Food Engineering

The refined science of heat transfer

August 31, 2005
Anyone can boil or freeze water. The fun begins when scientists and engineers must predict when boiling or freezing will occur with additional components under different conditions.

Freezing technology can deliver low-fat ice cream with full flavor, but so can food chemistry, a reality that has slowed adoption of cold extrusion systems. Source: Tate & Lyle.
Given the cavemen's seminal work in fire as a food processing tool, it's not surprising that heat transfer is the red-headed stepchild of food engineering. But understanding and controlling a phenomenon is a long road of discovery, and for heat transfer the journey continues.

The latest milestone was reached earlier this year when Campbell Soup Co. proved to the FDA's satisfaction that thermal destruction of microbial life was achieved in the fastest moving, low-acid particle in its Select Gold Label line of aseptic soup. Campbell first tackled this heat transfer challenge in the early 1980s, but abandoned the effort because of the validation issues involved. The company renewed its quest in the 1990s.

Movement off retailers' shelves where the upscale soups are beginning to appear will be a milestone of another sort. If the Combibloc cartons move as quickly in the US as they did in Canadian test markets, Campbell executives will be sufficiently pleased to continue to push the aseptic envelope, and that in turn will ignite further research and development into the measuring and monitoring of heat transfer in a continuous flow process.

Campbell's Kahlid Abdelrahim served as process authority for Select Gold, and like Ringo Starr, he had a little help from his friends. In fact, he had a lot of help, though modesty and confidentiality agreements forbid heat-transfer experts from taking credit where due. Still, the dots are there to connect, and the heat-transfer validation dots are extremely limited. "We used a combination of mathematical simulations and analyses of finished product," according to a senior research scientist at Campbell. The roadmap for building an empirical database from those simulations and analyses was laid out eight years ago in Tetra Pak's potato soup filing with the FDA, which elicited a letter of no objection. The difference today is that companies like Campbell get an e-mail of no objection.

Configuring an aseptic heat exchanger for product flow, whether it’s high-acid liquid or low-acid with particles, requires complex computations. Modeling software helps speed the engineering challenge. Source: American Purpac Technologies.
"In terms of multi-phase (liquid and solids), AseptiCAL is the only aseptic modeling software commercially available," notes process engineer Dilip I. Chandarana, a vice president and process authority with Creative Research Management Corp. (CRM), Stockton, CA. CRM was founded five years ago by former principals in California Natural Products, a leading aseptic processor of rice- and soy-based beverages for firms such as White Wave, Starbucks and Imagine Foods. "Apart from aseptic, there are tools available for measuring and modeling residence time, and that pretty much is it. There really isn't a lot of commercial demand."

AseptiCAL was commercialized a year after the potato soup filing, and while the logarithms and other mathematical calculations are essentially the same as Chandarana's Fortran program, the Windows-based program delivers more easily understood graphic presentations of findings. "We are probably the only model accepted by the FDA," says Jun Weng, a research fellow with FMC FoodTech in Madera, CA, and a developer of AseptiCAL. Chandarana's Fortran model was distributed to members of the National Food Processors Association (now the Food Products Association) in the late 1990s, but it has been relegated to the dustbin of neglect.

When engineers size a heat exchanger, most of the number crunching can be performed with AseptiCAL. During the validation process, models generated by the program are compared with microbial testing involving 299 particles inoculated with surrogate bacteria to a load sufficient to demonstrate a five log microbial kill. Scientific workshops leading up to the potato soup filing fixed 299 as the number of intact particles sufficient to declare commercial sterility, says Weng.

After commercial production begins, the model receives input on process deviation to determine if a product is safe for human consumption. The calculations are done off-line. An on-line tool has been developed, he says, though FDA review is on hold. "If the low-acid particulate category grows, the on-line option will be available sooner," Weng says.

Campbell purportedly installed two FMC FranRica aseptic lines at its Canadian facility to produce Select Gold, but FMC officials refuse to confirm. FranRica's dimple-tube system maximizes heat transfer and delivers gentle agitation for minimal product abuse.

