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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Institute of Food Technologists Series
- Sprache: Englisch
Part of the IFT (Institute of Food Technologists) series, this book discusses multiphysics modeling and its application in the development, optimization, and scale-up of emerging food processing technologies. The book covers recent research outcomes to demonstrate process efficiency and the impact on scalability, safety, and quality, and technologies including High Pressure Processing, High Pressure Thermal Sterilization, Radiofrequency, Ultrasound, Ultraviolet, and Pulsed Electric Fields Processing. Ideal for food and process engineers, food technologists, equipment designers, microbiologists, and research and development personnel, this book covers the importance and the methods for applying multiphysics modeling for the design, development, and application of these technologies.
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Seitenzahl: 882
Veröffentlichungsjahr: 2011
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Table of Contents
Cover
Table of Contents
Half title page
IFT Press
Title page
Copyright page
Titles in the IFTPress series
Preface
Contributors
Chapter 1 Introduction to Innovative Food Processing Technologies: Background, Advantages, Issues, and Need for Multiphysics Modeling
1.1. Introduction
1.2. Multiphysics Modeling
1.3. Innovative Food Processing Technologies
1.4. Modeling Challenges
1.5. Concluding Remarks
Chapter 2 The Need for Thermophysical Properties in Simulating Emerging Food Processing Technologies
2.1. Introduction
2.2. Definitions and Methods to Determine Thermophysical Properties
2.3. Final Remarks and Future Recommendations
Notations
Chapter 3 Neural Networks: Their Role in High-Pressure Processing
3.1. Brief History of NNs
3.2. Basis of NNs
3.3. How NNs Are Helping the Chemical Industry
3.4. The Role of NNs in the Food Industry
3.5. NNs in High-Pressure Processes
3.6. A Macroscopic Model for Thermal Exchange in an HPP System
3.7. Conclusions
Acknowledgments
Notation
Chapter 4 Computational Fluid Dynamics Applied in High-Pressure Processing Scale-Up
4.1. Introduction
4.2. Thermofluiddynamic Phenomena under High-Pressure Conditions
4.3. Mathematical Modeling and Numerical Simulation of High-Pressure Processes
4.4. Prediction of Process Impact and Control of High-Pressure Treatment
4.5. Conclusions and Outlook
Acknowledgment
Notation
Chapter 5 Computational Fluid Dynamics Applied in High-Pressure High-Temperature Processes: Spore Inactivation Distribution and Process Optimization
5.1. Introduction
5.2. Description of an HPHT Processing System
5.3. Developing a CFD Model for an HPHT System
5.4. Prediction of Temperature Uniformity and Flow by Means of CFD Modeling
5.5. Distribution of Process Sterility by Coupling with Kinetic C. botulinum Inactivation Models
5.6. Dimensionless Parameters to Express the Process Performance
5.7. Overview and Future Challenges
Notation
Chapter 6 Computer Simulation for Microwave Heating
6.1. Introduction
6.2. EM Wave Equations
6.3. Solutions to Maxwell’s Equations
6.4. MW Heating Equations
6.5. Computer Simulation of MW Heating
6.6. Simulation Model for MW Sterilization
6.7. Conclusion
Notation
Chapter 7 Simulating and Measuring Transient Three-Dimensional Temperature Distributions in Microwave Processing
7.1. Introduction
7.2. Microwave Thermal Modeling
7.3. Temperature Measurement (Mapping) Methods in Microwave Fields
7.4. Examples of Validated Microwave Heating Models
7.5. Summary, Conclusions, and Outlook
Notation
Chapter 8 Multiphysics Modeling of Ohmic Heating
8.1. Introduction
8.2. Electrical Heating of Foods: Governing Processes
8.3. Modeling and Validation
8.4. Further Development of Ohmic Heating and Appropriate Modeling
8.5. Conclusions
Notation
Chapter 9 Basics for Modeling of Pulsed Electric Field Processing of Foods
9.1. Introduction
9.2. Governing Equations for Multiphysics Simulation of PEF Processing
9.3. PEF System and Modeling of PEF Processing Parameters
9.4. Relevant Food Engineering Properties for PEF Simulations
9.5. Selected Inactivation Models for Numerical Simulations
9.6. Conclusion and Outlook
Acknowledgments
Notation
Chapter 10 Computational Fluid Dynamics Applied in Pulsed Electric Field Preservation of Liquid Foods
10.1. Introduction
10.2. PEF Treatment Chamber Design
10.3. Experimental Validation of PEF Simulations: A Review
10.4. Unsteady Simulation of PEF Batch Processing
10.5. Steady-State Simulation of Continuous PEF Processing
10.6. Improvement of the PEF Treatment Homogeneity
10.7. Turbulence Simulation in PEF Processing
10.8. Characterization of PEF Processing Conditions Assisted by Numerical Simulations
10.9. Conclusion and Outlook
Acknowledgments
Notation
Chapter 11 Novel, Multi-Objective Optimization of Pulsed Electric Field Processing for Liquid Food Treatment
11.1. Introduction
11.2. Modeling and Simulation of Fields Induced by PEF in Food Capable of Flowing
11.3. Numerical Optimization of Treatment Chamber and PEF Process
11.4. Conclusions and Outlook
Acknowledgments
Notation
Chapter 12 Modeling the Acoustic Field and Streaming Induced by an Ultrasonic Horn Reactor
12.1. Introduction
12.2. Modeling the Acoustic Field—Linear Acoustics
12.3. Nonlinear Acoustics, Sound Absorption, and Acoustic Streaming
12.4. Low-Power Acoustic Streaming Rayleigh, Nyborg, and Westervelt (RNW) Streaming Theory
12.5. High-Power Ultrasonic Streaming—Jet Flow Behavior
12.6. Final Remarks and Outlook
Notation
Chapter 13 Computational Study of Ultrasound-Assisted Drying of Food Materials
13.1. Introduction
13.2. Computational Design of the Power Ultrasonic System: Numerical Study by FEM of the Constituent Elements
13.3. Development of the Power Ultrasonic Transducer Prototype
13.4. Ultrasonically Assisted Convective Drier
13.5. Computational Analysis of Ultrasonically Assisted Convective Drying
13.6. Future Trends
Acknowledgments
Notation
Chapter 14 Characterization and Simulation of Ultraviolet Processing of Liquid Foods Using Computational Fluid Dynamics
14.1. Introduction: UV Light Processing of Liquid Foods and Beverages
14.2. Mathematical Modeling of UV Processing of Liquid Foods Using CFD
14.3. Application of CFD Modeling to Predict Microbial Performance of the Taylor–Couette Reactor
14.4. Validation of CFD Models
14.5. Future Research Needs for Understanding of Performance of UV Reactors through CFD Modeling for Food Applications
Notation
Chapter 15 Multiphysics Modeling of Ultraviolet Disinfection of Liquid Food—Performance Evaluation Using a Concept of Disinfection Efficiency
15.1. Introduction
15.2. Modeling UV Processing
15.3. Fluence
15.4. Simulation Examples
15.5. Conclusions
Acknowledgments
Notation
Chapter 16 Continuous Chromatographic Separation Technology—Modeling and Simulation
16.1. Introduction
16.2. True Moving Bed (TMB) Chromatography
16.3. Modeling Chromatographic Separations
16.4. Column Models
16.5. Modeling of the SMB process
16.6. Adsorption Isotherms
16.7. SMB Design
16.8. SMB Model Parameter Determination
16.9. Simulation and Validation
16.10. Conclusion
Notation
Chapter 17 The Future of Multiphysics Modeling of Innovative Food Processing Technologies
