Advanced software continuously models, predicts and maintains lethality in batch-retort operations

Commercial sterility -- the destruction of Clostridium botulinum spores -- is an absolute requirement for producing low-acid, in-container shelf-stable food products. The thermal process for achieving commercial sterility is based on the accumulated time/temperature relationship needed to achieve lethality of C. botulinum.

For over 75 years, however, the major methods for calculating lethality were the General Method and the Ball Formula Method, both developed in the early 1920s. According to Jeff Cleek, vice-president for technology at AmeriQual Foods and a recognized thermal process authority, the General Method is based on initial product temperature and is accurate but specific to individual process conditions. "The General Method is very accurate but has little to no flexibility," said Cleek at Food Engineering's Food Automation 2001 Conference last February.

The Ball Formula, which correlates retort temperature and product temperature as a function of process time, is more flexible and "allows some portion of thermal credit in come-up and cooling times, but it doesn't take full advantage of available lethality," Cleek continued. This can result in overprocessing a product to achieve the lethality value needed. Nevertheless, the Ball Formula is still the most widely-used method for calculating microbiological lethality in a retort process.

AmeriQual Foods, a major co-packer located in Evansville, Ind., applied the Ball Formula until 1999, said Cleek, when it installed FMC FoodTech's NumeriCAL On-Line predictive modeling software to control its batch-retort operations. According to Cleek, NumeriCAL combines the accuracy of the General Method with the flexibility of the Ball Method.

"We chose NumeriCAL because we saw that its flexibility leads to a better processing environment," he reported. "It's as accurate as the General Method, and we've demonstrated that quite a few times in product development. It takes true temperature data and compares it to a known thermal rate. When we compared NumeriCAL with the General Method, we were fairly amazed at how closely the two approximate each other."

Cleek also compared NumeriCAL to the Ball Method, which is based on fixed values and is much more conservative. "You overprocess the product quite a bit when you use the Ball Method," he observed.

Versatile co-packer

AmeriQual Foods processes, packages and thermally sterilizes in-container a wide range of low-acid and acidified products at a USDA-inspected, 160,000-sq.-ft. plant employing about 400 people at Evansville. Products include entrees and sauces for several major food companies as well as MRE (Meal Ready-to-Eat) combat rations for the U.S. Dept. of Defense. Examples: cheese dips, toddler meals, pasta, stew and chili entrees packaged in 6- and 7-oz. retortable, single-serving microwavable plastic Omnibowls; pasta sauces in 16-oz. glass jars; and 32 different MREs such as beef steaks with gravy, turkey breast with potatoes, chicken and salsa in 5- and 8-oz. retort pouches.

AmeriQual is also equipped to package foods in the institutional-size retort pouch developed to replace the No. 10 can. "We can produce pouches for anybody, not just the military," observes Michael C. Billig, vice-president/marketing and sales at AmeriQual.

AmeriQual doesn't can foods. "We focus on the more difficult, specialty products in niche packaging which most processors can't do," says Billig.

Predictive control

Because of the many variables which must be controlled in optimizing product quality, AmeriQual Foods controls its entire process from ingredient batching, blending and cooking through packaging, sterilization, secondary packaging and distribution.

AmeriQual operates 13 batch retorts in batch-continuous modes where several retorts are sequenced in process stages, allowing one to be manually emptied and re-loaded with dollied baskets as others are in-process, for total capacity of up to 120 retort loads per day. The retort lines include:

  • Nine Allpax Rotopax rotary full water-immersion units with capacity of four carts per load.

  • Three FMC static Waterspray steam/water retorts with capacity of seven carts per load.

  • One FMC rotary Waterspray steam/water unit, with capacity of four carts per load. This unit is equipped with a chain conveyor for semi-automated loading and unloading.

All 13 retorts are controlled via FMC's NumeriCAL On-Line predictive modeling software, running on FMC LOG-TEC controllers integrated with a host PC which records process profiles for each batch.

Accepted by FDA and USDA, NumeriCAL continuously models and predicts on-line in real time the total lethality (Fo) delivered to a food product within a container, and automatically corrects Fo when a deviation occurs in the sterilization process. A process deviation can occur, for example, when utility disruptions cause the heating medium (hot water or steam) to supply inadequate energy and thus non-uniform lethality to each container in a batch retort. Any deviation which could reduce target lethality is alarmed, and an equivalent model is instantly calculated and applied.