Metal measurement

Determining residence time for particles of varying sizes in a continuous stream proved to be more of an economic challenge than an engineering challenge for the food industry. Researchers at the National Center for Food Safety and Technology in suburban Chicago patented a process using magnetic resonance imaging (the same technology used to great advantage in capturing detailed images of the human heart and other vital organs). Unfortunately, MRI machines cost $1.5 million or more. With no managed-care providers to pick up the tab, food companies balked at MRI as a heat-transfer validation tool.

Creamy mouthfeel with low-fat milk is made possible by ultra-low temperature ice cream extrusion systems such as Tetra Pak Hoyer’s Deep Blue, which requires high-speed freezers like the LF 1000-F1.
Magnetic thermometry is the solution envisioned by Kenneth Swartzel and Josip Simunovic, researchers at North Carolina State University in Raleigh who were issued five US patents for their technology from 1999 to 2004, with more on the way. Their multi-dimension solution delivers real-time validation of residence time and temperature with thermomagnetic implants that replicate the heat transfer to the core of a given particle. Eutectic solder alloys which flow when sterilization temperature is reached allow tiny magnets inside surrogate particles to realign and emit a signal that is read by sensors in the heat exchanger. The surrogates emulate the behavior of different particles, such as chunks of beef, potato, carrot, etc.

"We can monitor and map nine or ten different magnetic particles at a time, slashing testing time dramatically," says Simunovic, "and processors can adjust hold-tube times on the fly, depending on what the readings are."

The ability to "revalidate the product each time you start the process" can slash huge amounts of waste for food processors, he points out. At the system validation stage alone, companies are faced with 10 hours of run time, with 50 gallons of food a minute moving through a system. Raw material costs easily climb into six figures. Minimizing those and ongoing validation costs should save manufacturers millions of dollars over the life of a system, Simunovic says.

"Campbell's product is really a milestone and has created a feeding frenzy in the industry," Simunovic adds. "Now that the fear factor of regulatory validation has been removed, there is a lot of interest."

Decades of false starts on the multi-phase heat transfer front have left others leery of similar predictions. Yes, Campbell has advanced the aseptic process, but packaging remains a huge limitation. The SIG Combibloc fillers in Campbell's Etobicoke, ON, plant can only handle particles measuring less than one-quarter inch in any dimension-not exactly the stock of chunky soup. Fill speeds of 100 units per minute also negate the advantage of a continuous process over batch processing of retorted cans. More problematic is the premium price: at close to $3 per 500 ml (about 16 oz.), Select Gold is competing with retorted Campbell products selling for about $1. And meat tastes better after a long, slow simmer than a short- time, high-temperature process.

More compelling aseptic success stories can be found elsewhere, CRM's Chandarana believes. "There aren't enough validated machines out there to fill demand for flavored dairy creamers," he says. Bulk packages of freshly harvested produce are another success story, particularly in the case of Gerber Products Co.'s First and Second Foods. Because the foods are recooked before final packaging, less severe heat transfer models can be applied when processing raw materials for storage in 300-lb. totes. The aseptic products have helped propel Gerber's market share to well over 80 percent of the baby food segment.

Uniform heating is a process advantage with aseptic, but cooling products before packaging remains a challenge, Chandara and FMC's Weng agree. "Maybe 60 or 70 percent of the loss in flavor, texture, color and nutritional value comes in the cooling side of the system," says Chandarana. "The problem we run into is the cooling technology that isn't there yet."

Flash cooling by means of a vacuum inside the heat transfer vessel offers some advantages, Weng points out, and evaporative cooling remains the fastest technology. "We cool as fast as we can to preserve quality," he says.

The iceman cometh

Not long after the caveman mastered the rudiments of fire control, he was exposed to the flip side of heat transfer: the ice age. Hundreds of thousands of years after the fact, archaeologists continue to thaw out the remains of large mammals trapped in ice, proving that colder is better.

The benefits of rapid freezing certainly have been born out with food, which loses moisture the longer it takes to freeze. And the link between rapid chilling of milk and low bacterial cell counts has prompted many dairies to install state-of-the-art cooling systems on their farms. Within the four walls of the processing facility, the latest freezing technology, also known as the ULTICE process, is applied to ultra-low temperature ice cream extrusion.

Although the template for gaining FDA approval for low-acid multiphase aseptic processing was laid out eight years ago, Campbell Soup had few options to validate the commercial sterility of its new Select Gold Label soups.
Ice cream typically is processed at 40