VARIABLE CONTROL of A BATCH RETORT and PROCESS SIMULATION FOR OPTIMIZATION STUDIES

A microcomputer was interfaced with a batch retort for digital PI control of the retort temperature. Six retort temperature profiles (three isothermal, sinusoidal, step, and ramp) were examined for processing pea puree to nearly the same lethality values in 211 × 400 cans. Thiamine content and steam consumption were measured. Thiamine retentions in non-isothermal processes were greater than in the isothermal processes, but there was no significant difference in the steam consumption.
Lethality and thiamine retention were also simulated using a finite difference model for the same heating profiles, with good agreement with experimental results. the simulation suggests that differences in thiamine retention due to differing heating regimes are significant only for large lethality values and smaller cans. For F₀ values smaller than 10 min, the simulation suggests that the sinusoidal heating regime will provide better thiamine retention. Limitations of the model and suggestions for future improvement are also discussed.     

COMPLEX METHOD FOR NONLINEAR CONSTRAINED MULTI-CRITERIA (MULTI-OBJECTIVE FUNCTION) OPTIMIZATION of THERMAL PROCESSING

The goal in a multi‐objective function optimization problem is to optimize the several objective functions simultaneously. the complex method is a powerful algorithm to find the optimum of a general nonlinear function within a constrained region. the objective of this study was to apply the complex method to two different shapes (a sphere and a finite cylinder) subjected to the same thermal processing boundary conditions to find a variable process temperature profile (decision variable) to maximize the volume‐average retention of thiamine. A process temperature range of 5 to 150C was used as an explicit constraint. Implicit constraints were center temperature and accumulated center lethality of the sphere and the finite cylinder. the objective functions for both shapes were combined into a single one using a weighting method. Then, the previously developed complex algorithm was applied using Lexicographic Ordering to order the objective functions with respect to their significance. the results were reported as optimum variable process temperature profiles using the given geometries and objective functions. the thiamine retentions were also compared with a constant process temperature process, and 3.0% increase was obtained in the combined objective function. the results showed that the complex method can be successfully used to predict the optimum variable process temperature profiles in multi‐criteria thermal processing problems.     

NONLINEAR CONSTRAINED OPTIMIZATION OF THERMAL PROCESSING: I. DEVELOPMENT OF A MODIFIED ALGORITHM OF COMPLEX METHOD

The complex method, a general and flexible optimization algorithm, was used to determine the optimum process temperature profiles during thermal processing to maximize (optimize) the objective function of volume average retention of thiamine in conduction heated foods with restrictive explicit and implicit constraints. Implicit constraints were the target lethality at the coldest point, and a threshold below which the center temperature must reach at the end of the process. A process temperature range of 5–150C was used as an explicit constraint. Another explicit constraint was that the process temperature had to reach 5C at the end of the process. Control variable for the optimization was process temperature profile discretized at equidistant time steps throughout the process. All calculations were performed using a computer program written in Microsoft Visual Basic V. 6.0. This Windows‐based software calculated the optimum temperature profiles with resulting objective functions and implicit constraints. The developed algorithm for the Complex Method was found to be satisfactorily usable for the optimization of thermal processing of conduction heated foods.     

OPTIMIZATION OF QUALITY RETENTION IN CONDUCTION‐HEATING FOODS OF CONICAL SHAPE1

A cone frustum is an alternative shape for packaging thermally processed foods that can be useful in modeling the increasing number of microwaveable, ready‐to‐eat conical‐shaped food containers seen on supermarket shelves. Thermal processing in a cone frustum can be optimized by using numerical models for heat transfer to predict temperature distribution profiles, together with thermal destruction kinetics of target organisms and nutrient/quality factors. Iso‐lethality curves, showing combinations of process time and retort temperature that deliver equal lethality, were developed for each of three different cone frustum geometries (different dimensional proportions of major and minor diameters and height). Total volume average quality retention was determined for equivalent process time‐temperature combinations for quality factors with assumed thermal degradation kinetic parameters (D and Z‐values). Response of quality retention to the equivalent process combinations (designated by their retort temperature) revealed optimum process conditions that delivered maximum quality retention. The effect of kinetic parameters, thermal properties, and surface heat transfer coefficient on quality retention response to equivalent process conditions was also studied and compared with findings reported in the literature for the case of more traditional finite cylinder shapes.     