17.1. Introduction
17.2. What Can Usefully Be Modeled?
17.3. What Extra Data Are Needed?
17.4. Where Shall We Stop?
17.5. Conclusions
Index
Color Plates
Innovative Food Processing Technologies: Advances in Multiphysics Simulation
The IFT Press series reflects the mission of the Institute of Food Technologists—to advance the science of food contributing to healthier people everywhere. Developed in partnership with Wiley-Blackwell, IFT Press books serve as leading-edge handbooks for industrial application and reference and as essential texts for academic programs. Crafted through rigorous peer review and meticulous research, IFT Press publications represent the latest, most significant resources available to food scientists and related agriculture professionals worldwide.
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Library of Congress Cataloging-in-Publication Data
Innovative food processing technologies: advances in multiphysics simulation / [edited by] Kai Knoerzer ... [et al.].
p. cm.—(IFT Press series)
Includes bibliographical references and index.
ISBN 978-0-8138-1754-5 (hardback)
1. Food industry and trade–Mathematical methods. 2. Food industry and trade–Simulation methods. I. Knoerzer, Kai.
TP370.9.M38M85 2011
664'.020113–dc22
2010044300
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This book is published in the following electronic formats: ePDF 9780470959411; Wiley Online Library 9780470959435; ePub 9780470959428
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Titles in the IFT Press series
Preface
The food industry is an increasingly competitive and dynamic arena, with consumers now more aware of what they eat and, more importantly, what they want to eat. Important food quality attributes such as taste, texture, appearance, and nutritional content are strongly dependent on the way the foods are processed.
In recent years, with the aim to improve, or replace, conventional processing technologies in order to deliver higher-quality and better consumer-targeted food products, a number of innovative technologies, also referred to as “emerging” or “novel” technologies have been proposed, investigated, developed, and in some cases, implemented. These technologies take advantage of other physics phenomena such as high hydrostatic pressure, electric and electromagnetic fields, and pressure waves. Some of the most promising innovative technologies, in various stages of development and adoption, are discussed in this book, namely high-pressure processing (also in combination with heat), microwave processing, ohmic heating, pulsed electric field processing, ultrasound processing (liquid- and airborne), ultraviolet light (UV) processing, and enhanced continuous separation.
These innovative technologies provide the opportunity not only for the development of new foods but also for improving the safety and quality of conventional foods through milder processing. Different physical phenomena, utilized by these technologies, can potentially reduce energy and water consumption and therefore assist in reducing the carbon and water footprint of food processing, thus playing an important role toward environmental sustainability and global food security.
Apart from the underlying thermo- and fluid-dynamic principles of conventional processing, these innovative technologies incorporate additional Multiphysics dimensions, for example, pressure waves, electric and electromagnetic fields, among others. To date, some of them still lack an adequate, complete understanding of the basic principles of intervening in temperature and flow evolution in product and equipment during processing. Their proper application, development and optimization of suitable equipment and process conditions still require a significant amount of further knowledge. Computational Fluid Dynamics (CFD) is already established as a tool for characterizing, improv- ing, and optimizing traditional food processing technologies. Innovative technologies, however, provide additional complexity and challenges for modelers because of the concurrent interacting Multiphysics phenomena. In order to differentiate Multiphysics modeling from CFD modeling, the word “Multiphysics” will be capitalized throughout the book.
Four symposia were organized at two consecutive Annual Meetings of the Institute of Food Technologists (IFT) in 2008 and 2009 (New Orleans and Anaheim, respectively) to gather Multiphysics modeling experts in innovative technologies to present and discuss the latest advances in their respective fields. These symposia highlighted the importance and key role of Multiphysics modeling to further advance the development of each innovative technology and facilitate their introduction into the food industry.
Written by international experts from world-class research centers, academia, and industry, this book explains and discusses how Multiphysics modeling—that is, the simulation of the entire process comprising the actual equipment, varying process conditions, and the thermophysical properties of the food to be treated—can be applied in the development, optimization, and scale-up of innovative food processing technologies. The most recent research outcomes are shown to demonstrate benefits to process efficiency and the impact on scalability, safety, and quality.
The first part of this book includes two chapters introducing the rationale of the book and some common themes to all chapters. Chapter 1 is the introductory chapter outlining the range of innovative processing technologies covered, briefly describing the technologies and making the case for the necessity of Multiphysics modeling for their design, development, and application. Chapter 2 discusses the importance of determining the relevant (common and technology-specific) thermophysical properties and their essential role for accurate model prediction.
The second part of the book is an extensive collection of chapters devoted to the various case studies on the modeling of innovative food processing technologies. For clarity and convenience, they are divided into subsections focusing on high-pressure processing (Chapters 3–5), technologies utilizing electric and/or electromagnetic effects (microwave, ohmic heating, and pulsed electric field processing; Chapters 6–11), processes using ultrasound waves (in liquids or air) (Chapters 12 and 13), ultraviolet light (UV) processing (Chapters 14 and 15), and finally, one chapter on innovative chromatographic separation technologies (Chapter 16).
