FPE 4 - Heat and Mass Transfer in Foods PDF

AGBE 3313 FOOD PROCESS ENGINEERING HEAT&MASS TRANSFER ENGR. MARC GOMERSON A. CORPUZ Institute of Agricultural and Biosystems Engineering Bataan Peninsula State University – Abucay Campus HEAT TRANSFER Heat transfer is a dynamic process in which heat is transferred spontaneously from one body to a...

AGBE 3313 FOOD PROCESS ENGINEERING HEAT&MASS TRANSFER ENGR. MARC GOMERSON A. CORPUZ Institute of Agricultural and Biosystems Engineering Bataan Peninsula State University – Abucay Campus HEAT TRANSFER Heat transfer is a dynamic process in which heat is transferred spontaneously from one body to another cooler body. The rate of heat transfer depends upon the differences in temperature between the bodies, the greater the difference in temperature, the greater the rate of heat transfer. Temperature difference between the source of heat and the receiver of heat is therefore the driving force in heat transfer. An increase in the temperature difference, increases the driving force and therefore increases the rate of heat transfer. HEAT TRANSFER APPLICATIONS Understanding heat transfer in food processing is important for the development of energy efficient thermal processes and ensures the production of safe and high-quality food. The food industry is known for its high consumption of energy in processes such as: Cooking, blanching, baking, sterilization, evaporation, pasteurization, freezing, and drying HEAT TRANSFER Heat transfer processes are important for almost all aspects of food preparation and play a key role in determining food safety. 1. For preservation, i.e. to kill bacteria and inactivate enzymes, such as the pasteurization of milk and the sterilization of canned food. Here the aim is to deliver the required microbial kill with as little damage as possible. HEAT TRANSFER 2. To develop taste and flavour, such as when cooking meats and vegetables, where in addition to sterilization heat is required to carry out physical changes to the food. HEAT TRANSFER 3. To develop the structure of the material, such as in baking of bread or biscuits, where heating acts both to change the starch structure and function and also to develop the bubble structure within the material Thermal Properties of Foods The thermal properties of food products determine their ability to transfer and store heat. The thermal properties are: specific heat , Cp (J/kg K), thermal conductivity , k (W/m K), and thermal diffusivity , α (m2 /s).  Specific Heat Specific Heat Specific heat is a measure of the amount of energy required by a unit mass to raise its temperature by a unit degree. So, specific heat is the quantity of heat that is gained or lost by a unit mass of product to accomplish a unit change in temperature, without a change in state. It can be calculated as follows: where Q is the heat gained or lost (kJ), m is the mass (kg), DT is the temperature change in the material (K), and Cp is the specific heat (kJ/kg/K)  Specific Heat One of the earliest models to calculate specific heat was proposed by Siebel (1892) and Charm (1978). Siebel’s model is described by the following equation: where W is the water content expressed as a fraction of the total base.  Thermal Conductivity The thermal conductivity of a food material is an important property used in calculations involving the rate of heat transfer. Thermal conductivity k is the rate of heat transfer q through a unit cross-sectional area A when a unit temperature difference ( T1 –T2 ) is maintained over a unit distance L :  Thermal Conductivity Riedel’s (1949) model predicts thermal conductivity of fruit juices, sugar solutions, and milk: Here W is the mass fraction of water. This formula gives an error in the temperature interval between 0 and 180°C of approximately 1%.  Thermal Conductivity For fruits and vegetables with water content greater than 60% the following equation was proposed by Sweat and Haugh (1974) :  Thermal Diffusivity Thermal diffusivity, α, is a ratio involving thermal conductivity, density, and specific heat and can be expressed as Here a is the thermal diffusivity (m2 /s) and ρ is the density (kg/m 3 ).  Thermal Diffusivity If thermal conductivity, density, and specific heat are known, thermal diffusivity can be calculated. Thermal diffusivity is strongly influenced by the water content as shown by the following models: Modes of Heat Transfer There are three modes of heat transfer: conduction , convection , and radiation. Any energy exchange between bodies occurs through one of these modes or combinations of two or all three modes.  