INFLUENCE OF THE GEOMETRY ASPECT OF JARS ON THE HEAT TRANSFER AND FLOW PATTERN DURING STERILIZATION OF LIQUID FOODS (original) (raw)
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THEORETICAL ANALYSIS OF THERMAL STERILIZATION OF FOOD IN 3-D POUCHES
The objective of this chapter is to present the theoretical analysis of the process of thermal sterilization of canned liquid food in a three-dimensional (3-D) pouch. The prediction of temperature distribution and the migration of the slowest heating zone (SHZ) during natural convection heating in a pouch are presented for the first time in literature. Such information may be used to optimize the industrial sterilization process with respect to sterilization temperature and time. As a result of this investigation the companies involved in thermal retorting will be able to predict the necessary sterilization time required for any pouch containing any new liquid food products. This optimization process will save both energy and time, which are of great value for thermal retorting. Sterilization of food in cans has been well studied both experimentally and theoretically, but little work has been done on sterilization of food in pouches, which has recently been introduced to the market. The two different liquid food materials used in this study (carrot-orange soup and broccoli-cheddar soup) were some of the products of Heinz Watties Australasia located at Hastings, New Zealand. The computational fluid dynamics (CFD) code PHOENICS used in the study of cans was also used here. Saturated steam at 121 • C was assumed to be the heating medium. The partial differential equations (PDEs), describing the conservation of mass, momentum, and energy, were solved numerically together with bacteria and vitamin concentrations, using the finite volume method (FVM). The liquid foods used in the simulation were assumed to have temperature-dependent viscosity and density, while other physical properties were assumed constant. In this chapter, the following cases are discussed: 1. Temperature distribution, velocity profiles, and the migration of the SHZ during sterilization of broccoli-cheddar soup 2. Temperature distribution, velocity profiles, and the migration of the SHZ during sterilization of carrot-orange soup 3. Simulation for the same pouch but on the assumption of pure conduction heating to illustrate the effect of natural convection heating in pouches 4. The effect of the cooling period of the pouch on the sterilization of carrot-orange soup The results of these simulations are compared with the measurements of temperature distribution in Chapter 8. The results of the simulations show that the velocity of the liquid food in the pouch is low due to the small height of the pouch and high viscosity of the soup. In all the simulations, the SHZ was found to migrate toward the bottom of the pouch into a region within 30–40% of the pouch height, 93
Computer Simulation of Thermal Processing for Canned Food Sterilization
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A computer-aided engineering model is described that is capable of simulating the thermal sterilization processing of canned foods. The use of the model to find optimum processing conditions, physical properties and container geometry is reported. This work describes the use of thermal properties and physical properties in the development of computer models that simulate conduction and convection heat transfer in canned foods, for one application in automatic control or on-line computer control in real time. The present research in thermal properties it was compared by real-processes obtaining satisfactory results.
Although CFD models have been applied long time ago to different processing industries, it is only in recent years that they have been applied to food processing applications. A number of commercial software packages of practical use to the food industry have now become available, such as PHOENICS used in our simulations. CFD models can be of great use in a variety of food engineering applications. It can be used for predicting convection patterns in chillers or ovens, or determining the flow patterns of airborne microorganisms in a clean-room factory environment. The objective of this work is to analyze the process of thermal sterilization of liquid food in a three-dimensional pouch and to predict transient temperature, and concentrations of bacteria and vitamins profiles as heating progresses. The migration of the slowest heating zone during natural convection heating in a pouch heated from all sides was simulated and presented. The model liquid (carrot-orange soup) was assumed to have constant properties except for the viscosity (temperature dependent) and density (Boussinesq approximation). The governing equations of continuity, momentum and energy equations were solved numerically, together with that of bacteria and vitamins concentration, as expressed by a reaction kinetics model. A computational fluid dynamics (CFD) code PHOENICS was used, which is based on finite volume method of solution. The results of the simulations showed that, natural convection plays an important role in the heat transfer within the liquid food in pouch. The SHZ was found to migrate toward the bottom of the pouch into a region within 30-40% of the pouch height, closest to its deepest end. The profiles of the relative bacteria and different vitamin concentrations were also found from the simulation, which maybe used to optimize the industrial sterilization process with respect to sterilization temperature and time.
