The Heat is On

Food packaging runs the gamut of methods and techniques, some tried and true and some pushing the envelope of contemporary packaging. Pouches fall into this latter category, with an even more targeted segment dealing with retort packaging.

With retort pouches, the food is processed (cooked) in the actual package. Therefore, the pouch construction must withstand the rigors of high temperatures and pressure, testing the robustness of all aspects of the package—seals, closures, inks, laminations, and adhesives.

All this means is that retort pouches fit the age-old adage, “you don’t get something for nothing.” The “something” you get is fairly substantial.

Tobin Hass, director of technical marketing, Ampac Flexibles, lists several benefits provided by retort pouches. “These include improved product quality due to shorter cook times, package differentiation, lighter weight, improved aesthetics, higher print quality, and safety (no sharp edges).

The impact pouches have can be readily seen up and down the aisle of any grocery store and maybe some places that many consumers don’t see. “We are seeing retort pouches replacing jars, bottles, and metal cans—smaller cans in the consumer food market, as well as large industrial-sized cans in food service,” reports Ehud Safrai, president, CLP Packaging Solutions.

What retort pouches bring to the picture says Safrai, are excellent barrier properties that protect against oxidation, moisture, and other factors that affect food products. The shorter cook times are the result of efficient heat transmission and the fact that the mass of the contents in the pouch is not a thick cylinder as it is in cans and jars. “That means that the product can be cooked more quickly and gently, which helps manufacturers maintain more delicate flavors. Simply put, food in pouches tastes better.”

Rodney Weaver, research and development associate for Sonoco Flexible Packaging, touches on another critical factor—consumer convenience. “With today’s on-the-go lifestyles, consumers want good tasting foods that are either ready to consume or can be consumed with a minimum of preparation. Retort pouches are the perfect packaging medium as they are designed for shelf-stable and processed foods.”

Another important convenience factor that pouches meet is that they are easy to open—for adults and children, says Weaver. “One segment that truly appreciates this benefit of the easy-to-open pouch is the pet food segment. Parents like that children can feed the family dog or cat by squeezing the food out of a pouch.”

The convenience factor is further enhanced when retort pouches are also microwaveable. “Certain retort pouches are suitable for microwave heating, allowing the individual to create a portable meal, an even greater convenience,” says Philip Dabbert, business development manager, Toray Films Group.

Retort pouch construction

Retort pouches are made up of multiple layers. “Traditionally, this has been between three and five, however some companies now are pushing to reduce costs by asking that desired objectives be met with two layers,” reports Hass. Various materials that make up these layers include PET, foil, nylon, polypropylene, and specialty materials such as aluminum oxide, silica oxide, polyacrylic acid, or nanocomposite coated films.

What really stresses these multi-layer structures, says Dabbert, is the need to withstand the sterilization process, which involves high heat and pressurized steam. “That necessitates a package that has excellent heat seal strength, impact strength, and barrier to oxygen and moisture,” he says.

Weaver believes that retort pouches face some of the toughest challenges of any packaging format. “During processing, filled pouches must be able to withstand temperatures up to 275°F [135°C] for periods of 60 minutes or longer, while enduring changes in both internal and external pressures. The laminate and adhesives are under tremendous stress; any failure will adversely affect the hermetic nature of the pouch.”

Because of the demanding environments faced by retort pouches, David Idan, R&D manager, CLP Industries (parent company of CLP Packaging Solutions), says that each element of the retort package must be optimized. “Each layer plays at least one role in the performance of a retort package. At the core, a barrier layer of aluminum or coated PET protects the contents from spoilage,” he says. “Inside, a sealing layer, usually polypropylene, maintains the integrity of the seals, which is vital for pouch performance. Outside, a PET layer is often reverse-printed to provide rich graphics.”

Microwaveable pouches provide ­additional challenges that must be met. “Microwaveable pouches can be subjected to even higher temperatures than retort,” says Haas, “however, for a much shorter period of time, and only after the consumer gets the package home.”

Because metal cannot be used in microwave ovens, the use of an aluminum foil layer is not feasible in microwaveable pouch constructions. A common alternative to provide this barrier layer is to use PET coated with silica or aluminum oxide, says Idan. However, this can put additional challenges on the table for converters.

“That’s not an easy change,” he reports. “Just a few percentage points of stretch during the converting process can destroy the barrier properties of coated PET, so it takes a great deal of finesse and excellent machinery to create a good microwaveable retort pouch.”

The inks and adhesives used in retort pouch applications also have to be carefully selected. “The majority of retort pouches are rotogravure printed,” notes Weaver. “Specially designed inks and adhesives are required for retort applications, primarily because of the high temperatures and pressures of processing.”

On the horizon

Devlopment efforts to reduce the layers required in retort pouch structures are ongoing. “Of course, the goal is always to develop lighter pouches that require less material in their construction,” says Idan. “In many cases, we have succeeded in going down from four-layer laminates to three-layers, and we continue to work toward even greater efficiency.”

Safrai has a positive view of the retort pouch market. “The retort pouch sector is growing nicely. I also think we will see adoption of pouches for water—now that we have been able to create laminates that do not impart a taste to the water—and industrial products such as oil and machinery fluids. In addition, now that we have developed laminates that withstand hard-to-hold ingredients such as ­alcohol and iodine, pouches are also being used increasingly for sterile products such as patient care materials, wound dressings, and medical equipment.

“The U.S. has begun to pick up as a market for pouches lately, and I think it is getting ready to really hit its stride as consumer convenience features continue developing, and as manufacturers see the benefits of using lightweight packaging that is far less expensive to transport and store. The speed of filling and sealing machinery is also improving, which is making pouches even more attractive to food and beverage companies.” pP