Technology Update: New Frontiers in Thermal Processing

April 16, 2003
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Process engineers continue to refine old as well as develop new applications for heat.

Versatility is engineered into the interior of today’s steam water-spray retorts. Photo: FMC FoodTech.
Thermal processing traces its roots to man’s mastery of fire a million years ago, so it lacks the glamour of more avant-garde food preparation methods. But process engineers continue to refine old and develop new applications for heat.

Retort is synonymous with thermal processing, and it illustrates thermal’s constant evolution and change. Since the saturated steam retorts of the early 20th century, systems have evolved to full emersion, steam air with fan agitation and, finally, the steam water sprays that surfaced a decade ago. Systems are constantly being modified to meet different industry demands, whether it’s greater energy efficiency, new packaging types or foods with better taste and appearance.

“Automated batch retorting systems continue to be refined to operate more efficiently and to provide greater flexibility for the containers and processes desired by the consumer,” according to Rick Eleew, technical applications manager with FMC FoodTech. “Engineering improvements and more advanced process controls have significantly boosted system capabilities and capacities. Processors now have the flexibility to run a variety of containers and recipes in a single production day.”

Product and packaging diversity have made flexibility the mantra of the modern food processor. “Marketing tends to dictate where changes will occur in retort,” observes Bill Cornelius, an adjunct professor at Ohio State University who also serves as the head of the Institute for Thermal Processing Specialists. “Thermal is inexpensive, but if the products don’t change, you risk depleting your business. When I was at Dole, we were dictated by what marketing could sell. If they could sell it, I could sterilize it.”

New isn’t always what sells, of course, so a retort that can handle glass, metal, plastic and other packaging materials can pay big dividends. For example, tomato juice in glass was a bust because metal imparts a saltier taste, says Cornelius, and that’s what consumers have grown to expect. Other products successfully migrate from metal to other packaging, but glass can’t tolerate a saturated steam retort, so an agitated or nonagitated airstream system with overpressure capability would facilitate a packaging change without requiring the installation of a new line.

“Consumers are looking for crisper, fresher foods with more vibrant colors, and retort system manufacturers have had to respond to that,” says Eleew. That’s why processors are opting for advanced real-time controls platforms such as FMC FoodTech’s LOG-TEC Momentum that automatically account for critical shifts in temperature and other process variables without operator intervention.

An operator slides another batch into a retort at Ameriqual Foods, an Evansville, Ind., copacker that recently added all-plastic containers to its packaging repertoire.

Paperboard retort

Flexible pouches and all-plastic rigid containers are two packaging options that retorts have adapted to in recent years. The next will be cartons: Tetra Pak Inc. is conducting FDA validation work and presenting documentation from a European application involving a retortable carton (see related story in Food Packaging, page 16). It hopes to have a North American carton retort system in operation later this year.

Hoping to parlay its success in fluid foods into the solids segment, Tetra Pak is working with several suppliers of filling equipment and retorts to offer processors a turnkey system that serves as an alternative to metal can retorts. “Our goal is to offer 24-month shelf life, but that is product dependent,” says Stephen P. Hellenschmidt, Tetra’s general manager of prepared foods. While the company makes no claims that a carton can deliver a superior-tasting product, it does believe the carton can outperform metal during cooling.

“Canned food makers are very concerned about potential failure of the epoxy in double-seam cans, because cooling water can be drawn into the vacuum of the can,” he maintains. “This carton is hermetically sealed, so there’s no risk of any fluids from outside the container being drawn in.”

A laser score at the top of the carton allows consumers to open the carton with their bare hand without the possibility of laceration. That’s the same benefit the flexible pouch brought to the foodservice market, though there are no immediate plans to offer a large enough carton to compete with the #10 can.

A carton line capable of producing 24,000 14-oz. units an hour would require about 10,000 sq. ft. of floor space. To help processors conduct product development and shelf life analysis before making such a commitment, Tetra Pak developed a mini-line that produces 10 units per minute. It is shipped in a 24-ft. long container. “It’s a mobile retort system for low-volume test runs,” says Hellenschmidt.

