What have the ThrustSSC land speed record, brain scans and a pair of ladies' tights got to do with the food industry? The answer is that they have all contributed to better ingredient mixing and blending.
When the ThrustSSC set a new world land speed record of 763mph in October 1997 it became the first land vehicle to break the sound barrier. And staff at engineering company Inbis were jubilant at their role in this British success story. The firm had been selected by ThrustSCC project director Richard Noble to design the nose cone and engine intakes for his vehicle - it used two Rolls-Royce Phantom II jet fighter engines.
And it was this expertise in aerodynamics and fluid dynamics at Inbis (now part of Assystem) that led to the company being asked to come up with a novel design for a low-shear mixer and blender for the food industry a couple of years ago.
The Food Processing Faraday Partnership, now the Food Processing Knowledge Transfer Network (KTN) wanted Inbis to apply its brains to the problem of the low-shear mixing of delicate ingredients. The result was the 'tumble tube' - a horizontal drum or tube tilted up slightly at one end with corrugations set around the inside surface. Fluids are poured in at the upper end and flow down the tube and out the other end. But as the tube rotates, the corrugations cause the fluid to rise up the side of the tube and then fall back to mix it.
"It improves the agitation of a fluid but in a nice gentle, almost folding way," says Brian Gilbert, project manager at Assystem. "Even we were surprised at how effective it was."
The company built a prototype and for demonstrations used a translucent starch mixture about the consistency of yoghurt. "We would introduce a red dye into it and within about 20cm it was really very well mixed," says Gilbert. A typical application for the tumble tube, he says, might be for the mixing strawberries into yoghurt without bruising the fruit or destroying the structure of the yoghurt.
The prototype was shown earlier this year at the Foodex Meatex show in Birmingham by the Food Processing KTN, which is now looking for a partner, either an equipment maker or a food manufacturer, to take up the idea.
It has been said that mixing and blending is one of the most common processes in food manufacture. Yet when the Department for Environment, Food and Rural Affairs gave the green light to an £800,000 Food Link project three years ago, little research had been done into how efficiently commercial cooking vessels mix and heat their contents.
Poor mixing of a batch of soup or a sauce could mean the contents would not be properly heated, with the risk that the batch would not be properly pasteurised. And over-heating it, just to make sure, could mean an overcooked soup.
What was needed was a way of measuring how the mixing processes affected the distribution of heat throughout the mass of the food in a cooking and mixing vessel. The Link project set out to use the technique of time-temperature-integrators (TTIs) being developed by Campden & Chorleywood Food Research Association (CCFRA). TTIs are small capsules of alpha-amylase enzyme solution that can be attached to mixer blades, or left free to travel with the food, and which are then recovered after mixing and heating and analysed. The degradation of the enzyme gives an accurate record of the temperature a capsule has been subjected to and for how long.
Modelling behaviour
For the first time TTIs gave a picture of the heating and mixing process inside a cooking vessel. However, the Link project partners, led by CCFRA, wanted to know where and how hot and cold spots were formed during mixing. So they turned to Birmingham University.
But it wasn't aerospace engineering or land speed records that attracted them. Instead it was the university's use of brain scan technology. By adapting a couple of old brain scan cameras, the university's chemical engineering department is using the technique of positron emission particle tracking (PEPT) to track tiny radioactive particles inserted into TTIs. By putting some of these tracer TTIs loose into a mixer, PEPT gives a three-dimensional picture of what is happening inside the mixer.
And so by combining the results of PEPT at Birmingham and the work on TTIs at CCFRA, the Link project is now close to providing the definitive guide to how foods are mixed and heated in cooking vessels. Project partner, cooking vessel manufacturer Giusti, is sponsoring a PhD student at Birmingham to use the work to help improve the design of its mixing, blending and cooking vessels.
Garry Tucker, process development manager at CCFRA, who led the award-winning work on TTIs, says you would expect the worst mixing to occur around the central mixing shaft and worst thermal processing to occur there, too, since the shaft is the furthest away from the cooking vessel's heating jacket.
"We fixed TTIs to places where we thought there would be the least thermal processing - along the central shaft and on the mixer blades. It's not rocket science, but the TTI devices allowed us to get some data rather than relying on just a gut feeling."
As a result of the work, Giusti has now made changes to the design of its mixers by incorporating a baffle to improve mixing and heating along the central shaft area, says Tucker. "It is all about taking the hotter layers from the outside of the vessel and mixing them with the colder layers from the inside to get a uniform mixture."
According to Gareth Cure, md at Giusti in Burton on Trent, the research vessel, a 250 litre Vesuvio cook-cool plant is now on permanent loan to CCFRA for use in commercial TTI trials for CCFRA members. "What we are doing is trying to get back to first principles and understand the mixing technology. What it has told us so far is that the on-board temperature thermocouple we use gives a pretty good idea of what's going on in the vessel. The position of the thermocouple in relation to the agitation in the vessel is good," he says.
"The use of TTIs prevents overcooking. We have been doing some more trials with Birmingham using two different TTI capsules tuned to two different temperatures - one for the temperature at the vessel wall and one for the temperature in the centre of the mixer." Birmingham has also been using a transparent vessel and new three dimensional visualisation technique, particle image velocimetry, to physically see where the TTI capsules are going, says Cure.
But the key inference from the work, he says, is that if you haven't got efficient mixing you aren't going to get temperature homogeneity across the vessel. "We want to resist change for the sake of change. Instead we want to improve our fundamental understanding of the science and engineering of mixers to allow us to continually improve our mixers."
CCFRA has now developed TTIs that will measure just a few minutes exposure at 70°C all the way up to tens of minutes exposure at 100°C. "So provided the thermal contribution to a mixing process is within that range, you can use TTIs to determine how successful any mixing is," suggests Tucker.
And the ladies tights? Ah, says Tucker. Now, although it can be quite valuable to put some TTI capsules into the food and let them loose to move freely throughout the mixer, there are problems, he says. Getting them to stay in critical areas such as around the central mixing shaft can be difficult unless you constrain them in some way.
"We found that putting a mixture of food and TTIs into what I suppose is technically a pair of tights, and then tying them to the central shaft was very effective. And the TTIs were also easy to get out afterwards. Sometimes quite basic devices work extremely well." FM
Key Contacts
- Assystem 01772 645449
- Birmingham University 0121 414 5354
- CCFRA 01386 842000
- Food Processing KTN 01664 503649
- Giusti 01283 566661
- Hosokawa Micron 01928 755100