EVALUATION OF THERMAL PROCESSES FOR CONDUCTION HEATING FOODS IN PEAR-SHAPED CONTAINERS

A numerical model was developed to study transient heat conduction in a pear-shaped can. The model was extended to perform bacterial lethality and nutrient retention calculations for foods processed in pear-shaped containers. Transient temperature distributions and lethality predictions obtained from the numerical model when applied to finite cylinders, compared favorably with previously accepted solutions. Calculations of integrated F_s values of processes for pear-shaped containers indicated up to 50% overprocessing by the single point lethality concept. An "equivalent cylinder" was defined as one having a geometry index and characteristic heat transfer length corresponding to those of a pear-shaped container. Process evaluations for "equivalent cylinders" were in agreement with solutions obtained for the pear-shaped model.     

Verification of a formula for estimating average quality factor degradation in thermally processed foods in cylindrical containers

Summary A formula suggested by Jen et  al. (1971) may be suitable for estimating the average retention of quality factors in conduction heated foods. This hypothesis was tested by comparing the results obtained for thiamine retention with calculations based on analytical and finite difference solutions of transient heat conduction in cylindrical containers. For a high Biot number, agreement was within 10.8% over a wide range of processing conditions (can size, retort temperature, initial temperature, etc).     

SELECTION OF VARIABLE RETORT TEMPERATURE PROCESSES FOR CANNED SALMON

Variable retort temperature processes (VRT), in which retort temperature is a function of time, were developed for sterilization of Pacific salmon in 307 × 115 steel cans, with objectives of minimizing loss of quality criteria such as surface quality or thiamine, or minimizing process time, all while maintaining constant center-point lethality (F). A finite difference computer model of conduction within a finite cylinder was used to test different temperature-time profiles. Processes were constrained to include steam vent schedules and to exclude temperature decrease during the heating process as well as temperature modulation during cooling, thereby improving compatibility with typical salmon canning facilities. Rho, the fraction of total lethality accumulated at steam-off time, was found to be a function of final unaccomplished temperature, retort temperature, salmon thermal diffusivity and container geometry. The search for favorable processes was aided by Random Centroid Optimization (RCO). Minimum surface cook in constant retort temperature (CRT) processes varied with retort temperature and z of surface quality, but the best VRT process was consistently better than the best CRT process. VRT reduced operator's process time from 64 min to 54 min and maintained equal F₀ and surface quality. Thiamine losses were reduced from 19.6% loss by CRT to 16.8% loss by VRT.     

Improvement of Thiamin Retention in Canned Low-Acid Foods Through pH Adjustment

The effect of pH adjustment, in the range 5.0–6.9, on the thiamine retection after thermal processing of canned pea, corn, pork and beef liver pureés was studied. In all foods tested, reduction of pH, in the range studied, dramatically increased the retention of thiamin after processing. Retention of thiamin in processed pea and corn purees adjusted to pH 5.6 increased by 95% and 63%, respectively, as compared to samples having normal pH (6.9 for pea and 6.5 for corn).     

MODELING AND OPTIMIZATION OF CONSTANT RETORT TEMPERATURE (CRT) THERMAL PROCESSING USING COUPLED NEURAL NETWORKS AND GENETIC ALGORITHMS

Coupled artificial neural network (ANN) models and genetic algorithms (GA) were applied for developing prediction models and for optimization of constant temperature retort (CRT) thermal processing of conduction heating foods. ANN prediction models were developed for process time (Pt), average quality retention (Qv), surface cook value (Fs), equivalent energy consumption (En), final temperature difference (T_g) at can center, and lethality ratio (p, heating/total lethality). The processing conditions as inputs for ANN models were as follows: retort temperature (RT = 110–140C), thermal diffusivity (α= 1.1–2.14*10^?7m²/s), volume of can (V = 1.64–6.55*10^?4m³), ratio of height to diameter of can (R_dh= 0.2–1.8), total desired lethality value (F₀= 5–10 min) at can center and quality kinetic destruction parameters: decimal destruction time (D_q– 150–300 min) and their temperature dependence (z_q= 15–40C). The data for training and testing ANN models were obtained from a finite difference computer simulation program. A second order central composite design was used for constructing the experimental data for training ANN models, while an orthogonal experimental design with 6 factors and 3 levels was used for the generalization of trained ANN models. ANN model linked Genetic Algorithms (GA) were employed for searching for the optimal quality retention and corresponding retort temperature, and for investigating the effects of main processing factors. ANN‐based prediction models successfully described the various outputs of CRT thermal processing (correlation coefficients: R² > 0.98; relative errors: Er ≤ 3%). The coupled ANN‐GA models, verified under several typical processing conditions, could be effectively used for optimization of CRT thermal processing. The main processing conditions and their interactions in the order of their importance with respect to the optimal quality retention and corresponding retort temperature were: V >z_q >R_dh >; and z_q >F_d > R_dh >V, respectively.     

