says Dennis Fehner. The water vapour in the BILB’s test chamber is so fine that it penetrates the frozen fish and increases the heat conductivity of the flesh enormously. The fish defrosts more quickly. Depending on the size and weight of the frozen item, it can defrost up to 30 per cent more quickly than using conventional defrosting methods. This means the process also saves energy.

Researchers harness the properties of ceramic

Fehner pushes the metal trolley into the chamber and closes the door. “We need to wait just half an hour for the piezo ceramic to do its work,” he says, matter-of-factly – before realising further explanation is required. “Piezo ceramics are used, for example, in loudspeakers and electric guitars. The ceramic converts electricity into ultrasonic vibrations which are used to produce the water vapour from the water bath.”

Melting ice crystals damage the fish

During a normal thawing process on a plate in the open, the ice crystals melt in the fish and damage the structure of its flesh. The fish defrosts slowly from the outside in. By the time the process is complete, a skin has already formed on the outside – the fish is then dry in those places, and in some cases still frozen on the inside. “But our method allows the fish to defrost evenly so it tastes much better – as if freshly caught,” says Dennis Fehner. This can be beneficial if the fish is going to be processed and made into ready meals.

The system is already being used in production

The new method is attracting a lot of attention in the food industry. “One company in Leipzig has already installed a large system and is successfully using our method in production,” says Florian Stukenborg, head of research in the food department at BILB. The advancements made in food processing have led to a collaborative project between science and business. “We are currently part of a group working on making the fish and food industry more efficient, more innovative and more cutting edge,” explains network manager Benjamin Küther, who leads the group from his base in Bremerhaven. A total of 15 companies and research partners from all over Germany have come together to work on this. “We meet twice a year to discuss the latest developments in science and business. It is a completely new working group and we hope that the collaboration will provide a boost for technological innovation.”

Pollock looks fresh and has succulent flesh

The timer on the defrosting chamber beeps loudly to announce that the 30 minutes are up. “So,” says Dennis Fehner, rubbing his hands. “That’s it.” He opens the door and pulls the trolley out of the mist into the room. On the metal tray lie four pieces of pollock – fresh and glossy with succulent flesh. “If we had a frying pan now, the fish would practically leap out of it,” he jokes.

Only for use in industry

Will this innovative system be available for home use any time soon? For example when cooking fish fingers that remain frozen from the catch to the frying pan. “At the moment our method is only intended for use by industry, and of course cost is a consideration when designing a scaled-down version to be manufactured in small quantities,” says Dennis Fehner. “However, our system can be used both to defrost and to freeze food.” The fine water particles raise the general heat conductivity of a product. So any kind of temperature can be conducted – whether hot or cold. “It would be entirely possible to use this technology to freeze fish on the boat as soon as it has been caught,” says Dennis Fehner. “Then the fish in the kitchen at home would be so fresh that you could clearly taste the benefits of the ‘nebuliser’ – and that would also work for fish fingers.”

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