Chapter 3 discusses two fundamentally different modeling approaches to characterize high-pressure (low-temperature) systems. It introduces the reader to the very promising modeling technique known as artificial neural networks (ANN), as well as the more generalized visual programming approach referred to as macroscopic modeling. In Chapters 4 and 5, “conventional” CFD modeling approaches for high-pressure processes at both low and high temperatures are discussed and their application for equipment design, scale-up, and optimization are highlighted. Also described is their application to present the process outcomes in terms of safety and quality of the processed foods. Chapter 6 and 7 covers the extension of classical CFD with a further Multiphysics dimension, electromagnetic radiation, and the implementation for designing and characterizing microwave heating processes. Chapter 7 also discusses various temperature mapping techniques and introduces the use of magnetic resonance imaging (MRI) for the determination of microwave-induced three-dimensional heating patterns. In Chapter 8, historical and new developments of Multiphysics modeling applied to ohmic heating are presented. Chapters 9, 10, and 11 are devoted to modeling of pulsed electric fields processing, covering the basics of the technology, its application for predicting liquid food pasteurization, and the “multiobjective” optimization of the technology for liquid food processing. Chapters 12 and 13 present two distinctly different ultrasound applications. Chapter 12 covers liquid-borne ultrasound, including a review on its use in food processing, followed by an extensive review of the mathematics and physics involved in this technology, and this is concluded with a novel approach of modeling ultrasound-induced streaming. Chapter 13 details the use of airborne ultrasound for the improvement of drying processes at low temperatures. The complex mathematics is described and the chapter is concluded by experimental studies, highlighting the advantages and commercial potential of this innovative drying technology. Chapters 14 and 15 both describe UV processing for liquid food disinfection/pasteurization as an effective alternative to thermal treatments. Chapter 14 focuses on the characterization of several alternative reactor designs by Multiphysics modeling, whereas Chapter 15 compares the performance of different commercially available reactors using Multiphysics modeling and the introduction of the concept of “disinfection efficiency.” The final technology chapter (Chapter 16) introduces an innovative continuous separation process based on the chromatographic simulated moving bed principle. It outlines the procedure of modeling these types of technologies and highlights the advantages over conventional column or bed-based separation processes.
Chapter 17 is the take-home message of this book, which concludes with a summary on what was presented in the chapters before and provides an outlook on future trends in Multiphysics simulation of innovative food processing technologies. Three questions are posed: (1) What can be usefully modeled today?; (2) What extra data is needed?; and (3) How much detail is needed, or Where shall we stop? This chapter is not intended to provide definitive answers to these questions, but it suggests some future research directions and places where research ought to or is expected to arrive.
The editors wish to thank all collaborators in this book for their excellent contributions, and the time and effort they have devoted to making this book a comprehensive interdisciplinary reference source for engineers, technologists and scientists, and researchers from academia and industry alike. We believe that the value of this book is not limited to food engineering; it is also useful for other branches of process and chemical engineering. We would also like to thank the Institute of Food Technologist’s Nonthermal Processing Division, the International Division, and the Food Engineering Division for sponsoring the session symposia that led to the development of this book.
Kai Knoerzer
Pablo Juliano
Peter Roupas
Cornelis Versteeg
Contributors
Víctor M. Acosta
Grupo de Ultrasonidos de Potencia
Consejo Superior de Investigaciones Científicas (CSIC)
Serrano, 144, E28006
Madrid, Spain
Abdul Ghani Albaali
Princess Sumaya University for Technology
P.O. Box 1438
Al-Jubaiha 11941
Jordan
Serafim Bakalis
Centre for Formulation Engineering
School of Chemical Engineering
University of Birmingham
Birmingham B15 2TT
United Kingdom
Gustavo V. Barbosa-Cánovas
Department of Biological Systems Engineering
Washington State University
Pullman, WA 99164-6120
Juan Andrés Cárcel
Grupo de Análisis y Simulación de Procesos Agroalimentarios (ASPA)
Departamento de Tecnología de Alimentos
Universidad Politécnica de Valencia
Camí de Vera s/n, E46022, Valencia
Spain
Hao Chen
Department of Biological Systems Engineering
Washington State University
Pullman, WA 99164-6120
(Currently with Microsoft, Redmond, WA)
Antonio Delgado
Institute of Fluid Mechanics
Friedrich-Alexander University Erlangen-Nuremberg
Cauerstrasse 4, D-91058 Erlangen
Germany
Özgür Ertunç
Institute of Fluid Mechanics
Friedrich-Alexander University Erlangen-Nuremberg
Cauerstrasse 4, D-91058 Erlangen
Germany
Larry Forney
School of Chemical and Biomolecular Engineering
Georgia Institute of Technology
311 Ferst Drive, N.W.
Atlanta, GA 30332
Peter J. Fryer
Centre for Formulation Engineering
School of Chemical Engineering
University of Birmingham
Birmingham B15 2TT
United Kingdom
Juan A. Gallego-Juárez
Grupo de Ultrasonidos de Potencia
Consejo Superior de Investigaciones Científicas (CSIC)
Serrano, 144, E28006, Madrid
Spain
José Vicente García-Pérez
Grupo de Análisis y Simulación de Procesos Agroalimentarios (ASPA)
Departamento de Tecnología de Alimentos
Universidad Politécnica de Valencia
Camí de Vera s/n, E46022, Valencia
Spain
Henry Jaeger
Department of Food Biotechnology and Food Process Engineering
Technische Universität Berlin
Koenigin-Luise-Str. 22
D-14195 Berlin
Germany
Filip Janakievski
CSIRO Food and Nutritional Sciences
671 Sneydes Road
Werribee, VIC 3030
Australia
Pablo Juliano
CSIRO Food and Nutritional Sciences
671 Sneydes Road
Werribee, VIC 3030
Australia
Dietrich Knorr
Department of Food Biotechnology and Food Process Engineering
Technische Universität Berlin
Koenigin-Luise-Str. 22
D-14195 Berlin
Germany
Tatiana Koutchma
Guelph Food Research Centre, Agriculture and Agri-Food Canada
93 Stone Road West
Guelph, ON, N1G 5C9
Canada
Jens Krauss
Institute of Fluid Mechanics
Friedrich-Alexander University Erlangen-Nuremberg
Cauerstrasse 4, D-91058 Erlangen
Germany
Kai Knoerzer
CSIRO Food and Nutritional Sciences
671 Sneydes Road
Werribee, VIC 3030
Australia
Nicolás Meneses
Department of Food Biotechnology and Food Process Engineering
Technische Universität Berlin Koenigin-Luise-Str. 22 D-14195 Berlin Germany
Huachen Pan
Institute of Mechatronic Engineering
Hangzhou Dianzi University
310018 Hangzhou
China
Georgina Porras-Parral
Centre for Formulation Engineering
School of Chemical Engineering
University of Birmingham
Birmingham B15 2TT
United Kingdom
Cornelia Rauh
Institute of Fluid Mechanics
Friedrich-Alexander University Erlangen-Nuremberg
Cauerstrasse 4, D-91058 Erlangen
Germany
Marc Regier
Fachhochschule Trier
University for Applied Sciences
Schneidershof, 54293 Trier
Germany
Enrique Fernando Riera Franco de Sarabia
Grupo de Ultrasonidos de Potencia
Consejo Superior de Investigaciones Científicas (CSIC)
Serrano, 144, E28006, Madrid
Spain
Pedro D. Sanz
Malta Consolider Team
Department of Processes
ICTAN, CSIC
c/ José Antonio Novais, 10
28040 Madrid
Spain
Helmar Schubert
Universitaet Karlsruhe (TH)/Karlsruhe Institute of Technology (KIT)
Institute of Engineering in Life Sciences
Dept. I: Food Process Engineering
Karlsruhe, Germany
Juming Tang
Department of Biological Systems Engineering
Washington State University
Pullman, WA 99164-6120
José S. Torrecilla
Department of Chemical Engineering
Universidad Complutense de Madrid
Avenida Complutense s/n
28040 Madrid
Spain
Francisco Javier Trujillo
CSIRO Food and Nutritional Sciences
11 Julius Avenue
North Ryde, NSW 2113
Australia
Cornelis Versteeg
CSIRO Food and Nutritional Sciences
671 Sneydes Road
Werribee, VIC 3030
Australia
Zhengcai Ye
Bechtel Oil, Gas and Chemicals, Inc.