Conduction In conduction, the molecular energy is directly exchanged from the hotter to the cooler regions, the molecules with greater energy communicating some of this energy to neighboring molecules with less energy. The effectiveness by which heat is transferred through a material is measured by the thermal conductivity, k (W/m/K). The rate of heat transfer by conduction qcond (W) is given by  Conduction In conduction, the molecular energy is directly exchanged from the hotter to the cooler regions, the molecules with greater energy communicating some of this energy to neighboring molecules with less energy. The effectiveness by which heat is transferred through a material is measured by the thermal conductivity, k (W/m/K). The rate of heat transfer by conduction qcond (W) is given by Conduction Conduction Conduction Conduction Conduction Conduction  Convection Convection is the transfer of heat by the movement of groups of molecules in a fluid. The groups of molecules may be moved by either density changes or forced motion of the fluid. In a typical convective heat transfer a hot surface heats the surrounding fluid, which is then carried away by fluid movement. The warm fluid is replaced by cooler fluid, which can draw more heat away from the surface.  Convection The convection coefficient, h , is the measure of how effectively a fluid transfers heat by convection. It is determined by many factors, such as the fluid density, viscosity, and velocity, and the geometrical shape of the object undergoing heating or cooling. The convection heat transfer coefficient, h , is measured in watts per square meter per kelvin. The rate of heat transfer from a surface by convection is given by Newton’s law: Here A is the surface area of the object, Ts is the surface temperature, and T∞ is the ambient or fluid temperature.  Radiation Radiation heat transfer is the transfer of heat energy by electromagnetic waves, which transfer heat from one body to another, in the way that electromagnetic light waves transfer light energy. Radiation heat transfer occurs when the emitted radiation strikes another body and is absorbed.  Radiation The radiation emitted by all real surfaces is less than the radiation emitted by a blackbody at the same temperature, and is expressed as where ε is the emissivity of the surface. The property emissivity, whose value is in the range 0 < ε < 1, is a measure of how closely a surface approximates a blackbody for which ε = 1.  Radiation When a surface of emissivity ε and surface area As at a thermodynamic temperature Ts is completely enclosed by a much larger (or black) surface at thermodynamic temperature Tsurr separated by a gas (such as air) that does not intervene with radiation, the net rate of radiation heat transfer between these two surfaces is given by: Convection and Radiation Convection and Radiation Convection and Radiation Convection and Radiation  Methods of Applying Heat to Food The methods of applying heat to food may be classified as follows: Indirect heating by vapors or gases such as steam or air; liquids such as water and organic heat-exchange liquids; electricity, in resistance heating systems. In indirect heating, heat is applied to the food through heat exchangers, the products of combustion being isolated from the food. Direct heating using gas, oil, and solid fuels; using infrared energy; using electricity, by dielectric or microwave methods. In direct systems, the heat energy is passed directly into the food.  Indirect Heating Methods These systems comprise four components: a combustion chamber where the fuel is burned and combustion products disposed of; a heat exchanger where the heat of combustion is taken up by a heat transfer fluid; a transfer system where the heated transfer fluid is passed to the heat user; and a heat exchanger where the transfer fluid exchanges its heat with the food. Indirect heating is illustrated  Indirect Heating by Vapors or Gases  Steam may be used for heating in its saturated or its superheated form; it may be also used to operate electric generators and vacuum ejectors. No other material possesses these unique properties.  When saturated steam is applied as a heat transfer medium it has a high latent heat and a high thermal conductivity, which are advantageous. But saturated steam has some disadvantageous properties, such a high vapor pressure and a low critical point.  Steam is very suitable for food processes because it is nontoxic, fireproof and explosion-proof, and odorless; it is produced from a cheap and abundant raw material – water.  Indirect Heating by Vapors or Gases  In normal practice, saturated steam is used for food processing up to a temperature in the region of 200°C. Above this, the cost of the necessary high pressure equipment starts to become unduly high. Superheated steam finds little heating application in the food industry for sterilization.  Air is a poor heat transfer fluid since it has low specific heat and thermal conductivity. Nevertheless, air is used for the heating of canned food, for baking, for drying, and in fluidized- bed cooking.  In all these cases heat transfer is by forced convection. Air is, of course, nontoxic and noncontaminating although it can bring about deterioration in foods which are sensitive to oxidation.  