A Look at Retorting
August 1994 -- Design Elements
By: Lynn A. Kuntz
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.
Safety firstWhile 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 onIn 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."
P-P-P-ParametersAll 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 visa versa. Depending on whether or not the product is formulated for existing equipment may dictate some of the options.
Retort reportWhile 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.