3000 Post Oak Blvd
Houston, TX 77056
Chapter 1
Introduction to Innovative Food Processing Technologies: Background, Advantages, Issues, and Need for Multiphysics Modeling
Gustavo V. Barbosa-Cánovas, Abdul Ghani Albaali, Pablo Juliano, and Kai Knoerzer
1.1. Introduction
In a world that is demanding environmental sustainability and food security, innovation is a key requirement for the sustained growth of the food industry. Furthermore, product innovation is the response to the growing demand for value addition along with more sophisticated and diverse food products. Modern food technology provides a handful of novel processing options to explore, which could provide more diverse food industry products and more competitive and efficient processes. Many of these innovative technologies can provide new opportunities for the development of new foods and for the improvement of safety and quality of more conventionally manufactured foods through milder processing.
This book discusses innovative technologies that take advantage of physical forces and phenomena such as high hydrostatic pressure, electric and electromagnetic fields, and pressure waves, for example, high-pressure processing (also in combination with heat), microwave processing, ohmic heating, pulsed electric field (PEF) processing, ultrasound processing (liquid and airborne), and ultraviolet light (UV) processing. Innovative processing technologies present a number of hurdles that need to be addressed from concept development to implementation. In particular, proper application, development, and optimization of suitable equipment and process conditions require a significant amount of further knowledge and understanding. In this book, the basic principles, current research, challenges, and commercial applications of the respective technologies, as well as the development and application of computational fluid dynamics (CFD) and, more broadly, Multiphysics modeling as a tool for characterizing, improving, and optimizing innovative food processing technologies are covered.
Most innovative processing technologies have a common challenge, that is, to achieve a sufficient uniformity of the treatment or the process. This challenge is often already an issue at laboratory scale and it can become progressively worse when scaling up to pilot plants and, subsequently, to commercial equipment. Among other potential technology-specific issues, nonuniformity of the treatment is most commonly encountered. In fact, the nonuniformities of the process and the lack of process validation of innovative processes are the greatest limitations for industrial uptake.
Nonuniform treatment is, however, not specific to innovative processing technologies; conventional processing technologies often encounter the same problem. For example, in conventional heat treatment processes such as canning, the temperature at the product surface is significantly higher than at the product center during most of the processing time, and only after prolonged holding times are temperature gradients throughout the product diminished. Another clear example of nonuniformity in conventional processing is the drying process of particulates. In this case, spatial and temporal heterogeneities in temperature and water content in the food product can be even more pronounced. The product goes (1) through an initial linear drying phase with water removal from the product surface, (2) over the falling rate period with moisture flux from the inside of the product to its surface, and (3) to a stage of product and drying medium (moisture) equilibrium with almost no further change in water content. In drying food products other important factors often come into play, increasing the degree of nonuniformity: product shrinkage and reduced moisture transport (increasing viscosity of contained liquids) up to a stage where pores are blocked. In the case of many innovative processing technologies as described throughout this book, nonuniformities may be reduced through technology-specific effects. However, these nonuniformities may be more pronounced due to increased complexities influenced by additional Multiphysics phenomena.
This introductory chapter outlines the range of innovative food processing technologies covered in this book and gives a short overview of their benefits and advantages over traditional technologies. Some additional background information on the technologies, not covered in the respective technology-specific chapters, is provided. Furthermore, this chapter makes a case for the need for applying Multiphysics modeling in these technologies for their design, including scale-up and optimization. The chapter summarizes the problems and challenges faced by the modelers, particularly with respect to the prediction of temperature, flow and technology-specific field distributions (e.g., sound intensity and electric or electromagnetic fields), and the extent of microbial or enzymatic inactivation and their distribution in equipment and products.
1.2. Multiphysics Modeling
1.2.1. Definition
Multiphysics modeling is an extension of classical CFD. By definition, CFD is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. The geometry of the modeled scenario, including all components, is discretized into finite cells on which the governing partial differential equations (PDEs), namely the continuity, momentum, and energy conservation equations, are solved. This is detailed in the chapters specific to the respective technologies. Because these are PDEs, they cannot be solved analytically. Numerical techniques, such as finite differences, finite volumes, or finite element methods, must be applied to achieve an approximated solution (Sun 2007).
Multiphysics modeling is based on the same principles as conventional CFD, that is, geometry discretization, and solving the PDEs is performed in a similar manner. However, Multiphysics modeling comprises additional physical phenomena such as electromagnetic waves, electrical fields, and acoustic waves related to the innovative technologies discussed further in this chapter. These phenomena can also be described by physically based PDEs (specific to each innovative technology), which have to be solved simultaneously with the ones from classical CFD. In some cases, the expression of the process outcome based on the attributes of the processed food, that is, the remaining microbial load, enzyme activity, and chemical reaction products, is required. Within Multiphysics modeling, reaction kinetics (i.e., microbial inactivation, quality degradation, chemical reaction, and structural responses) can be coupled with the specific differential equations to provide the spatial distributions of reaction response.
Multiphysics models that concurrently solve the PDEs of classical CFD and the additional technology-specific physical phenomena and the differential equations describing the reaction response require significantly greater computational resources. The increase in affordable computational power in recent years has allowed the simulation of innovative processes.
1.3. Innovative Food Processing Technologies
1.3.1. Background
This section presents a brief description of each technology covered in this book. The major design problems and application limitations of these technologies are highlighted as an introduction to subsequent chapters. Ways in which Multiphysics modeling of innovative food processing technologies can assist in their development will be discussed.
1.3.1.1. High-Pressure Processing (HPP) and High-Pressure Thermal Sterilization (HPTS)
HPP has demonstrated wide applicability for producing high-quality foods. HPP has become accepted as an attractive alternative to traditional preservation methods utilizing preservatives or thermal processing (Hernando Saiz et al. 2008, Chapters 3–5).