In this study, two approaches to developing simple correlations have been described and analysed. The recent ‘effective thermal diffusivity’ model has been benchmarked and the advantages of the approach illustrated. The ‘uniqueness’ of this kind of approach has been discussed. The second approach, which is new and called the ‘effective velocity’ approach, has been described and tested, against the limited data sets available. It has been demonstrated that by preserving the first order effect (i.e. the convection effect) in the heating equation, the second approach gives an opportunity to correlate with good accuracy the experimental data (whether it is generated by CFD or field measurement). Here the essential features of the natural convection driven process is captured well. The predicted circulation velocity level matches well with the previous CFD simulations for the two dimensional situations, supporting the validity of the approach. More in depth and quantitative study is required before any of these models can be used in practice.
Abdul Ghani, A. G. (2006). A computer simulation of heating and cooling liquid food during sterilization process using computational fluid dynamics. Association for Computing Machinery New Zealand Bulletin, 2 (3) Abstract: In this study, a theoretical analysis of a heating and cooling cycle during sterilization of the three-dimensional pouch filled with carrot-orange soup was presented and analyzed. Transient temperature, the shape of the slowest heating zone (SHZ) during heating and the slowest cooling zone (SCZ) during cooling were presented and studied. The simulation covered the whole heating and cooling cycles of 3600 s and 1200 s duration respectively. The computational fluid dynamics (CFD) code PHOENICS was used for this purpose. Saturated steam at 121 o C and water at 20 o C were assumed to be the heating and cooling media respectively. The partial differential equations, describing the conservation of mass, momentum and energy were solved numerically, using the finite volume method. The liquid food used in the simulation has a temperature dependent viscosity and density. At the end of heating, the SHZ was found settled into a region within 30-40% of the pouch height above the bottom and at a distance approximately 20-30 % of the pouch length from its deepest end. In the cooling cycle, the slowest cooling zone (SCZ) was found to develop in the core of the pouch and gradually migrates toward the widest end. The vertical location of this slowest cooling zone was about 60-70 % of the pouch height.
In this study, a theoretical analysis of a heating and cooling cycle during sterilization of the three-dimensional pouch filled with carrot-orange soup was presented and analyzed. Transient temperature, the shape of the slowest heating zone (SHZ) during heating and the slowest cooling zone (SCZ) during cooling were presented and studied. The simulation covered the whole heating and cooling cycles of 3600 s and 1200 s duration respectively. The computational fluid dynamics (CFD) code PHOENICS was used for this purpose. Saturated steam at 121 o C and water at 20 o C were assumed to be the heating and cooling media respectively. The partial differential equations, describing the conservation of mass, momentum and energy were solved numerically, using the finite volume method. The liquid food used in the simulation has a temperature dependent viscosity and density. At the end of heating, the SHZ was found settled into a region within 30-40% of the pouch height above the bottom and at a distance approximately 20-30 % of the pouch length from its deepest end. In the cooling cycle, the slowest cooling zone (SCZ) was found to develop in the core of the pouch and gradually migrates toward the widest end. The vertical location of this slowest cooling zone was about 60-70 % of the pouch height. The experimental validation has been performed by measuring the temperature distribution in the pouch during heating and cooling, using thermocouples fixed at different locations. The predicted results were in good agreement with those obtained from the experiments.
STUDIES ON THERMAL STERILIZATION OF CANNED FOOD USING COMPUTATIONAL FLUID DYNAMICS
Sterilization of liquid food in cans was studied and analyzed using computational fluid dynamics (CFD). In all the simulations studied, saturated steam at 121oC was used as the heating medium. The different liquid foods studied in this work were assumed to have constant specific heat, thermal conductivity and volume expansion coefficient, while the viscosity was taken as a function of temperature. Density variations were governed by the Boussinesq approximation. The software package code PHOENICS was used, which is based on finite volume method of analysis (FVM). The results of the simulations were presented in the form of transient temperature and velocity profiles. The shapes and movement of the slowest heating zone were followed throughout the sterilization time. The simulations show clearly the action of natural convection, which forces the slowest heating zone (SHZ) to migrate towards the bottom of the can as expected.
