A Look at Retorting
August 1994 -- Design Elements
By: Lynn A. Kuntz
Putting the heat on
(A number of other continuous retort designs exist in which containers enter and exit through the same pressure lock. This provides agitation by rolling the containers on a track through the pressure chamber.)
- The container shape and type.
A cylindrical container can be put through almost any type of system. The odd-shaped containers and retort pouches require special designs.
- Product type.
Convection heating, conduction heating and products that benefit from an agitation influence the selection. "Products that are very sensitive to burn on will benefit from an agitating or continuous rotary system," says Heyliger. "At the other extreme, products that heat strictly by conduction don't require agitation. Having said that, however, we've been able to run certain products normally run in a hydrostatic or stationary retort in a continuous rotary stabilizer at a higher temperature, shorter time. They get just enough movement in the products so they won't burn on at that higher temperature."
- Throughput required.
The higher the throughput, the more efficient it is to use a continuous system.
- Energy efficiency.
Since the useful life of a retort is a minimum of twenty years, and there are many still in use at fifty years plus, according to Heyliger, the energy savings could be significant over the life of the retort. From a quality standpoint, it often is desirable to shorten the amount of time the product is exposed to heat. There are several areas to watch for potential pitfalls. For example, while agitation can aid product mixing and heat transfer, too much can produce centrifugal force that forces the contents to remain stationary, particularly in end-to-end mixing. Higher temperatures used in short time processes create several potential problems as well. One concern in overcooking, or burning the product nearest the outside, so agitation is critical to obtaining an acceptable product.
"In general, higher temperature processes have shorter times and this implies the need for greater control," warns Uebersax. "If you think of the lethality that is being applied per unit of time, it is accumulating lethality much more rapidly than it would at lower temperatures. A deviation in time or temperature when you're at a high temperature for only a short time becomes a more critical issue. There's a much smaller margin of error."
Some of the more significant recent advances in retorting have occurred in the heating medium. The cooling method also affects the finished product quality and safety.
"While cooling isn't normally part of the thermal process considerations," explains Gavin, "the Ball formula makes some assumptions about the cooling, so it's inherent in the calculation. However, it only takes into account a portion of the cooling -- the Ball formula is very conservative."
"Incorporating the cooling in the thermal process would also mean that you would have more critical factors to include in your process," adds Scott. "Every time you optimize a process, there will be more things that turn into critical control points."
Cooling generally requires pressure adjustments to assure that the outside pressure doesn't drop much more quickly than the headspace gases. If the pressure is not equalized, the container can buckle or bulge.
"Cooling probably doesn't get looked at as often as it should," notes Heyliger. "The focus is on the sterilization, but there are products that could be sensitive to the cooling method. If the product needs good dispersal of an ingredient or particulate, a rotary or agitated cool would probably give you a more uniform product than if you cool it stationary."
There are other undesirable effects that can occur through insufficient cooling according to thermal processing and regulatory consultant, William Coffin.
"Insufficient cooling can markedly affect the product," he says. "All containers should be cooled to a temperature below 100°F. Not only can you get poor quality product through stack burn, but I've seen some pretty dramatic thermophilic spoilage in some products that maintained a center temperature of up to 140°F."
The other concern that may develop from the cooling technique is the post-contamination of products. According to Scott, some of these result from container or seaming defects, but some are related to the fact that the seaming compound may be somewhat fluid at elevated temperatures.
"The can is still drawing a vacuum as it cools," she points out. "It's possible to draw something in even through a well-formed seam. What you're trying to do is minimize the chance of whatever has been drawn in containing a microorganism. That is the primary reason for the chlorination of cooling water."
Adding ingredientsThe actual formulation of a product influences the heat treatment required. A number of techniques can be used to decrease the time duration necessary and thus improve finished product quality. A number of these were discussed in Shelf Stability: A Question of Quality in the June 1994 issue of Food Product Design.
Because convection heating and fluid viscosity greatly reduce the time needed to reach the required internal temperature, using starches and gums with heat thinning properties improves heat transfer. Some specialty starches and many gums are less viscous when hot and, upon cooling, thicken to the original consistency. As agitation increases, shear thinning properties may also serve to increase the heat transfer rates. As mentioned, these types of systems may need special attention to cooling procedures if consistent distribution of particulates is required in the finished product.
Another area that decreases heat requirements is optimized food systems. These combine such parameters as pH, Aw, solutes and other characteristics that increase the microbial lethality of a given thermal process.
"Canned mushrooms, for example, are almost always acidified to maintain texture," says Coffin. "There are a number of processes available. One company was offering a system using a complex organic acid to reduce the thermal process time while contributing little or no acid flavor."
Some processes use a combination of ingredients, packaging and process techniques to improve product quality attributes. For instance, the Veri-Green(r) System developed by Continental Can brings together a high-temperature, short-time sterilization process, a high (12.0) pH blanch and specially coated cans to improve the flavor and texture of vegetables.
The size and number of particulates also affects the heat transfer. If the pieces are too large they impede the convection flow of the product. The more there are, the more they restrict the flow and increase the amount of time necessary to reach the required temperature level. In addition, the centers of solid ingredients heat by conduction -- the larger they are, the longer the come-up time.