HPP is commonly referred to as a nonthermal process of liquid and solid foods through application of high pressure in the order of 100–800 MPa (1,000 to 8,000 bar) and holding times of several minutes. HPP of foods is of increasing interest because it allows the inactivation of vegetative organisms at low or moderate temperature with minimum degradation (Abdul Ghani and Farid 2007). HPP offers opportunities for increased shelf life and preservative-free stabilization of meats, seafood, vegetable products, and juices. HPP can be used not only for preservation, but also for modifying the physical and functional properties of some foods.
More than 70 companies currently utilize HPP, producing more than 170,000 tons of products (Tonello 2010). Several HPP-treated food products, including juices, jams, jellies, yogurts, ready-to-eat meat, and oysters, are already widely available in the United States, Europe, Japan, New Zealand, and Australia. These successful applications have led to a pronounced increase in commercial-scale HPP units around the world during the past 10 years, as shown in Figure 1.1.
Figure 1.1. Number of commercial high-pressure equipment units around the world as of 2009 (Tonello 2010).
In addition to inactivation of microorganisms and some spoilage enzymes (Seyderhelm et al. 1996; Yen and Lin 1996), promising results have been obtained with respect to the application on gelation of food proteins (Ohshima et al. 1993), improvement of digestibility of proteins, and tenderization of meat products (Ohmori et al. 1991; Jung et al. 2000a, 2000b; Buckow et al. 2010b). These changes in proteins have been used successfully in fish meat; in Carpaccio and Carpaccio-like products, high pressure allows the “processing” of the product, while still maintaining its raw characteristics. However, because of the application of high pressures, these products have retained “fresh-like” qualities and texture compared with heat-processed food, are microbiologically safe, and have an extended shelf life compared with raw food. Gomez-Estaca et al. (2009) investigated HPP on fish products (such as salmon, tuna, and cod), showing superior sensory results.
If the aim of the process is the inactivation of microbial spores, high pressure alone is not sufficient. However, a combination of high pressure and elevated temperatures, also referred to as HPTS or pressure-assisted thermal sterilization, can result in synergistic inactivation of these spores at potentially lower temperatures or shorter processing times, thus improving the quality of the processed foods while potentially reducing energy consumption (Bull et al. 2009). In this application, the increase in pressure is used as a means to increase the temperature evenly and fast in the product.
There are two approaches to achieve high-pressure conditions. In the direct approach, a piston is utilized, which compresses the content of the high-pressure chamber. In the indirect approach, a pressure-transmitting liquid (e.g., water) is pumped into the treatment chamber (high-pressure vessel) using a high-pressure pump followed by a “pressure intensifier.” Liquids at extremely high pressures are compressible, requiring extra fluid to be pumped into the vessel.
During compression, the temperature of the processed food and the pressure-transmitting fluid increases due to the compression force working against intermolecular forces. The magnitude of the adiabatic temperature increase depends on a number of factors, such as the pressure medium and food product thermophysical properties (density, thermal expansion coefficient, and specific heat capacity) and initial temperature (see, e.g., Chapters 2, 4, and 5).
Higher fat content of the food and higher initial temperature, for example, lead to an increase in compression heating. The phenomenon of increasing compression heating at elevated initial temperatures is important; for example, in HPTS, the product and the pressure medium are preheated to achieve higher process temperatures, which in turn allows inactivation of microbial spores (Wilson et al. 2008).
In HPP, the greater the pressure level and time of application, the greater the potential for changes in the structure and appearance of the treated foods. This is especially true for raw high-protein foods, where pressure-induced protein denaturation may be visually evident. High pressures can also induce significant structural changes (or damages) in some sensitive foods, such as strawberries or lettuce. Cell deformation and cell membrane damage can result in softening and cell serum loss. Usually, these changes are undesirable because the food will appear to be processed and no longer fresh or raw.
Limitations of HPP and HPTS
Although great progress has been made in the development of economically viable high-pressure applications, the scientific community and the food industry recognized in the early 2000s that engineering fundamentals, including CFD models, were required to design, evaluate, optimize, and scale up high-pressure processes of foods (Hendrickx and Knorr 2001).
The limitation of HPP to date mainly lies in the limited throughput and, relative to heat processing, the high cost of equipment, labor (HPP is not yet a fully automated process), and maintenance. High maintenance costs are caused mainly by the extreme processing conditions. Furthermore, there are only a few large-scale commercial high-pressure equipment suppliers worldwide that have expertise in the food industry, including Avure Technologies, Inc. (Kent, WA), Kobelco (Kobe Steel Ltd., Kobe, Japan), and NC Hyperbaric (Burgos, Spain).
A common issue in both HPP and HPTS is the nonuniformity of some aspects of the treatment. HPP generates pressure waves in liquids, which travel at the speed of sound (sound in water travels at 1,500 m/s). Therefore, pressure is commonly assumed to be transmitted instantaneously and uniformly. However, treatment nonuniformities can occur during HPP not only as a result of different compressibilities of the various substances in the food product, including trapped air (also headspace), but also because of the food packaging material. In addition, if the purpose of the process is the inactivation of the vegetative microorganisms, a nonuniform treatment can occur because some microorganisms are supposedly more resistant to the pressure when embedded in a fat matrix. Foods with higher fat or oil content may, therefore, protect the microorganisms in some areas in the food where fat is contained.
In the case of processing above room temperature (initial temperature), for example, in HPTS, nonuniform treatment temperature is likely to be more pronounced. In addition to pressure, temperature is an important process variable. In heterogeneous food materials, with the contents exhibiting differences in compression heating, temperatures may not be uniformly distributed in the food products. Furthermore, the packaging material, the material of the product carrier, and the steel of the high-pressure vessel are not heated to the same extent as the food; therefore, temperature gradients are developed throughout the system, leading to heat flux from the products to the cooler areas (which are mainly the steel walls). These spatial temperature heterogeneities increase over the process time. Although, theoretically, the preheated product heats up uniformly during compression to sterilization temperatures, during pressure holding time temperatures may decrease in certain areas of the vessel. This can affect spore inactivation, and spores may survive the process if temperature loss is not prevented. Product carriers have been developed as a means of retaining heat throughout the vessel during both pressure come-up and holding times (Chapter 5). Multiphysics modeling can greatly assist in the characterization of temperature distribution, subsequent microbial distributions, and other quality changes as a result of temperature inhomogeneities. These models can also be applied to the redesign and optimization of equipment and determination of adequate processing conditions for optimum process/product performance.