THERMAL PROCESS TIME CALCULATIONS FOR THIN PROFILE PACKAGES: COMPARISON of FORMULA METHODS

The process time calculation accuracy of five formula methods were evaluated in relation to predictions from a computer model based on finite difference numerical solution to three-dimensional heat conduction equations with finite surface convection as applicable to processing of packaged foods in thin profile forms. Heat penetration data were obtained from the computer model for a range of package sizes, food properties and processing conditions. the centerpoint time-temperature data were used to calculate process times required for a target lethality of 5.0 min using the General method approach (employing numerical integration). the study indicated that process time prediction errors for the different methods calculated as percent deviations from those computed using the General method were relatively small (below 4% on average) within the range of experimental conditions.     

A NEURAL NETWORK APPROACH FOR THERMAL PROCESSING APPLICATIONS

The use of a neural network approach in thermal processing applications is presented. A four layer neural network with 3 inputs and 3 outputs was trained using a back-propagation algorithm. A finite difference computer program was used to predict nodal temperature responses of conduction heating model foods under thermal processing conditions. Equivalent lethality processes were obtained for a range of input variables (can size, food thermal diffusivity and kinetic parameters of quality factors) for sterilization temperatures between 110 and 134C (at 2C intervals). the computed optimum conditions and their associated quality changes were used as input variables for training and evaluation of the neural network. the trained network was found to predict optimal sterilization temperatures with an accuracy of ± 0.5C and other responses with less than 5% associated errors.     

Studies on some water-soluble vitamins retention in potatoes and cow peas as affected by thermal processing and storage

The effects of the commercial thermal processing and storage on the retention of ascorbic acid, thiamine and riboflavin in potatoes and cow peas have been studied. Samples have been collected after washing, blanching and thermal processing operations. Washing had no significant effect on the levels of these vitamins in both potatoes and cow peas. Either blanching (at 100 degrees C for 2 min) or thermal processing (at 121 degrees C for 20 min) had a great effect on the retention of ascorbic acid, thiamine and riboflavin. While the retention of ascorbic acid and thiamine had been affected by the storage conditions (at 25 degrees C for 6 month), there was no significant change of riboflavin content in potatoes and cow peas. Based on a laboratory study, leaching was largely responsible for losses of these vitamins during water blanching, thermal processing and storage.     

A Manual Method for Predicting Nutrient Retention During Thermal Processing of Conduction-Heating Foods in Rectangular Containers

The objective of this research work was to develop and evaluate simple manual procedure for the prediction of the retention of nutrients with first order kinetics of thermal degradation, during thermal processing of conduction-heating foods packed in rectangular containers. An equation was developed that permits the estimation of the integrated nutrient retention over the volume by means of Stumbo's formula method. The procedure was compared with literature data for thiamin retention in various thermal processes. Results obtained from the application of the method were within the 90% confidence interval of the experimental values.     

TIME-VARIABLE RETORT TEMPERATURE PROFILES FOR CYLINDRICAL CANS: BATCH PROCESS TIME,ENERGY CONSUMPTION,and QUALITY RETENTION MODEL1

The batch retort model developed uses a heat transfer equation for heat conduction in cylindrical cans, first order kinetics for microbial inactivation, first order kinetics for quality losses and a transient energy balance to estimate steam consumption. For a given retort, lethality process and quality retention, the transient energy balance equation in the model allowed the identification of feasible time-temperature profiles reducing energy consumption, total process time or both. In the examples analyzed and depending upon product specifications, time-variable retort temperatures reduced process time by 18–55 min. These examples suggested that a change from constant to time-variable retort temperatures could increase canning capacity by 20–50%.     