Even the properties of the ingredients themselves often have to be modified to suit a retort application. For example, putting normal pasta through a retort often results in overcooked clumps of noodles. According to Robert Vermylen, vice president of A. Zerega's Sons, Inc., Fair Lawn, NJ, three important variables must be considered in order to retort pasta successfully: shape, size and ingredients.
"Wall thickness and shape affect how well the pasta holds up -- smaller, heavier-walled pieces work much better in retort application," he explains. "Using 100% durum semolina or durum flour helps keep the pasta firm and reduces starchiness in the finished product. Adding egg albumen and glyceryl monostearate, which are allowed by the Federal Standards of Identity in pasta, can provide additional benefits."
Retort pasta should rehydrate more slowly than that designed for home use in order to reduce overcooking. The increased thickness and additional ingredients slow the cooking process. Egg albumen increases the protein content, which forms a water insoluble network. This entraps the starch granules, slowing down gelatinization. Glyceryl monostearate combines with the amylose to form insoluble complexes that minimize starch migration to the surface and decrease water absorption after cooking.
Package PropositionGlass and metal have been used as retort packaging materials since its beginnings. In this century, plastic technology has given us a wider range of choices, from flexible pouches to rigid containers. A number of considerations that drive the packaging choice: cost, appearance, end use (such as microwavability), product compatibility, and -- probably the most common limiting factor -- the retort itself.
Not all packages can be run in all retorts. Most retorts can handle cylindrical containers. Pouches or non-cylindrical containers cannot be run on a continuous rotary system. Glass is susceptible to breakage from thermal shock or excessive pressure. Process conditions may cause plastic to deform or lose its seal. Most plastics are not heat tolerant about 250°F.
"Lighter weight materials, even thin gauge cans, require overpressure processing during both heating and cooling. Most sterilizing equipment uses steam as the heating medium," explains Heyliger. "Steam at 15 psi is about 250°F. When a container heats, the internal pressure will exceed that. A container with a thin wall needs additional pressure. So you use pressurized air on top of that. You have to make sure the air is mixed so that is doesn't become an insulator around the container. One way is to submerge the containers in water -- then the air is on top of the water. Some manufacturers use a steam mixture circulated by a fan. Some use hot water sprays to mix things around. Each of these methods will give you slightly different heat transfer."
To keep retortable pouches from expanding to the point of bursting during retorting, they need to be kept in racks or constraints. Evacuating the air from the packages also reduces expansion and helps them to remain hermetically sealed.
Glass has special heating and cooling requirements. It can handle about a 100°F temperature difference before cracking.
"Jars are almost always given a water cook. You can do them in steam, but that requires an overriding air pressure," says Coffin. "You have to carefully control the temperature or you'll end up with a retort full of broken glass. We don't use borosilicate glass, but I believe we'll be able to use a completely different technology for glass packaging in the future."
Plastic packaged retort foods tend to be more expensive to produce than metal or glass. Contrary to popular belief, it's not that the packaging cost is significantly higher, it's the manufacturing and quality assurance costs.
"On most of the heat-sealed packages, you need 200% inspection on the seams," Heyliger says. "There is a tremendous amount of rejections. One of the biggest problems is that, because it's a hot seal, you have to keep that surface absolutely clean and free of contaminants. That is difficult to do with the filling process."
In the retort process, the heat must be transferred through the wall of the container to the product. Different packaging has different thermal conductivity, metal the highest and plastic the least.
"While these materials have different heat conductivities and different insulating effects, the geometry has a much greater effect on the heat transfer," notes Uebersax. "With pouches, in particular, if you minimize the thickness so the heat transfer to the center is much more rapid, you really can achieve a much less processed product."
On the back burnerWhile industry experts agree that retorting food products is a mature technology, that does not mean it is stagnant. As scientists and engineers know, room for improvement always can be found. Still, in terms of processing, the options are somewhat limited.
"There are only so many ways you can cool and heat in a retort system," points out Heyliger. "You can use steam, steam with air, water sprays and sprays with a mixture of water and air. There isn't anything revolutionary in commercial use, except that alternatives to pure steam are becoming prevalent. Fifteen years ago or so no one even talked about steam/air. They were taught all their life to keep the air out of a retort system."
Other technologies are under investigation, however. Microwaves have been applied to the process in various means -- as a blanch, a preheat or even the entire thermal source. Energy costs do not make this last choice a practical option at this time. Also, some sort of pressure is still required to bring the temperatures up to acceptable process levels.
Other, athermal, methods of sterilization hold some interest. Irradiation can be used to sterilize foods, but it does result in some product degradation, including vitamin loss and off flavor generation. Extremely high pressure can rupture and inactivate microorganisms. Researchers use levels of around 600 atmospheres to accomplish this and the technique shows some promise in high acid foods, according to Heyliger. The drawback with this method is that pressure does not inactivate the enzymes in foods.
The use of computerization has expanded in thermal processing. Not only has computerization brought new advances to thermal processing calculation, (See Computer Modeling: Solutions to Product Design in the July 1993 issue of Food Product Design), but it has improved data acquisition, process control and test methods, too.
"Computerization is another big change we're seeing," notes Gavin. With computerized control you can optimize the processes more readily and eliminate operator error."
So while it may be impossible to duplicate the flavor and texture of a fresh-picked peach in a retorted product, there are some means available to improve the process.
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On the back burner
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