THERMAL STERILIZATION OF FOOD Historical Review
Thermal sterilization has been used to achieve long-term shelf stability for canned foods and is now used for a broad range of products. The majority of shelf-stable foods are thermally processed after being placed in the final containers. A relatively small percentage of shelf-stable foods are processed before packaging, using aseptic filling (Heldman and Hartel, 1997). Thermal sterilization of canned foods has been one of the most widely used methods for food preservation during the twentieth century and has contributed significantly to the nutritional well-being of much of the world's population (Teixeira and Tucker, 1997). The objective of thermal sterilization is to produce safe and high-quality food at a price that the consumer is willing to pay. It is a function of several factors such as the product heating rate, surface heat transfer coefficient, initial food temperature, heating medium come-up time, Z value for the quality factor, and target F ref value (Silva et al., 1992). The sterilization process not only extends the shelf life of the food but also affects its nutritional quality such as vitamin content. Optimal thermal sterilization of food always requires a compromise between the beneficial and destructive influences of heat on the food. One of the challenges for the food canning industry is to minimize these quality losses, meanwhile providing an adequate process to achieve the desired degree of sterility. The optimization of such a process is possible because of the strong temperature dependence of bacteria inactivation as compared to the rate of quality destruction (Lund, 1977). For this reason an estimate for the heat transfer rate is required in order to obtain optimum processing conditions and to maximize product quality. Also, a better understanding of the mechanism of the heating process will lead to an improved performance in the process and perhaps to energy savings. Basic principles for determining the performance of different but related processes have been presented by May (1997) and Wilbur (1996). In thermal sterilization of food, the heating medium temperature (steam or hot water) can deviate significantly from the design value during the heating phases. Such deviations may seriously endanger public safety due to under-processing of food (under-sterilization), waste energy, or reduce quality because of overprocessing of food (Datta et al., 1986). For these reasons, online retort control in thermal sterilization has been well studied by Datta et al., 1986; Gianoni and Hayakawa, 1982; Teixeira and Manson, 1982; and Teixeira and Tucker, 1997, to assure safety, quality, and process efficiency of thermally processed canned foods. In the design of thermal food process operations, the temperature in the slowest heating zone (SHZ) and the thermal center of the food during the process must be known. Traditionally this temperature is measured using thermocouples. The placement of thermocouples to record the temperature at various positions in a container during heating disturbs the flow patterns, causing errors in the measurements (Stoforos and Merson, 1990). Also, it is difficult to measure the temperature at the SHZ because this is a nonstationary region, which keeps moving during the heating progress, as
Thermal sterilization of canned food in a 3-D pouch using computational ¯uid dynamics
Sterilization of food in cans has been well studied both experimentally and theoretically, but little or no work has been done on sterilization of food in pouches. The food pouches have only been recently introduced to the market. In this study, transient temperature, velocity pro®les and the shape of the slowest heating zone (SHZ) have been established for a uniformly heated three-dimensional pouch containing carrot±orange soup, using saturated steam at 121°C. The computational ¯uid dynamics (CFD) code PHOENICS was used for this purpose. The liquid food used in the simulation has temperature-dependent viscosity and density. From the simulations, the maximum axial velocity of the soup was found to be 10 À2 À 10 À4 mm s À1 , which was due to the small height of the pouch and high viscosity of the soup. The SHZ was found to migrate into a region within 30±40% of the pouch height above the bottom and at a distance approximately 20±30% of the pouch length from its widest end. The experimental measurements were conducted at Heinz Watties Australasia based in New Zealand. The measured temperature at dierent locations in the pouch was compared with that predicted. Both results were found to be in good agreement. The results of a simulation done for the same pouch geometry and material considering pure conduction mechanism were also presented for the purpose of comparison.
Thermal sterilization of canned food in a 3-D pouch using computational fluid dynamics
Journal of Food Engineering, 2001
Sterilization of food in cans has been well studied both experimentally and theoretically, but little or no work has been done on sterilization of food in pouches. The food pouches have only been recently introduced to the market. In this study, transient temperature, velocity profiles and the shape of the slowest heating zone (SHZ) have been established for a uniformly heated three-dimensional pouch containing carrot–orange soup, using saturated steam at 121°C. The computational fluid dynamics (CFD) code PHOENICS was used for this purpose. The liquid food used in the simulation has temperature-dependent viscosity and density. From the simulations, the maximum axial velocity of the soup was found to be , which was due to the small height of the pouch and high viscosity of the soup. The SHZ was found to migrate into a region within 30–40% of the pouch height above the bottom and at a distance approximately 20–30% of the pouch length from its widest end. The experimental measurements were conducted at Heinz Watties Australasia based in New Zealand. The measured temperature at different locations in the pouch was compared with that predicted. Both results were found to be in good agreement. The results of a simulation done for the same pouch geometry and material considering pure conduction mechanism were also presented for the purpose of comparison.