P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come 38 Thermal Processing Principles Yetenayet Bekele Tola and Hosahalli S Ramaswamy Introduction Thermal Processing Basics Introduction Reaction Rate Zero-Order Reaction First-Order Reaction Second-Order Reaction Common Food Microorganisms Thermal Resistance of Food Microorganisms Kinetics of Microbial Death Decimal Reduction Time and Thermal Death Time Temperature Dependency of Kinetic Parameters Concepts of Process Lethality Thermal Process Determination Methods General Method Original General Method Improved General Method Formula Methods Characterization of Heat Penetration Data Heat Penetration Parameters The Retort CUT, f h and j h Values Stumbo’s Method Quality Optimization High-Temperature Short-Time and Ultra-High Temperature Processing Agitation Processing Aseptic Processing Thin Profile and Retort Pouch Processing Novel Thermal Food Processing Technologies Microwave and Radio Frequency Heating Ohmic Heating Novel Nonthermal Processing Technologies High-Pressure Processing Pulsed Electric Field Retort Types for Commercial Application Batch Retorts Steam Heating Retort Water Heating Retort (Immersion and Spray Modes) Steam-Air Heating Retort Continuous Retorts References Abstract: Food intended for human consumption must be produced in safe and stable forms to ensure availability, distribution, as well as normal growth and development Thus, foods are processed in various forms to achieve these desired effects Thermal processing entails the application of heat energy for food transformation into the desired safe and stable forms This chapter describes the basic principles of thermal processing and surveys conventional versus novel thermal processing methods currently available for food transformation INTRODUCTION Food processing is used to transform and/or preserve raw ingredients from the farm into various food forms for consumption by human being Food processing often takes clean, harvested produce or edible portions and uses them to produce attractive and marketable food products Fresh agricultural products (plant and animal origin), due to their high moisture content, are highly perishable and can be kept only for a short period of time Especially during peak harvesting seasons, due to large volume of these products, unless and otherwise pertinent measures are taken, the problem leads to spoilage and high economic losses Furthermore, food products should be available throughout the year in market to fulfill the demand of consumers in all seasons To this, we need to minimize postharvest losses and preserve agricultural products in a safe way with no or minimum quality loss The major emphasis of food processing is shelf life extension by preventing undesirable changes in the wholesomeness, nutritive value, and sensory qualities (Ramaswamy and Marcotte 2006) Food Biochemistry and Food Processing, Second Edition Edited by Benjamin K Simpson, Leo M.L Nollet, Fidel Toldr´a, Soottawat Benjakul, Gopinadhan Paliyath and Y.H Hui C 2012 John Wiley & Sons, Inc Published 2012 by John Wiley & Sons, Inc 725 P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm 726 Printer Name: Yet to Come Part 7: Food Processing Various food processing operations make the food products more attractive, satisfying, safer, and easier to eat Common food-processing techniques include pasteurization, sterilization, cooking, drying, cooling and freezing, fermentation, addition of preservatives and reduction of water activity, and use of two or more of the above techniques to inhibit or stop chemical, biochemical, and microbiological activities Most foods available in the market are subjected to some form of thermal processing Although the canning process started from Nicholas Appert’s time in 1809, food processing via the canning process still provides a universal and economic method for preserving and processing foods Thermal processing 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 two very common industrial conventional thermal food processing technologies in production of canned products are pasteurization and sterilization Pasteurization is the process of heating liquids and/or solid foods for the purpose of destroying principal pathogens capable of growing under aerobic conditions and reducing the level of spoilage vegetative bacteria, protozoa, and fungi from high acidic foods (pH < 4.5) When pH > 4.5, foods produced by this process should be stored at low temperature to avoid further growth spore forming microorganisms However, commercial sterilization refers to an intensive heat treatment process that effectively kills or eliminates all pathogens and vegetative microorganisms as well as bulk of spore-forming bacteria from the low acid foods (pH ≥ 4.5) Products produced through this method can be shelf stable for up to two years In order to reduce the process severity, the thermophilic spore formers are not targeted to be completely eliminated, instead the canned product is advised to be stored at temperatures below 30◦ C to prevent the growth of thermophiles Because of the high safety implications and severe processing conditions, the conventional thermal sterilization has been accepted to result in considerable product quality degradation However, many improvements have been implemented to improve thermal processing operations and techniques, and novel processing approaches have been introduced in recent years to improve the quality of thermally processed foods without compromising safety of these products Therefore, the main focus of this chapter is to describe the basic principles of thermal processing operation in terms of production of safe and better quality products THERMAL PROCESSING BASICS Introduction The major objective of thermal