Lecture -02 (05-05-24) food unit operations
- 1. Food Unit Operation-103 Lecture – 02 ( 05-05-2024) By: M. Qazafi
- 2. Contents • Objective • Definitions • Examples of unit operations • Measurement system • Properties of food • Unit Operations: Classifications • Mass Transfer Unit Operations • Heat Transfer Unit Operations • Simultaneous Mass–Heat Transfer Unit Operations • Thermal Properties of Food materials
- 3. Objective of Unit Operation • The main objective of Unit operation in Food engineering: • to study the principles and laws governing the physical, chemical, or biochemical stages of different processes, and the apparatus or equipment by which such stages are industrially carried out. • The studies should be focused on the transformation processes of agricultural raw materials into final products, or on the conservation of materials and products
- 4. Unit Operations • Number of basic operations/steps are involved in the production of food products and called unit operations.
- 5. Examples of unit operations • Examples of unit operations common to many food products include • Cleaning • Coating • Concentrating • Controlling • Disintegrating • Drying • evaporating • Fermentation • Forming • heating/cooling (heat exchange) • materials handling • mixing, packaging • pumping, separating, and others. These operations are listed alphabetically, not in the order of their natural sequence or importance
- 6. Measurement system • A physical entity, which can be observed and/or measured, is defined qualitatively by a dimension. • For example, time, length, area, volume, mass, force, temperature, and energy are all considered • Dimensions like unit of length may be measured as a meter, centimeter, or millimeter.
- 7. Measurement system • Primary dimensions, such as length, time, temperature, and mass, express a physical entity. • Secondary dimensions involve a combination of primary dimensions (e.g., volume is length cubed; velocity is distance divided by time). • Physical quantities are measured by variety of unit systems. • The most common systems include the Imperial (English) system; the centimeter, gram, second (cgs) system; and the meter, kilogram, second (mks) system.
- 8. Measurement system • International organizations have attempted to standardize unit systems, symbols, and their quantities. • As a result of international agreements, the Systeme International d’Unites, or the SI units have emerged. The SI units consist of seven base units, two supplementary units, and a series of derived units.
- 9. Base Units
- 10. Derived Units • Derived units are algebraic combinations of base units expressed by means of multiplication and division. For simplicity, derived units often carry special names and symbols that may be used to obtain other derived units.
- 11. Derived Units
- 12. Properties of food Raw food materials are biological in nature and they have • Irregular shapes commonly found in naturally occurring raw materials • Properties with a non-normal frequency distribution; • Heterogeneous composition • Composition that varies with variety, growing conditions, maturity and other factors • Affected by chemical changes, moisture, respiration, and enzymatic activity.
- 13. Rheological properties • The majority of industrial food processes involve fluid movement. • Liquid foods such as milk and juices have to be pumped through processing equipment or from one container to another. • A number of important unit operations such as filtration, pressing and mixing are, particular applications of fluid flow. • The mechanism and rate of energy and mass transfer are strongly dependent on flow characteristics. • The flow properties and deformation properties of fluids are the science called ‘rheology’ or the relationship between stress and strain is the subject matter of the science known as rheology
- 14. Mechanical Properties • Mechanical properties are those properties that determine the behavior of food materials when subjected to external forces. • Mechanical properties are important in processing (conveying, size reduction) and consumption (texture, mouth feel).
- 15. Thermal Properties • In the food industry every process involves thermal effects such as heating, cooling or phase transition. • The thermal properties of foods are important in food process engineering. • The following properties are of particular importance: • thermal conductivity, • thermal diffusivity, • specific heat, • latent heat of phase transition and emissivity
- 16. Electrical Properties • The electrical properties of foods are particularly relevant to microwave and ohmic heating of foods and to the effect of electrostatic forces on the behavior of powders. • The most important properties are electrical conductivity and dielectric properties. • Ohmic heating is a technique whereby a material is heated by passing an electric current through it.
- 17. Production of Fruit Juice Concentrate
- 18. Unit Operations: Classifications Physical stages: • Grinding • Sieving • Mixture • Fluidization • Sedimentation • Flotation • Filtration • Absorption/adsorption • Extraction • Heat exchange • Evaporation/drying, etc.
