In some ways, retorting has come a long way since the days of its inception. Yet, essentially the process has remained the same. The underlying concept -- heating foods prone to microbial spoilage in hermetically sealed containers to extend the shelf life -- remains the operating principle which defines as well as limits the process. Although formulation plays an important role in these products, it is the process that ultimately affects the finished product quality. Three factors must be in a delicate balance in order to design a retorted product: safety, quality and economics.
While retort sterilization may not have the cachet of some newer methods of food preservation, it is still a complex technology. The foremost consideration is that of safety. While processing mistakes in other disciplines can mean repercussions in product quality, and, at worst, repercussions to your career, miscalculations in the retort process have the potential to kill someone.
The goal of retort processing is to obtain commercial sterilization by the application of heat. The microorganism of greatest concern is Clostridium botulinum, a gas forming anaerobe that produces a lethal exotoxin. Additionally, any spoilage organisms present need to be inactivated. While the thermal process is designed to destroy or inactivate these organisms, certain bacteria may survive the process, so the product is safe, but not necessarily sterile.
"Commercial sterilization is an inactivation of organisms of significance to both public health and spoilage under normal conditions of storage," states Jenny Scott, chief microbiologist of the processing technology and microbiology center of the National Food Processors Association located in Washington, D.C. "Clostridium botulinum is the most heat-resistant organism of public health significance. But most thermal processes are designed to inactivate spoilage organisms with the understanding that they will be more resistant than C. botulinum. In most cases, you're looking at Clostridium sporogenes."
In addition to the putrefactive spoilage created by this and similar organisms, other types of spoilage need to be considered, primarily flat sour spoilage or "T.A." spoilage (thermophilic anaerobe, not producing hydrogen sulfite). If the spores are thermophilic, they may survive the process. Under normal circumstances this does not cause problems. But, if a retorted product is held at elevated temperatures for long periods of time, spoilage can occur due to inadequate cooling or hot storage temperatures.
"A classical 'hot process' would have an Fo of 3, while that for a typical commercial sterility process would be on the order of 5 to 6," explains Scott.
"There are some thermophilic organisms that are very heat stable -- the Fo is on the order of thirty."
Simply put for the purposes of this discussion, the F value is the number of minutes to destroy the organism at 250°F. This varies with the microorganism. The Fo value describes the amount of time to reduce the microbial population by a factor of 1012. For C. botulinum, a heating time of 2.45 minutes at 250°F reduces the population by this factor. The Fo value is used to compare heat treatments.
This explains why spoilage problems occurred in some of the retorted products used by the military in the recent Persian Gulf War. The thermal processes these products underwent were not designed to take long periods of hot storage into account. The remaining thermophilic organisms found the high storage temperatures quite conducive to growth. To prevent spoilage, products destined for hot storage conditions such as hot vend packs require more severe treatments than typical products.
Putting the heat on
In order to ensure commercial sterility, the entire food mass must undergo the required temperature for the required time. Because extended exposure to heat affects the finished product quality -- usually adversely -- it often becomes a consideration in designing the product and process.
"In determining the heat process, the number one thing is that we must make certain that the product is safe," says Terry Heyliger, manager, thermal processing, FMC Corporation, Madera, CA. "The cooked quality has to take a back seat to that. Depending on the limiting factors -- the type of equipment, the product or the package, there are times you have to make trade-offs to get the proper lethality."
The following critical factors influence the rate of heat transfer and thus the time the product must be exposed to the heat: The type of process, equipment design, the size and shape of the container, product viscosity, particulates and headspace.
Assuring product safety while optimizing product quality can take several avenues, depending on the limiting factors. But first, a word about thermal lethality and heat transfer.
The higher the temperature, the faster the kill rate and therefore the shorter the exposure necessary. This technique is commonly used in pasteurization and is becoming more prevalent in retort processing.
"The relationship is that, if your killing power at 250°F is one, then your killing power at 268°F is ten," explains Heyliger. "That's going to dramatically shorten the cook time and most retorted products will benefit from a high temperature, short time process."
The second phenomenon to consider is the heat transfer process. In retort containers, after the heat passes through the container via conduction, it can heat the food in two ways. Conduction is appropriate for solid pack foods such as pumpkin and dog food, or for foods with solid particulates. Convection heating is best used for products or portions of the food exhibiting some degree of flow.
For conduction-heated products, the center of the container is the slowest heating point and the time it takes to reach the thermal processing temperature -- the come-up time -- is significantly longer than it is for products processed by convection heat transfer. In products heated entirely by convection, the coldest point will fall near the bottom of the container. Many foods, however, do not exhibit a single mode of heat transfer and the cold point must be determined by measuring various points in the can with thermocouples.
"There are two aspects in determining heat treatments, "advises Austin Gavin, chief scientist, processing at the NFPA. "There's the microbiological side -- we have to know what the resistance is of the particular organisms of concern. That will determine the amount of heat needed. On the processing side, we're going to apply that knowledge and determine how much heat the product is getting. Bridging the two together gives us the thermal process."
"There are two types of calculation methods," continues Gavin. "One is a formula method and the other is a numerical method. The formula method takes the heat penetration data and, based on a mathematical formula, determines the product's heating rate and predicts the lethalities. The numerical method is based on the heat transfer properties of the product and is essentially going to calculate temperatures by a numerical formula."
Once the lethality is determined, the thermal process should be measured using actual process conditions to confirm its validity. The two methods used are temperature measurement with thermocouples or microbiological validation with spore count reduction or inoculated packs.