processing is production of safe and stable products that consumers are willing and able to buy To achieve this goal, it is necessary to understand the scientific basis on which the process is established Silva et al (1992) indicated that commercial thermal processing is a function of several factors, such as product thermo-physical properties (product heating rate), surface heat transfer coefficient, initial food temperature, retort temperature, heating medium (hot water or steam), heating medium come-up time (CUT), temperature resistance of food microorganisms and quality factors, and target degree of lethality or safety level we need to achieve The success of thermal processing does not depend on the elimination of the entire microbial population, because this would result in low product quality due to the long heating required Instead, all pathogenic and most spoilage-causing microorganisms in a hermetically sealed container are destroyed, bulk of the spore formers is killed, and an environment is created inside the container that does not support the growth of remaining spore formers There are different mechanisms that enable one to control the germination and growth of such type of spores in canned foods These are based on the microbial growth and inactivation with respect to oxygen requirement, pH preference, and temperature sensitivity In general, canned foods have a 200-year history and are likely to remain popular in the foreseeable future owing to their convenience, long shelf life, and low cost of production The technology is receiving increasing attention from thermal-processing specialists to improve both the economy and quality of some canned foods (Durance 1997) However, the sterilization process not only extends the shelf life of the food, but also affects its nutritional and sensorial qualities Process optimization is therefore necessary in order to promote better quality retention without sacrificing safety Reaction Rate During thermal processing of foods, several types of chemical reactions occur Some reactions result in a quality loss and such type of reactions must be minimized, whereas others result in the development and formation of a desirable flavor, taste, or color, and these ones must be optimized to obtain the best product quality (Toledo 2007) In order to maintain quality of food products through optimization of processing conditions, predictive mathematical models are very important To realize this goal, information is needed on the rates of destruction of microbes as well as quality parameters and their dependence on variables such as temperature, pH, light, oxygen, and moisture content, which can be expressed by mathematical models A better understanding of kinetics of food products can provide better opportunities for developing food processes to maximize quality parameters and ensuring safety Each reaction undergoes on its own rate and the rate of reaction is described by the reaction kinetics Kinetics is the study of the rate at which compounds react Reaction kinetics (rate theory) deals to a large extent with the factors that influence the reaction velocity The rate depends on several factors, including the contact between the reacting components, their concentration, temperature, and pressure at which the reaction takes place The “collision theory” implies that the molecules need to collide with each other in order for the reaction to take place If there are a higher number of collisions in a system, there is a greater chance for the reaction to occur The reaction will go faster, and the rate of the reaction will be higher In collision theory, two main things are to be considered: the activation energy, which P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come 727 38 Thermal Processing Principles is the minimum energy required to initiate the collision, and the statistical probability for collisions between certain molecules that possess an adequate energy level for the reaction to occur at a given temperature To measure a reaction rate, it is necessary to monitor the concentration of one of the reactants or products as a function of time Therefore, the rate of reaction can be expressed as a rate of change in concentration to change in time On the basis of this concept, the rate law is an expression relating the rate of a reaction to the concentrations of the chemical species present, which may include reactants, products, and catalysts Many food reactions follow a simple rate law, which takes the form r = k[A]a [B]b [C]c Zero-Order Reaction A zero-order reaction is independent of the concentration of the reactants A higher concentration of reactants will not speed up the zero-order reaction This means that the rate of the reaction is equal to the rate constant, k, of that reaction Zero-order reaction is described as dA Rate = r = − = k[A]0 (2) dt After separating variables and integrating both sides of Equation t dA = − A0 kdt (3) t0 This provides the integrated form of the rate law [A] = [AO ] − kt (4) where k is the rate constant of the reaction, A is concentration at time t In a zero-order reaction, when concentration data [A] is plotted versus time (t), the result is a straight line First-Order Reaction A first-order reaction is one where the rate depends on the concentration of the species to the first power Most of the reactions involved in the processing of foods are of first-order reactions For a general unimolecular reaction, the decrease in the concentration A over time t can be written as dA Rate = r = − (5) = k[A]1 dt Rearranging the equation − dA = kdt A A A0 dA =− A t0 kdt (7) t0 and the linear for of Equation is: ln[A] = ln[A0 ] − kt (8) where [A] is the concentration at time t, [A0 ] is the concentration at time t = 0, and k is