1.3.1.2. Microwave and Radio Frequency Processing
Microwave heating refers to the use of electromagnetic waves of certain frequencies to generate heat in a material (Metaxas and Meredith 1983; Roussy and Pearce 1995; Metaxas 1996). Typically, microwave food processing uses frequencies of 2,450 and 915 MHz. In domestic ovens, 2,450 MHz frequency is commonly utilized, while in industrial heating application both frequencies are used, depending on the product to be treated, that is, product size and composition, associated with the relevant thermophysical properties (Chapters 2, 6, and 7).
Microwave heating has been proposed as an alternative to traditional heating methods in many food manufacturing processes, such as (re)heating, baking, (pre)cooking, tempering of frozen food, blanching, pasteurization, sterilization, and dehydration (Metaxas and Meredith 1983; Decareau 1985; Buffler 1993; Metaxas 1996; Schubert and Regier 2005; Tang et al. 2008).
Microwave and radio frequency heating for pasteurization and sterilization are rapid; therefore, less time is required for come-up to the desired process temperature compared with conventional heating. This is particularly true for solid and semisolid foods that depend on slow thermal diffusion process in conventional heating. Microwave and radio frequency heating can approach the benefits of high-temperature short-time (HTST) processing, whereby bacterial destruction is achieved, while thermal degradation of the desired components is reduced.
Heating with microwaves primarily involves two mechanisms. Water in the food is often the main component responsible for dielectric heating. Due to their dipolar nature, water molecules follow the alternating electric field associated with electromagnetic radiation. The second major mechanism is through the oscillatory migration of ions in the food under the influence of the alternating electric field. Such oscillatory motion of water molecules and ions and the associated intermolecular friction lead to a conversion of electromagnetic energy to thermal energy.
The dielectric properties, namely the dielectric constant and the loss factor (Chapter 2), determine the strength of the electric field inside the food and its conversion into heat. These properties strongly depend on the composition (or formulation) of the food, with moisture and salt being the two primary determinants of interest (Mudgett 1985, 1986; Sun et al. 1995; Nelson and Datta 2001). The subsequent temperature rise in the food depends on the duration of heating, the location in the food, convective heat transfer at the surface, and the heat conduction and extent of evaporation of water inside the food and at its surface.
Although the final objective of each process differs, an increase in product temperature is seen as a common theme. There has also been some speculation on the so-called nonthermal effects of electromagnetic waves in the microwave frequency range. Four theories have been proposed to explain “nonthermal” or nondirect thermal effects of microwaves on, for example, microorganisms: selective heating, electroporation, cell membrane rupture, and magnetic field coupling (Kozempel et al. 1998). The selective heating theory states that solid microorganisms are heated more effectively by microwaves than the surrounding medium and are thus killed more readily. Electroporation is caused when pores form in the membrane of the microorganisms due to electrical potential across the membrane, resulting in leakage (this is similar to one of the theories on the effect of PEF processing for cold pasteurization). Cell membrane rupture is related to the voltage drop across the membrane, which causes it to rupture, which is also a theory in PEF processing. In the fourth theory, cell lysis occurs due to coupling of electromagnetic energy with critical molecules within the cells, disrupting vitally important internal cell components.
Although researchers have repeatedly reported nonthermal effects of microwave processing, the general consensus (Heddleson and Doores 1994; Heddleson et al. 1994) is that the reported nonthermal effects are likely to be due to the lack of precise measurements of the time–temperature history and its spatial variations. A number of studies have shown that thermal effect is the essential contributor to the destruction of microorganisms (Goldblit and Wang 1967; Rosen 1972; Fujikawa et al. 1992). Therefore, to date, it is presumed that only thermal effects on microbial inactivation are effective, and microbial inactivation caused by microwave processing is essentially the same as in conventional thermal processing. Of course, the rates of heating and temperature distributions are quite different.
Limitations of Electromagnetic Heating
Volumetric microwave and radio frequency heating is theoretically more uniform than conventional heating (Datta and Hu 1992). There are, however, a number of microwave-specific factors that induce nonuniform heating patterns. First, electromagnetic field distribution inside a microwave cavity is, in most cases, not uniform. Placing dielectrics (i.e., food products) into the microwave field leads to a change in the field distribution. Therefore, differences in the products, for example, product size, shape, and particularly composition with varying dielectric properties, will almost certainly lead to changes in process outcomes. However, not only do the field variations in the cavity cause nonuniform processing, the field characteristics inside the product are also heterogeneous.
The heterogeneous composition of the different food components (and different dielectric properties) is an important factor in the heating of foods. Differences in dielectric properties lead to differences in temperature increases, even in a perfectly homogeneous microwave field. As these properties are in most cases strongly temperature-dependent, changes in temperature may compensate or may increase the nonuniformity. In particular, in cases where increasing temperatures lead to increasing loss factors (the imaginary part of the complex dielectric permittivity; Chapter 2), a so-called thermal “runaway” phenomenon can occur. With increasing temperature the rate of converting the electromagnetic energy into thermal energy increases as well; therefore, the gradients between hot and cold areas in the product become more pronounced.
Another important factor in heating is the so-called focusing effect of the microwaves into specific areas in the product. This phenomenon is strongly dependent on the geometrical properties of the product. For example, a spherical product that does not exceed a certain size (due to limited penetration) can exhibit a pronounced hot spot in its geometrical center.
Other phenomena causing uneven heating patterns include edge and corner overheating (caused by the penetration and absorption of the microwaves from more than one direction) and the development of standing waves inside the product (which is mainly dependent on the dielectric constant (the real part of the complex dielectric permittivity; Chapter 2).
The time–temperature history at the coldest point for a conventional thermal process is generally predictable for a food that is all solid or all fluid. For example, for a conduction-heated (solid) food, it is usually the geometric center. In microwave heating, even for a solid food, it is less straightforward to predict the coldest point and it can change during the heating process depending on temperature-dependent material properties and oven characteristics (Fleischman 1996; Zhang et al. 2001).
A number of approaches have been proposed to improve the uniformity associated with microwave heating. These include rotating and oscillating the food in the microwave cavity (Geedipalli et al. 2007), providing an absorbing medium (such as hot water) surrounding the product (Chen et al. 2008; Chapter 6), equilibrating after heating (Fakhouri and Ramaswamy 1993), and cycling the power (Chapter 7). Success to date is limited due to the dependence of the materials’ properties on temperature and the nonuniform distribution of the electromagnetic field inside the food and the microwave cavity. Utilizing a lower microwave frequency of 915 MHz and radio frequencies to improve uniformity of heating have the potential to improve the evenness of heating (Chen et al. 2008), as the penetration depth into the food is greater and the field nonuniformities are less pronounced. Combinations of microwave and conventional technologies in many different configurations (e.g., hot air, vacuum, or infrared heating) have also been used to improve treatment uniformity; (Contreras et al. 2008; Turabi et al. 2008; Abbasi and Azari 2009; Kowalski and Mierzwa 2009; Kowalski and Rajewska 2009; Seyhun et al. 2009; Uysal et al. 2009). These approaches can be successful for some applications, especially where the cold spot is located at the food surface (Chapter 7); however, in food products with high salt or sugar content, the cold spot is usually within in the food, as the penetration depth of the electromagnetic waves is reduced.