Total thermal emersion

Four recalls involving 101.4 million pounds of ready-to-eat meats for possible Listeria monocytogenes contamination in recent years have riveted attention on the need for new safeguards. While a number of different remedies are being tried, one of the most workable involves hot-water submersion of packaged meats.

Peter Muriana, a food microbiologist at Oklahoma State University, has conducted numerous validation studies of various post-packaging pasteurization processes, including a submersion heating system to address the danger of Listeria in deli meats. The 50-gallon lab unit emulates a 20-ft. long emersion unit called the Aquaflow created by David Howard, president of Unitherm Food Systems Inc. Water flow and temperature are the only variables that have to be monitored to ensure that the system delivers a 3-log pathogen reduction in meats inoculated with Listeria, Muriana says. With steam systems, pressure also impacts performance, thereby complicating performance monitoring.

A water bath also is a good conductor of energy, adds Howard, and is more effective in reducing surface pathogens than steam or sprayed water. He offers the analogy of a man running through a room filled with steam heated to 200 degrees. Severe burns would be likely, but areas of the man’s skin would be untouched. Sprayed water also would leave untreated patches of skin, though it would kill the man. But no area of the body would be unaffected by a bath.

“There are a lot of nooks and crannies where the bugs can hide” in deli meats, points out Muriana. “Different products react differently to post-package pasteurization. A netted ham has grooves you could hide pencils in,” so the time-and-temperature treatment needed for those products is significantly different than a roast. The composition of a particular type of deli meat also can have dramatic consequences: a 40 F degree difference exists above and below the skin of a submerged turkey, underscoring the need for extensive research before determining the proper treatment to achieve lethality.

“Don’t force the product to fit the process that you’ve got,” Howard advises. The effectiveness of hot-water emersion for various packaged deli meats is well documented, and FSIS accepts the results, he says. That’s why inspectors did not require a recall of 40 million pounds of thermally treated turkey meats two years ago from a Texas plant that had to recall 16.7 million lbs. of untreated turkey that possibly was contaminated with Listeria.

Modifications were made to conventional filling and carton-handling equipment for Tetra Pak’s new retortable cartons, with a new form-and-seal machine the only additional piece of equipment necessary. Photo: Tetra Pak Inc.

Killing them softly

While several oyster processors have adopted ultra high pressure to pasteurize and partially shuck their catch, a Franklin, La., firm applies mild heat pasteurization to give customers both great taste and peace of mind from fears of Vibrio contamination. Food scientists worked with AmeriPure Processing Co. Inc. to develop the process, which has been shown to reduce Vibrio vulnificus and Vibrio parahaemolyticus bacteria to undetectable levels.

“The oyster dies during the process, but it looks like a live oyster, and we knock out 99.9 percent of the bacteria,” explains AmeriPure owner Patrick Fahey. “High pressure blows the oyster open; with our process, the oyster stays shut and retains its juices, which is where all the flavor is.”

A rubber band is placed around each oyster, a labor-intensive step that accounts for much of the company’s 8 cents per oyster processing cost. Oysters then are loaded onto baking trays and conveyed to the processing unit. From that point on, everything is automated.

The heart of the system is a 7,500 gallon stainless steel tank. It accommodates 12 trays at a time, or a total of about 1,500 oysters. The water bath is 126 F degrees, slightly higher than a hot tub, and 10 minutes at that temperature is sufficient to kill the Vibrio bacteria. The oysters then are submerged in a 40 F degree bath.

Twenty deaths were attributed to Vibrio last year, Fahey says, and the frequency of oyster recalls is increasing. California requires restaurateurs who serve Gulf oysters to post draconian health warnings because of the danger. Because of the validation work AmeriPure has done and its HACCP documentation, the warning mandate is waived for AmeriPure’s clients, he says.

As the oyster business illustrates, new technologies are becoming available to help food processors cope with quality and safety issues. But even after a million years of use, heat mixed with a little ingenuity can provide effective solutions, too.

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