Running on NumeriCAL algorithms, the LOG-TEC controller adjusts process time or temperature to compensate for the temperature-deviation effect on product lethality in the slowest-heating or fastest-cooling zone of the retort. It applies maximum thermal credit for come-up time and cooling time, precisely controlling retort conditions to just above the minimum temperature and pressure setpoints for the given process. This reduces heating times, steam consumption, final product temperatures, cooling times and cooling-water consumption to boost throughput and - through less overprocessing - product quality. NumeriCAL tracks lethality through each stage of the process, and displays accumulated lethality (Fo) on the LOG-TEC screen.

"Many variables must be considered in calculating a predictive model," said Cleek. In addition to product-formulation variables such as ingredients, water activity and pH, factors which must be considered include container size, shape, filling and sealing methods; product heating rate; type of retort and orientation of the containers within the retort. For example, the coldest spot, (or slowest-heating spot) in a retort is package-specific and typically in the center of a basket, Cleek observes. To determine the location of the cold spot in each retort, a series of temperature-distribution tests must be conducted using thermocouples. When determined, the cold spot is one variable used to configure a product's process, or recipe.

NumeriCAL has memory capacity for up to 999 recipes, each characterized by time, starting temperature, ending temperature and heating rate.

Improved throughput, quality

In a typical retort process at AmeriQual, the internal temperature of a sample package is taken via thermocouple to determine the initial temperature (IT) for the pre-heat stage, which is 50 degrees F above the temperature of the IT. "This is the base for the NumeriCAL-controlled cook-time," explains Quality Engineer Jason Reising. Dollied baskets are then manually pushed-into the retort (or semi-automatically loaded, in the case of the FMC rotary unit). LOG-TEC senses when the retort door is closed and prompts the operator to enter product identity, number of containers, and the IT. LOG-TEC computes process time based on the IT, target temperature of 250 degrees F and the appropriate operating overpressure. The operator then hits "Enter" on the keypad to start the process, and NumeriCAL takes over, controlling the process through water-fill, pre-heat, come-up, holding and cooling. LOG-TEC displays time and temperature required to achieve setpoints for sterilizing (i.e., 250 degrees F) and cooling, and integrates with a host computer to document time/temperature profiles for each batch. Each retort is equipped with a chart recorder verified with a mercury-in-glass thermometer. FDA accepts the LOG-TEC record as the operator's log required by 21 CFR 113.

Cleek estimates that NumeriCAL model-predictive control has resulted in energy savings of 12 to 20 percent, and throughput gains of 7 to 10 percent. The system also prevents oversterilization of temperature-sensitive products. "If you're running Alfredo sauce, for example, you want it white, not brown."

New technologies

AmeriQual will soon upgrade retort control to running NumeriCAL on the new Windows-based LOG-TEC Momentum system, said Cleek. According to Rick Eleew, technical applications manager at FMC FoodTech (Madera, Cal.) , LOG-TEC Momentum replaces the earlier DOS-based LOG-TEC system with Microsoft Windows CE, NT and 2000 operating systems - "CE for the controller, NT and 2000 for the host" - on an Ethernet LAN, allowing multiple viewers and data connectivity through off-the-shelf Microsoft products such as Excel and Access. Momentum is capable of faster computing speeds, allowing the user to optimize advanced mathematical algorithms like FMC's NumeriCAL, HydroCAL and TunaCAL (FE April, '01).

In developing Momentum "we maintained the same fundamental view on safety, that is, on FDA and USDA regulatory acceptance and HACCP requirements," said Eleew. "The system is validatable because it has full historic audit-trail capability and on-line electronic record review."

At the time of Food Engineering's plant visit mid-May, AmeriQual Foods was expanding its plant by 90,000 sq.-ft. to accommodate new packaging equipment and more warehouse space. AmeriQual is currently installing a Silgan Poly-Weld 600 spin-friction closure machine capable of sealing up to 600 Omnibowls per minute with retortable tear-off barrier-plastic lids to replace the current aluminum lid, offering a totally micro-wavable and more easily recyclable container. "We're hoping it will drive the industry toward an all-plastic container," said Cleek.