reaction rate constant (s−1 ) Plotting ln [A] with respect to time (t) for a first-order reaction gives a straight line with the slope of the line equal to −k, where the rate constant is calculated (1) that is, the rate (r) is proportional to the concentrations of the reactants (A, B, C) each raised to some power The constant of proportionality, k, is called the rate constant The power a particular concentration is raised is the order of the reaction with respect to that reactant Note that the orders not have to be integers The sum of the powers in Equation is called the overall reaction order A Integrate both sides of the equation (6) Second-Order Reaction A second-order reaction is one where the rate depends on the concentration of the species to the second power The reaction rate expression for a unimolecular second-order reaction is dA (9) = −k[A]2 dt Separation of variables and integration of both sides of equations will give us A A0 dA = −kdt [A]2 t A =− kt [A] t0 1 = + kt [A] [A0 ] (10) (11) (12) Second-order unimolecular reaction is characterized by a hyperbolic relationship between concentration of the reactant or product and time A linear plot will be obtained if 1/A is plotted against time Second-order bimolecular reactions may also follow the following rate equation: A+B→P dA Rate = = −k[A][B] (13) dt where A and B are the reactants After separating variables, the differential equation may be integrated by holding B constant to give dA = −k[B]dt [A] A t0 dA =− k dt A0 A t0 1 = −kt [A] [A0 ] (14) (15) (16) k is a pseudo-first-order rate constant: k = kB A second-order bimolecular reaction will yield a similar plot of the concentration of the reactant against time as a first-order unimolecular reaction, but the reaction rate constant will vary with different concentrations of the second reactant (Table 38.1) P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm 728 Printer Name: Yet to Come Part 7: Food Processing Table 38.1 Summary of Zero-, First-, and Second-Order Reaction Rates (M -molar Concentration) Rate Law/Order Zero First Second Differential Form Integral Form Linear Plot to Determine k dA = −k dt dA = −k[A]1 dt dA = −k[A]2 dt [A] = [Ao ] − kt [A] vs t ln[A] = ln[Ao ] − kt ln[A] vs t 1 = + kt [A] [Ao ] vs t [A] Common Food Microorganisms Foods by their very nature involve complex biological molecules that strongly support the survival and growth of food microorganisms One of the major causes for food deterioration is the growth and activity of microorganisms Microorganisms can contaminate the food before and after processing from various sources The major categories of microorganisms involved in food spoilage and deterioration are fungi and bacteria Each microorganism has its own optimum temperature, pH, and oxygen level to grow Broadly, food microorganisms can be categorized as pathogenic and spoilage ones Food pathogenic microorganisms are those groups that can cause illness in human being or animals due to food poisoning or infection Food poisoning and food infection are different, although the symptoms are similar True food poisoning or food intoxication is caused by eating food that contains a toxin or poison due to bacterial or fungal growth in food The bacteria or fungi that produced and excreted the toxic waste products into the food may be destroyed during processing, but the toxin they produced can cause the illness or digestive upset to occur Staphylococcus aureus and Clostridium botulinum are two common species of bacteria that cause food poisoning Food infection is the second type of foodborne illness It is caused by eating food that contains certain types of live bacteria, which are present in the food Once the food is consumed, the bacterial cells themselves continue to grow and illness can result Salmonellae, C perifringens, Vibiro Units of Reaction Constant (k) M s s M.s parahaemolyticus, Yersinia enterocolitica, and Listeria monocytogenesis are common infectious bacteria of food The other category includes food spoilage microorganisms Food spoilage can be defined as the process or change leading to a product becoming undesirable or unacceptable for consumption This can be chemical, physical, or microbial spoilage or the combination of these factors Food spoilage is more of an economic concern than safety issue Food microorganisms can be grouped based up on their oxygen and temperature requirement for survival and growth (Table 38.2) In foods that are packaged under vacuum, low oxygen levels are intentionally achieved Therefore, the prevailing conditions not support the growth of microorganisms that require oxygen (obligate aerobes) Likewise, the thermophilic microorganisms require temperatures much higher than 30◦ C for their growth Acidity level of a food is another intrinsic food factor that determines the survival and growth of food microorganisms From thermal processing standpoint, foods are divided into three pH groups: (i) high-acid foods (pH < 3.7), (ii) acid or medium-acid foods (3.7 < pH < 4.5), and (iii) low-acid foods (pH > 4.5) The most important distinction in the pH classification, with reference to thermal processing, is the dividing line between acid and low-acid foods Most laboratories concerned with thermal processing have devoted attention to C botulinum, which is a highly heat-resistant, rod-shaped, spore-forming, anaerobic Table 38.2 Classification of Microorganisms and Bacteria Based on Their Oxygen Demand and Optimum Temperature for Growth Classification Groups of Microorganisms Oxygen-based classification Obligatory aerobes (require oxygen) Obligatory anaerobes (require absence of oxygen) Facultative anaerobes (tolerate some oxygen) Optimum temperature for growth Thermophilic (55–35◦ C) Mesophilic (40–10◦ C) Psychrophilic (35–