"As an industry, thermocouples are probably the most widely used tool," Heyliger notes. "But there are certain instances when you would want to use the biological methods -- if it is difficult to reproduce the exact product in a laboratory setting versus a commercial setting, or if it's difficult to run heat penetration studies in the commercial unit, for example. Maybe someone isn't comfortable with the data from the thermocouple. In one product we were getting large lumps of dry grain, so we made some inoculated product to make sure we were getting enough kill inside the lump. I always try to remind people that we're not trying to sterilize thermocouple tips."
All these critical points affect the heat treatment required. The heat treatment required affects the finished product. Therefore, the heat treatment parameters directly affect the quality of the finished product. Within the boundaries of acceptable time/temperature scenarios, there exists the opportunity to optimize the product through the process, the product and the packaging.
In canned peas, excessive heat is a negative; in the case of baked beans, its hard to get too much of a good thing -- extensive heating is required to adequately cook the product. In the best tradition of robbing Peter to pay Paul, achieving microbial stability by retorting comes at the expense of flavor, texture and nutritional loss. Vitamin C and thiamine degrade fairly readily with exposure to heat, as do many of the naturally occurring pigments such as chlorophyll and lycopene.
"The destruction of microorganisms is predicted on a z value or rate of reaction with a temperature change of 18°F and many times that will create a difference for quality components," asserts mark A. Uebersax, Ph.D., professor and associate chairperson, department of food science and nutrition, Michigan State University, East Lansing. "If you look at the thermal processes available for microbial stability and they range from 240 to 280°F, the 240°F process would be about 50 minutes long. With that, you would lose about 50% of the thiamine in cream-style corn. If you took a process of equivalent lethality at 270°F, you'd be down to two or three minutes and you would only lose about 5% of the thiamine. There's a rate of destruction for vitamins as well as other quality factors that is different from the microorganisms with regard to temperature. This can give products microbial stability with improved quality at higher process temperatures."
Since heat destroys many quality factors, limiting the time of exposure makes sense from a quality standpoint. One of the first things that comes to mind is the elimination of the blanching step in fruits and vegetables. After all, the high heat during retorting is sufficient to inactivate the enzymes. While that is true, there are more important reasons to continue this step.
Blanching prior to retorting serves several purposes. Most importantly is removes tissue gases. This increases the level of vacuum in the container and reduces the amount of oxygen present, both of which extend the shelf life. Additional blanching softens the product, which facilitates filling and acts as a preheating step prior to retorting.
"Additionally, generally the density of the blanched product will be greater because you've replaced the gas with water," notes Uebersax. "When you ask if all this advance preparation is necessary, invariably the answer turns out to be 'yes'."
Since blanching is required, we need to examine the process, product and packaging to determine which parameters will optimize the finished product from both a safety and a quality perspective. The three parameters are closely related -- the process can dictate the package or product that can be run and vice versa. Depending on whether or not the product is formulated for existing equipment may dictate some of the options.
While grandma may use a pot on the stove for canning, manufacturers cook their hermetically sealed products in retorts. But like grandma's, retorts come in a number of different variations. These variations dictate the rate of heating and therefore, the finished product quality.
Still retorts are either horizontal or vertical batch systems that sterilize the product using steam. The heat transfer rate of the product, whether conduction or convection, determines the length of the heating process. Water is the cooling media. Often overpressure is required to prevent the internal pressure inside the package from buckling or otherwise destroying the integrity of the package.
The product is usually loaded into crates for handling, although some crateless models exist. These are designed to automate the handling process and can provide some energy savings as well.
"The crateless retorts have been receiving a lot of attention lately because they can take advantage of regenerated cooling," notes Uebersax. "You load the cans into preheated water that serves as a cushion. Once filled, the water is purged by injecting steam. Typically that water is held in an insulated vessel and reused. After cooking, the pressure is reduced. Then you just open the retort and they fall into a cooling canal. If you have a series of these and have each at a different stage in the process, it's a quasi-continuous system from the filler and seamer."
Batch retorts also may contain rotating racks that agitate the product in an end-over-end manner. This speeds the heat transfer in products that flow by agitating the contents by the movement of the air bubble created by the headspace. Lacking this bubble, there is little if any advantage in using this system for solid pack foods.
"The headspace bubble is essentially what stirs the product -- the larger the headspace, the more mixing you get and the faster a product heats," Gavin reports. "End-over-end agitation in and of itself is more efficient than side-over-side agitation, which is a more efficient means than a static process. But with so many retort designs, there is more at stake than the length of the thermal process. Manufacturers are interested in yield, throughput and things like that."
Continuous retorts increase the production rate and lower the labor costs, as well as increase the rate of heat transfer in fluid products. The most common design feeds cylindrical container through a mechanical pressure lock and conveys them through the retort on a horizontal, spiral conveying system. Typically, the cans rotate on their own axis for a portion of the spiral. The two types of rotation provide agitation, incorporating the headspace bubble and improving the heat transfer.
A variation on this type of retort locks the cans in place, eliminating the axial rotation. The headspace bubble takes an elliptical path through the container, which significantly increases the rate of heat transfer. This method can improve the quality by shortening process times of very viscous products in large containers, such as creamed corn in #10 cans.
Hydrostatic retorts are vertical systems that use a water leg as a steam valve. The height of the water column counteracts the pressure from the steam in the steam dome. A steam pressure of 15 psi requires a 33-foot water column. A conveyor chain lowers the cans into heated water, in which the temperature gradually increases to within a range of 225 to 245°F. This is an advantage for containers subject to thermal shock. They are conveyed through a steam chamber for the appropriate thermal process treatment and exit through another water leg, gradually cooling and again minimizing heat shock. These retorts can include multiple chains for processing different sizes or using different speeds simultaneously. Some manufacturers may provide an agitation method.
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.
According to Heyliger, the major criteria that would direct the selection of a specific retort style include:
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.
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."
The higher the throughput, the more efficient it is to use a continuous system.
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."
The 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.
Glass 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 burner
While 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.