Establishing Safe Thermal Processes Methods

Establishing Safe Thermal Processes Method

Making decisions in the design of safe and suitable thermal process schedules in processing systems of great variety and capability requires exceptional knowledge, judgement, skill, equipment, facilities, and resources on the part of the process authority/specialist. Here the discussion is about the differences in three calculation methods used throughout the thermal processing industry to establish safe processes. Blind use of these calculation tools in the wrong situation can lead to a costly and health threatening situation in which the company or consumer is the victim.


The General Method is used to determine lethality equivalents at known internal container temperatures measured directly by the use of thermocouples positioned within the test container. The General Method is a procedure used to integrate the lethal effect of a time/temperature history resulting from a thermal process at a position located in the slowest heating zone in the container. The methods vary but may include a graphical representation of the data, a measurment of surface area under the lethality curve, a determination of paper weight under the lethality curve, or the addition of lethal rate values under the lethal rate graph. The principles of this method were introduced by Bigelow in 1920 who used a graphical representation to arrive at lethality value.

The General Method is used as the baseline target upon which all other formula calculation method performances should compare. It should be recognized, however, that even with the General Method approaches, lethality results can vary slightly depending upon the calculation technique being used. Perhaps the best known and most widely used General Method approach was proposed by M. Patashnik (Food Technology, July 20, 1952).

The General Method is not useful for calculating scheduled process times and extrapolating process lethality (F values) for different retort temperatures and product initial temperatures or changes within the process delivery system such as changes in heating or cooling ramp temperatures or times and cooling water temperature. A process developed by the General Method, (and thus direct thermocouple measurement) is critically dependent on all product, container, and process delivery system conditions as studied at the time of the test. Precise temperature distribution profiles must be ensured for each and every production cycle. Extrapolation outside the given process condition yields undefined, invalid lethality results. Additional in-container thermocouple testing must be performed to verify the sterilizing value delivered under the altered conditions. The thermocouple must be placed in the slowest heating zone of the container. Care must be exhibitied to adequately determine that the original cold spot determination has not changed by the altered conditions.

If and when a condition outside the boundary of the General Method scheduled process results, the product must be placed on hold unless other direct thermocouple measurement information is available which “fits” the deviation condition. Extrapolation is not accurate or valid. In most instances, the deviation condition must be reproduced, or simulated, (under commercial conditions, and in the commercial unit) under the exacting deviation conditions. Such thermocouple simulation, which must be made, could interrupt production efficiency, unnecessarily distract thermal processing personnel in order to perform additional testing, and require that the same product and container conditions are achieved in the simulations. This kind of simulation requirement is non-flexible, very costly, time consuming, unpredictable, and provides for “hit or miss” results. For all of the above reasons, the USDA and FDA are not comfortable with the food processor’s ability to control these factors. The food processor using General Method processes then becomes vulnerable to close regulatory scrutiny on the verification procedures used to confirm adequate achievement of process lethalities on production products.


C.O. Ball, in “Thermal Process Time for Canned Food” (National Research Council Bulletin 7, Part 1, No. 37 [1923]) proposed a formula method which would allow for the extrapolation of process time independent of a required direct thermocouple measurement once time and temperature data were collected by direct measurement. The Ball Formula Method is the most widely used process calculation procedure in the U.S. today. However, the pitfalls, limitations, and theory of the calculation method is not well understood by many in the thermal process industry.

The basic assumptions in the Ball Method result from empirically defined terms in the method which allow for an “estimation” of both process time or sterilizing value. Unfortunately, these assumptions often overestimate process lethality and, although conservative, do not accurately define the actual lethality delivered in the process. The Ball Method utilizes heating and cooling factors to predict the internal product SHZ temperature. The following assumptions are fixed within the method for your products:

1 jc = 1.41 (hyperbolic cooling lag)
2. fc = fh (inverse slope of linear logrithmic cooling =
inverse slope of linear logrithmic heating)
2 RT -CWT = (m + g) = 180o F (steam) or 130o F (full immersion water overpressure)
3 0.58 x CUT in calculation of j value
4 No further product heating after cooling starts
5 A constant retort temperature method only
6 A constant cooling water temperature method only

The above assumptions make Ball flexible but not accurate, with most lethality errors occurring in cooling. In the case of conduction heating products, there is often as much as a 100 percent increase in resulting lethality than that targeted in the scheduled process, often unacceptable to product quality. However, in retortable pouches and other low profile containers (cans, barrier trays, etc.) the use of the Ball Formula Method may overestimate cooling lethality, and thus not be an appropriate method to employ under certain circumstances.