It remains a challenge to uniformly treat food products with microwaves and to achieve the targeted process outcomes; Multiphysics models, however, will greatly assist in designing microwave processes by evaluating process performance and developing appropriate control strategies (Chapters 6 and 7). Accordingly, Multiphysics models (including temperature-dependent properties of foods) need to be developed and subsequently validated to ascertain the location of the point of lowest integrated time–temperature history (Chapter 7).
1.3.1.3. Ohmic Heating
Ohmic heating is defined as a process wherein electric currents are passed through foods or other materials with the primary purpose of heating them. The heating occurs in the form of internal electric energy dissipation within the material. Ohmic heating is distinguished from other electrical heating methods by the presence of electrodes contacting the food, the frequency of the current, or the waveform. The main purpose for the development of ohmic heating processes was to allow for HTST sterilization of solid–liquid mixtures (Chapter 8).
Applications of ohmic heating in the food industry to date are scarce, although there are a number of advantages over other (conventional) heating methods. The main advantages for ohmic heating are the associated rapid and relatively uniform heating of the food product, depending on the electrical conductivity of the food components. This is expected to reduce unwanted thermal effects on the product that often occur in conventional heating applications, caused by the need to heat the product by the transfer of thermal energy from a heating medium to a low temperature product, where excessive treatment times are necessary for sufficient heat penetration from the surface of a solid product to its core.
Potential applications for ohmic heating include its use in blanching, evaporation, dehydration, fermentation, and extraction. At present, the primary type of application is a heat treatment for microbial control, for example, for the pasteurization of milk, and also for processing of sauces, fruits, and tomatoes (Chapter 8).
The principal mechanisms of microbial inactivation in ohmic heating are thermal in nature. Recent literature, however, indicates that a mild electroporation mechanism may occur during ohmic heating (similar to the effects utilized in PEF processing (Lebovka et al. 2005; Kulshrestha and Sastry 2006). The principal reason for the additional microbial inactivation effect to heating of ohmic treatment may be its low frequency (50–60 Hz), which allows cell walls to build up charges and form pores. This is in contrast to high-frequency methods such as microwave or radio frequency heating, where the electric field is essentially reversed before sufficient charge buildup occurs at the cell walls.
Nevertheless, temperature is the principal critical process factor in ohmic heating. As in conventional thermal processes, the key issue is identifying the slowest heating zone. Fundamentally, there is only one critical factor: the temperature–time history of the coldest point. Since the primary critical process factor is the thermal history and location of the cold spot, the effects on microbial inactivation are the same as for thermal processes. Locating the slowest heating zones during ohmic heating, however, cannot be extrapolated from current knowledge of conventional heating, and requires special consideration.
Several factors significantly affect the temperature within an ohmic process. The critical parameters in continuous flow ohmic heating systems include electrical conductivities of the respective phases of the food, temperature dependence of the electrical conductivity, design of the heating device (e.g., location and orientation of the electrodes), extent of interstitial fluid motion, residence time distribution, thermal properties of the food, and electric field strength (Chapter 8).
Limitations of Ohmic Heating
The main limitation of ohmic heating is the heterogeneous nature (in composition) of the food products and their corresponding electrical conductivities that leads to differences in the conversion of the electrical current into thermal energy. As in microwave heating, in ohmic heating, thermal runaway can also occur, because electrical conductivity, which is the property that influences electrical energy dissipation, usually increases with increasing temperature. Therefore, especially in stationary (i.e., not moving in a stream) solid products, there may be areas that are very hot (usually areas close to the electrodes), which in some instances may even be burned, while in other areas (with initially lower electrical conductivities, or farther away from the electrodes) almost no heating occurs.
Uniform heating with ohmic processing is theoretically possible, but at the same time challenging due to the various factors impacting on the slowest heating zone and the time–temperature history throughout the product. Multiphysics modeling (including the temperature-dependent properties of the foods: mainly the electrical conductivity) can greatly assist the evaluation and optimization of ohmic heating systems to achieve heating uniformity (Chapter 8).
1.3.1.4. PEF
PEF processing is an innovative nonthermal processing technology mainly for liquid and pumpable foods (including emulsions, suspensions, and semisolids such as sausage meat), predominantly used for the inactivation of microorganisms at ambient or mild temperatures, thereby preserving the fresh flavor, color, functional properties, and integrity of heat-sensitive compounds (Chapters 9–11). PEF can also be used to enhance extraction yield of juices and bioactives from plant sources. PEF is one of the most appealing nonthermal technologies for preservation of liquid foods due to reduced heating effects compared with traditional pasteurization methods (Barbosa-Cánovas et al. 1999).
In PEF processing, a liquid or other pumpable material is passed through an electrode arrangement where the PEF is applied. For microbial inactivation, foods are processed by means of brief pulses of a strong electric field with field strengths of around 15–40 kV/cm. For extraction of plant materials and pretreatment of meat for processing, only about 0.7 to 3 kV/cm is required (Toepfl et al. 2006). The utilization of PEF leads to the formation of pores (the so-called electroporation [temporary or permanent]), in the membranes of microbial or plant cells, which disturbs and damages the membrane’s functionality, leading to inactivation of the cells and the partial release of the cell contents to make extraction or other processing more efficient.
Membrane disruption occurs when the induced membrane potential exceeds a critical value of 1 V in many cellular systems, which, for example, corresponds to an external electric field of about 10 kV/cm for Escherichia coli (Castro et al. 1993). The most relevant factor affecting microbial inactivation and extraction enhancement by PEF is, therefore, the electric field intensity. The combination of electric field intensity, total treatment time during PEF and pulse shapes, and the associated temperature increase determine the extent of membrane disruption in bacterial and plant cells (Hamilton and Sale 1967). Other factors affecting the performance of the PEF process include the microbial entity to be inactivated (type, concentration, and growth stage of microorganism) and the treatment media (pH, antimicrobials, and ionic compounds, electric conductivity, and medium ionic strength).