Indirect Heating of Food by Liquids  Liquids such as water, mineral oils, chlorinated hydrocarbons, and fused salts are used for general process heating.  High-temperature water is a most useful medium at temperatures up to 200°C when advantage may be taken of its high specific heat and thermal conductivity.  The other liquids find application in higher- temperature processing since they have the advantage of low vapor pressures.  Indirect Heating of Food by Electrical Resistance  Electrical resistance heating is the generation of heat by the flow of current through a resistor. The resistors may be attached to the walls of the process vessels or immersed in the material to be heated (immersion heating). The heating elements are made from spirally wound, nickel–chromium wires. These elements work at temperature up to 800°C, so heat transfer into the food is primarily conductive.  Resistance-heated baking ovens are common; resistor banks are located within the oven, heat being conveyed to the food by a combination of conduction, convection, and radiation.  Direct Heating Methods  There are some risks using the direct heating of food by solid, gaseous, and liquid fuels but numerous direct-fired baking ovens, malt kilns, and driers are encountered in the food industry.  Direct heating, by means of electrode boilers, is used in special steam-boiling applications.  Infrared Heating of Food Infrared heating occurs by means of banks of radiant heaters located in a tunnel, where food is conveyed, or in a oven, where food is baked. Radiant heaters are of two types: medium- temperature heaters and high-temperature heaters.  Dielectric Heating of Food  Dielectric heating and microwave heating use high-frequency energy to avoid interference in radar, television, and radio transmissions. The permitted frequencies below 300 MHz, which are called radio frequencies, are used in dielectric heating and those above 300 MHz are called microwaves and are used in microwave heating.  Dielectric heating is defined as heating in an electrically insulating material by the losses in it when subjected to an alternating electric field. Heating is brought about by molecular friction due to the rapid orientation of the electric dipoles under the influence of the high-frequency alternation of the applied field.  Dielectric Heating of Food  Dielectric heating is clean, continuous in operation, and well suited to automatic control.  It is used to thaw frozen eggs, meat, fruit juices, and fish, to melt fats, chocolate, and butter, to bake biscuits, to heat peanuts, and to dry sugar cubes and crisp bread.  Microwave Heating of Food  Microwaves are regarded at electromagnetic radiation having frequencies in the range 300–300,000 MHz.  The quantum energy in microwaves is responsible for creation of heat as the microwave oscillates 2,450 × 10 6 times per second and the dipole molecules align to the electric field of the microwave at the same rate.  The alternating electric field stimulates the oscillation of the dipoles of the molecules (e.g., water) in the food. The heat is generated owing to the molecular friction between dipole molecules. Microwave Heating of Food  Microwave Heating of Food Microwaves are used to heat precooked, frozen food and, in conjunction with the browning effect of infrared heaters, for cooking in canteens and hospitals, where speed is important. Industrially, microwaves are used for precooking chicken, for apple juice evaporation, and in potato chips finishing. There are some investigational works for using microwaves in the pasteurization of fruit juices, in reducing mold counts in bread, cakes, and jam, in bread baking, and in accelerated freeze-drying.  Heat Methods of Food Safety and Food Preservation Blanching, pasteurization, and sterilization are heating process with the objective of ensuring the preservation of food; the last two also accomplish the objective of safety. Other processes such as baking, roasting, and frying also accomplish these objectives, but their main purpose is to transform food materials for consumption (Anon, 1984).  Blanching Blanching is an important heat process in the preparation of vegetables and fruits for canning, freezing, or dehydratation. Blanching inactivates enzymes or destroys enzyme substrates such as peroxides. During blanching the food is heated rapidly to a predetermined temperature, it remains at this temperature for a predetermined time, and then the food cools.  