A Method for Thermal Process Evaluation of Conduction Heated Foods in Retortable Pouches

A mathematical model was developed to evaluate thermal processing of conduction heated foods in retortable pouches. This model uses any given finite value of surface thermal conductance (h) which is encountered in commercial sterilization of retortable pouches using hot water as the heating medium. The model is capable of determining process time, mass average sterilizing value and nutrient retention for a prefixed critical point sterilizing value using any retort temperature profile. Results of thiamin assay in pea puree processed in retortable pouches indicated good agreement between experimental and model predicted values of thiamin retention.     

Computer-Based Retort Control Logic for On-Line Correction of Process Deviations

A control logic algorithm is described for use with computer-based control systems on batch retort operations in the thermal processing of canned foods. The system is capable of automatically adjusting process time during the cook cycle to compensate precisely for any unexpected deviation in retort temperature. It takes into account the cooling lethality contributed with conduction-heating foods, and does not rely on any test cans for monitoring product center temperature. Performance is demonstrated by computer-generated plots showing actual retort temperature history along with calculated center temperature and accumulated process lethality; and results are compared with other methods.     

SENSITIVITY ANALYSIS of ASEPTIC PROCESS SIMULATIONS FOR FOODS CONTAINING PARTICULATES1

A sensitivity analysis of a computer model, simulating heat transfer into particulate foods processed in a continuous aseptic system was conducted to determine how the model reacts to the variations in selected product and process input parameters depending on simulation types, namely, “Total”, “F₀ Hold” and “Hold Only”. Three major output variables, namely, holding tube required to destroy 6 D of Clostridium sporogenes (PA3679), destruction of thiamine and inactivation of peroxidase were selected for the sensitivity analysis. Theoretical results indicated that basically no difference between the “Total” system approach and the “F₀ Hold” approach was found on the model prediction. Particle size, particle thermal properties such as density and specific heat were the most sensitive parameters (within the range investigated) among product parameters which influence holding tube length required, thiamine and peroxidase retention while fluid properties such as fluid thermal conductivity and viscosity were less susceptible to affect the model prediction regardless of simulation types. Among process parameters, product flow rate and product initial temperature seemed to be the most critical parameters within the range covered in this study while rotor speed seemed to be one of the least critical parameters to influence the model prediction regardless of simulation types.     

Evaluation of the instrumental quality of pressure-assisted thermally processed carrots

ABSTRACT:  This study was conducted to compare the effectiveness of pressure-assisted thermal processing (PATP) in preserving the texture, color, and carotene content of carrot cylinders in the pressure range of 500 to 700 MPa and the temperature range of 95 to 121 °C. The effectiveness of PATP was compared with that of conventional thermal processing (TP) by matching carrot preprocess temperature history. Results indicated that under comparable process temperatures (up to 105 °C), PATP retained the carrot quality attributes such as color and carotene content better than TP. However, process and preprocess thermal history at 121 °C greatly influenced carrot textural change and pressure protective effects were less pronounced. This study demonstrated that PATP has the potential to produce low-acid foods with a relatively better quality than TP.     

Bigelow's General Method Revisited: Development of a New Calculation Technique

This article describes the development of a comprehensive procedure for broader application of the General Method in calculating thermal processes. In addition to the capability of calculating lethality (or process time to achieve a specified lethality) for a given set of heat‐penetration data, the procedure can also calculate process time or lethality for process conditions different from those of the original heat‐penetration tests. The new procedure was experimentally tested against the Ball Formula Method and numerical finite difference methods. It often safely predicted shorter process times than the Ball Formula Method by accurately accounting for slow come‐up and cool‐down phases of the retort process.     

Effect of Thermal Process Time on Quality of "Shrimp Kuruma" in Retortable Pouches and Aluminum Cans

ABSTRACT:  The effect of processing time on biochemical, sensory characteristics, and instrumental texture profiles of "Shrimp Kuruma" in retortable pouches and aluminum cans with equal pack weight and lethality was studied. Processing in pouches resulted in a 35% reduction in processing time compared to cans for equal lethality ( F ₀ 8.0). Thermal processing resulted in a 14% loss of water in canned samples compared to 9% in pouched products. Thermal processing resulted in an increase in the volatile compounds and a decrease in the oxidation products in both canned and pouched samples. Products processed in pouches were rated as lighter in color, more succulent, and more desirable in firmness compared to products in cans. Chewiness and hardness showed a significant difference ( P < 0.05) between the 2 treatments.