Numerical Modeling

Numerical modeling (finite difference) methods have been used to establish thermal processes and evaluate process deviations since the 1970’s. Since 1988, TechniCAL has used its propietary software, NumeriCALTM, to perform these calculations by using a finite differencing method to solve partial differential equations of unsteady state heat transfer problems. The finite element modeling is particularly well suited for irregularly shaped containers as well as convection heating foods. Today, researchers have reported that calculations required large computer workstations to perform these numerical solutions. TechniCAL has overcome this hurdle by using a unique software approach with calculation results quickly determined using desktop computers.

Mathematically, within the model, the temperature is a distributed parameter in that at any point in time during the heating or cooling, the temperature takes on a different value with respect to a location in the container; and in any one location, the temperature changes with time as heat gradually penetrates the product from the container wall toward the container slowest heating zone. The finite differences are discrete increments of time and space defined in small fractions of process time and container height and radius.

As a framework for computer iteration, the cylindrical container is imagined to be subdivided in volume elements which appear as layers of concentric rings having rectangular cross-sections. Temperature nodes are assigned at the corners of each volume element on a vertical plane.

By assigning appropriate boundary and initial conditions to all the temperature nodes (interior nodes set at initial product temperature, and surface nodes set at retort temperature), the new temperature reached at each node can be calculated after a short time interval that would be consistent with data which is obtained from heat penetration tests. This new product temperature distribution is then taken to replace the initial test, and the procedure is repeated to calculate the temperature distribution after another time interval. In this way, the temperature at any point in the container at any instant in time is obtained.

If the volume of elements and time intervals are taken in sufficiently small increments, the temperature in each volume element can be treated as uniform and constant over the time interval. In this way the temperature at the container center can be calculated after each time. The accomplished sterilization value (F) in the slowest heating zone can be calculated after each time interval to produce the final F value at the end of the process.

The greatest advantages of NumeriCAL are its accuracy and flexibility. NumeriCAL can be used to simulate slowest heating zone product temperatures under a condition of variable retort temperatures and boundary layers outside of the container tested.

NumeriCAL can more accurately determine true lethality in the process than can the Ball Method. NumeriCAL is convenient in that the method utilizes heating and cooling factors (jh, fh, jc, fc, etc.) and can be used to extrapolate equivalent process times or sterilizing (F) values as in the Ball Method. Because any of the factors related to the product heating and cooling parmeters can be altered, a precise simulation and temperature comparison to a set of directly measured time and temperature thermocouple data can be made.

As in the General Method, the NumeriCAL slowest heating zone simulated temperatures relate directly to integrated lethality summations and, is thus, highly accurate. However, unlike the General Method, the model can ,in an off-line or real time mode, simulate the temperature at the slowest heating zone without a direct thermocouple measurement of either retort temperature outside the container or slowest heating zone position inside the container for variables such as retort temperature, initial temperature, heating or cooling ramp temperatures or times, and cooling factors (jh, fh, jc, fc, etc.) and retort temperature profiles are established from data obtained by direct thermocouple measurement, there is no further requirement to directly measure process variations within reasonable extrapolated limits. This alone, will save time and effort on the part of the process authority to establish precise process lethality conditions when temperature variation in the process delivery system occur.

NumeriCAL results are accepted by both the USDA and FDA. TechniCAL has taught industry thermal processing specialists how to use this powerful tool in process determination through its course “Computer-aided Advanced Thermal Processing”. Unlike other models, NumeriCAL results work well for convection , broken heating, and conduction heating products. It also requires the use of heating factors which are widely recognized by process authorities/specialists who are familiar with the Ball Formula Method. Precise temperature distribution which yields uniform temperature and stability in the retort vessel ensures safe process reduction results and serves as the process profile with which the NumeriCAL model is applied.
User misuse of the calculation power within this software program can produce inaccurate results compared to the real process conditions. The heating factors generated by the Ball Method (CALSoft II) approach may not always apply and may need to be numerically generated. Ironically, understanding the limitations of the numerical calculation approach creates a combined greater flexibility and accuracy method when compared to the General, Ball, or Stumbo Methods. The degree of calculation result conservatism is left up to the discretion of the user, not fixed or assumed as in other formula calculation methods.

Precise process delivery and product critical factor controls, in conjuction with accurate and flexible process calculation methodology, ensure safe product delivery to the consumer demanding maximum quality benefits. The food processor should use a process authority who is intimately familiar with all requirements for producing quality LACF products. A total systems approach is the only conceivable way to produce acceptable quality in sensitive products while making the process acceptable to production and regulatory concerns. Utilization of TechniCAL’s LOG-TEC Process Management System, NumeriCAL process modeling software, and Process Technology Division services,(including heat penetration, temperature distribution and personnel training programs)is often necessary to gain maximum process design benefits. Modifying, changing, or omitting any of these process design requirements when using NumeriCAL, may jeopardize the opportunity for obtaining desired process design safety and product quality results.