- 19. Unit Operations: Classifications Chemical stages: • Refining • Chemical peeling Biochemical stages: • Fermentation • Sterilization • Pasteurization • enzymatic peeling
- 20. Other Unit Operations • Momentum Transfer Unit Operations: • These operations are controlled by the diffusion of a component within a mixture. 1. Internal circulation of fluids: study of the movement of fluids through the interior of the tubing. 2. External circulation of fluids: the fluid circulates through the external part of a solid. This circulation includes the flow of fluids through porous fixed beds, fluidized beds (fluidization), and pneumatic transport. 3. Solids movement within fluids: the base for separation of solids within a fluid. This type of separation includes: sedimentation, filtration, and ultrafiltration, among others
- 21. Mass Transfer Unit Operations • These operations are controlled by the diffusion of a component within a mixture. • Distillation: Separation of one or more components by taking advantage of vapor pressure differences. • Absorption: A component of a gas mixture is absorbed by a liquid according to the solubility of the gas in the liquid. Absorption may occur with or without chemical reaction. The opposite process is called desorption.
- 22. Mass Transfer Unit Operations • Extraction: Based on the dissolution of a mixture (liquid or solid) in a selective solvent, which can be liquid–liquid or solid- liquid. The latter is also called washing etc. • Adsorption: Also called sorption, adsorption involves the elimination of one or more components of a fluid (liquid or gas) by retention on the surface of a solid. Ionic exchange: substitution of one or more ions of a solution with another exchange agent.
- 23. Heat Transfer Unit Operations These operations are controlled by temperature gradients. They depend on the mechanism by which heat is transferred: Conduction: • In continuous material media, heat flows in the direction of temperature decrease and there is no macroscopic movement of mass. • The process by which heat or electricity is directly transmitted through the material of a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material.
- 24. Heat Transfer Unit Operations Convection: • The flow associated with a moving fluid is called convective flow of heat. • Convection can be natural or forced. • The movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat.
- 25. Heat Transfer Unit Operations Radiation: • Energy transmission by electromagnetic waves. . • No material media are needed for its transmission. • Thermal treatments (sterilization and pasteurization), evaporation, heat exchangers, ovens, solar plates, etc. are studied based on these heat transfer mechanisms
- 26. Simultaneous Mass–Heat Transfer Unit Operations In these operations a concentration and a temperature gradient exist at the same time: Humidification and dehumidification: • Humidification - The process in which the moisture or water vapor or humidity is added to the air. Common equipment used in this process is a humidifier. • Dehumidification - as the term suggests, is the opposite of humidification since dehumidification means removing the moisture from the air. Crystallization: • formation of solid glassy particles within a homogeneous liquid phase.
- 27. Simultaneous Mass–Heat Transfer Unit Operations Dehydration: • Elimination of a liquid contained within a solid. The application of heat changes the liquid, contained in a solid, into a vapor phase. • In freeze-drying, the liquid in solid phase is removed by sublimation, i.e., by changing it into a vapor phase.
- 28. Complementary Unit Operations • One series of operations is not included in this classification because these are not based on any of the transport phenomena cited previously. • These operations include grinding, milling, sieving, mixing of solids and pastes, etc.
- 29. Thermal Properties of Food materials Thermal Properties of Products • Heating • Cooling • Combination of heating and cooling Cooking, cooling, baking, pasteurization, freezing, and dehydration: all involve heat transfer
- 30. Thermal Properties of Food materials Examples • Grain dried for storage • Noodles dried • Fruits/Vegetables rapidly cooled • Vegetables are blanched, maybe cooked and canned • Powders such as spices and milk: dehydrated • Design of such processes require knowledge of thermal properties of material
- 31. Thermal Properties of Food materials Heat is transferred by • Conduction: Temperature gradient exists within a body…heat transfer within the body • Convection: Heat transfer from one body to another by virtue that one body is moving relative to the other • Radiation: Transfer of heat from one body to another that are separated in space in a vacuum. (blackbody heat transfer)
- 32. Thermal Properties of Food materials We’ll consider: • Conduction w/in the product • Convection: transfer by forced convection from product to moving fluid Moisture movement through agricultural product is similar to movement of heat by conduction • Moisture diffusivity • Volume change due to moisture content change
- 33. Terms used to define thermal properties • Specific heat • Thermal conductivity • Thermal diffusivity • Thermal expansion coefficient • Surface heat transfer coefficient • Sensible and Latent heat • Enthalpy
- 41. Next Lecture Preliminary Unit operation • Cleaning, sorting & Grading • Aims/objectives • methods and applications
- 42. Recommended Book • Unit Operations in Food Processing - R. L. Earle • Introduction to Food Engineering by R. Paul Sing 2. Food Process Engineering by Heldman • Hand book of Food Engineering By Dennis R. Heldman • Introduction to Food Process Engineering by P.G.Smith • Food Engineering Operation by J.G. Brennan • Unit operation in Food Engineering by Gustavo V. Barbosa-Cánovas • Fundamentals of Food Process Engineering by Romeo T. Toledo Books For Food EngineeringBooks For Food Engineering