PEF produces products with slightly different properties from conventional pasteurization treatments. Most enzymes are not affected by PEF. The fact that the maximum temperature reached is lower than in thermal pasteurization means that some of the flavors associated with the raw material are not destroyed. Spores, with their tough protective coats, and dehydrated cells are mostly able to survive PEF processing. The survival of spores and enzymes means that products have to be refrigerated after passing through PEF processing in order to slow the action of the enzymes and keep pathogens from growing; PEF alone is generally not capable of producing ambient shelf-stable products. However, acidic well-packaged products may have a useful ambient shelf life.
As indicated before, another potential application of PEF, which is gaining increasing interest, is the utilization of the technology for enhanced extraction of plant cell material. Because PEF induces electroporation in cell walls at relatively low energy inputs, allowing the cell contents to leak out, it holds promise as an efficient way of getting useful components out of cells and cell membranes (Corrales et al. 2008; Lopez et al. 2009a, 2009b; Loginova et al. 2010; Puertolas et al. 2010).
To date, however, PEF has been mainly researched to preserve the quality of foods, such as to improve the shelf life of orange juice, apple juice, milk, and liquid eggs, as well as the fermentation properties of brewer’s yeast. Martín-Belloso and Soliva-Fortuny (2010) have summarized the work of several researchers on food-borne pathogenic microorganisms in different food products.
Limitations of PEF Processing
Issues that may arise with PEF include electric arcing, dielectric breakdown of the treated food, and a pronounced temperature increase (caused by ohmic heating). Several factors play a role here, including the material’s electrical conductivity, the frequency of the pulses, their duration (width), adequacy of deaeration, back pressure, and the flow rate of the liquid (laminar or turbulent flow regime; residence time in the treatment chamber). Because the pulse duration is only in the range of microseconds and, therefore, the overall treatment time is short, temperature increases during treatment are often assumed to be minimal and temperature effects neglected in inactivation studies.
In processing liquids with PEF, a nonuniformity of the treatment can be a result of the interaction between the flow, heat transfer, electric field phenomena, and effects on microbial or plant cells. Predictions of the increase in temperature caused by the electric field are similar to ohmic heating and less complicated compared with the dissipation of electromagnetic energy in microwave processing. Moreover, the property influencing this dissipation effect, that is, the electrical conductivity, is easier to measure, and usually shows a less complex behavior with temperature than the two dielectric properties in microwave processing, that is, the dielectric constant and the loss factor (Chapter 2).
However, the purpose of the pulsed (potentially alternating) electric field is, unlike in microwave processing, not an increase in temperature. The temperature increase should be minimized in most PEF applications. The main aim is a nonthermal inactivation of vegetative microorganisms for cold pasteurization or a nonmechanical means of opening cells for enhanced extraction. In particular, for the purpose of cold pasteurization, a great degree of electric field uniformity is needed to ensure a similar treatment of the entire liquid product. Ideally, the same number of electric pulses and electric field strength is applied to all microorganisms present in the liquid. Typically, pasteurization requires inactivation of up to 99.999%, that is, 5 log of the target organism. If only a small fraction of microorganisms bypass proper treatment through regions of low electric field strength, it is not possible to reach the required extent of inactivation.
Achieving this uniformity, however, is very challenging; the electric field distribution is strongly dependent on the configuration of the treatment chamber (and to a lesser extent on the electrical conductivity and other thermophysical properties of the processed media). PEF chamber designs such as co-field, coaxial, or colinear electrode arrangements (Chapters 9–11) exhibit pronounced nonuniformities in flow, temperature, and electric field distributions. Uniform fields can be achieved in parallel plate configurations, which are mainly applied for batch processing. If the field is not uniform, the induced temperature increase is also uneven across the volume of the treatment chamber. Often, several treatment cells are arranged to process in series, which reduces the effects of imperfections in single treatment cells.
Thus, in processes for inactivation of specific microorganisms that show synergistic effects of temperature and electric field on inactivation, temperature nonuniformities will lower the performance of the process. Nonuniformities can be minimized, but to some extent will always occur.
To enable a comparable treatment history of the entire product, the flow pattern is very important. Laminar flow conditions, which can be found in low-throughput laboratory-scale systems, are to be avoided. In laminar flow, each microorganism follows a more or less straight path through the treatment chamber; therefore, pronounced differences in exposure to varying electric field strengths and temperatures will occur. Modifying the treatment chamber with grids (Chapter 10) or increasing the flow rate to give turbulent flow (Buckow et al. 2010a) can improve the uniformity of exposure of the product to the important treatment variables (e.g., temperature and electric field strength) and furthermore improve temperature uniformity due to increased (turbulent) thermal conduction and convective flows.
For characterizing process performance, information on the field distributions is essential. However, such local information inside the chambers is difficult and near to impossible to obtain experimentally. For further development of the PEF technology, numerical simulations can be applied to improve the fundamental understanding of the physical phenomena in the process and to optimize it with respect to the chamber design and operating conditions (Gerlach et al. 2008; Chapters 9–11).
1.3.1.5. Ultrasound Processing
This technology is based on pressure waves at frequencies exceeding 20 kHz, that is, more than 20,000 vibrations per second. It is considered as another innovative process that has been investigated for many different purposes over the last decades. While in the earlier work mainly the lower frequencies of around 20 kHz were studied, research and applications currently include frequencies of several hundred kHz, to several MHz (Chapter 12).
Ultrasound systems consist of a generator for turning electrical energy into high-frequency alternating current, a transducer for converting the alternating current into mechanical vibrations, and a delivery probe for conveying the sonic vibrations into a medium to couple sonic vibrations to the treated material. The transducers may take the shape of a rod, plate, bar, or sphere, and are usually manufactured from titanium, aluminum, or steel.
The ultrasonic transducer can be mounted outside on the wall of a vessel or flow cell and be in indirect contact with foods, or it can be inserted into a treatment chamber or flow cell of specified geometry to transmit energy directly into a food system with better energy efficiency (Feng and Yang 2005). There are also transducers that are designed for effective transmission into air (Chapter 13).
Ultrasound has attracted considerable interest in the food industry due to its useful effects in food structure modification (e.g., emulsification, extraction, crystallization, and viscosity alteration), food preservation, and enzyme modulation (Patist and Bates 2008). As one of the innovative and advanced food processing technologies, it can be applied to develop gentle but targeted processes to improve the quality and safety of processed foods and, thus, offers the potential for improving existing processes as well as for developing new process options.
Ultrasound alone has some effects on the inactivation of vegetative organisms in liquid food products. The bactericidal effect of ultrasound is generally attributed to intracellular cavitation (Hughes and Nyborg 1962). It is proposed that micro-mechanical shocks and jet streaming are created by microscopic cavitation bubbles induced by the fluctuating pressures under the ultrasonication process (Chapter 12). These shocks and microjets disrupt cellular structural and functional components up to the point of cell lysis.