Blanching Two methods of blanching are used:  Immersion blanching involved passing the food at a controlled rate through a perforated drum, and rotation in a tank of water thermostatically controlled to the blanching temperature (75–95°C). This method leads to high loss of soluble nutrients in some food. (Selman, 1987)  Steam blanchers utilize saturated steam at atmospheric or at low pressure (150 kPa). The food is conveyed through the steam chamber on a mesh belt, and the residence time is controlled by the conveyer speed. The blanched product is discharged through an outlet lock to a washer and cooler. Steam blanching gives lower blanching losses than immersion blanching. Steam blanchers are easier to use for sterilization  Sterilization  The purpose of sterilization is to destroy all microorganisms present in the food material to prevent its spoilage and to ensure it is safe for consumption. Microorganisms are destroyed by heat, but the amount of heating required to kill different organisms varies.  The time and the temperature required for the sterilization of food are influenced by several factors, including the type of microorganisms found on the food, the size of the container, the acidity or pH of the food, and the method of heating (Encyclopedia Britannica, 2006).  Pasteurization  Pasteurization is a heat treatment which is sufficient to inactivate pathogenic microorganisms present in foods. This heating method of treating food is less drastic than sterilization.  It aims to inactivate pathogenic organisms such as bacteria, viruses, protozoa, molds, and yeasts, but not harm the flavor or quality of the food. The process was named after its inventor, French scientist Louis Pasteur.  Nowadays milk, wine, beer, and fruit juices are all routinely pasteurized. Pasteurization is also used with cheese and egg products. The most common application is pasteurization of liquid milk.  Pasteurization There are two methods of pasteurization of milk which are commonly used.  In the conventional method food is heated at least at 63°C and kept at that temperature for at least 30 min.  In the other method the temperature is higher (71°C) and food is kept at that temperature for at least 15 s. Unlike sterilization, pasteurization is not intended to kill all microorganisms in the food. Instead, pasteurization aims to achieve a “log reduction” in the number of viable organisms, reducing their number so they are unlikely to cause disease (assuming the pasteurized product is refrigerated and consumed before its expiration date). MASS TRANSFER If there are differences in concentrations of constituents throughout a solution or object, there will be a tendency for movement of material to produce a uniform concentration. Such movement may occur in gas, liquid, or solid solutions, Movement resulting from random molecular motion is called diffusion. Mass transfer occurs during various food processing operations, such as humidification and dehumidification, dehydration, distillation, and absorption. MASS TRANSFER The driving force for transfer is a concentration difference or a concentration gradient. These methods are: Distillation Gas absorption Dehumidification Liquid extraction Leaching drying Mass Transfer  Mass Transfer Leaching A process of mass transfer that occurs by extracting a substance from a solid material that has come into contact with a liquid/ solvent. The desired component diffuses into the solvent from its natural solid form. Applications: Sugar industry for removing sugar from beets (water is solvent) Oilseeds industry for removing oil from soybeans, etc. (hexane or similar organic solvents)  Mass Transfer Distillation: Distillation is an operation in which the constituents of a liquid mixture are separated using thermal energy. The difference in the vapor pressure is responsible for a separation. Distillation, particularly important in petroleum refining operations.  Mass Transfer Drying Refers to an operation in which moisture of a substance is removed with the help of thermal energy Both heat and mass transfer occur simultaneously Heat is transferred from the bulk of the gas phase (drying medium) to the solid phase and mass is transferred from the solid phase to the gas phase in the form of liquid and vapour  Mass Transfer Drying Need for Drying: For reducing the transport cost For purifying a crystalline product so that the solvent adhering to the crystals is removed To meet the market specifications of solid products set by the customer For making a material more suitable for handling and storage Preventing corrosion arising due to presence of moisture.  Mass Transfer Mass Diffusion Under steady-state conditions, the diffusion of moisture and nutrients in food may be described by Fick’s first law: where ma is the mass flow (kg/s), D is the diffusion coefficient (m2/s), and Ca is the concentration of the diffusing materials (kgm3). The flux is sometimes expressed in moles instead of mass  Mass Transfer Mass Transfer by Convection Nutrients or moisture diffuse inside the pores of the food at a rate of ma, according to Fick’s law, and then are transported from or to the surface by convective mass transport, similar to heat transport by free convection: where hm is the mass transfer coefficient and Cs and C∞ are the species concentrations at the surface of the food and in the bulk of the fluid.