A. FOOD ENGINEERING AND RHEOLOGY
1. Food Engineering
Food engineering is the application of engineering principles, scientific concepts, and unit operations to the processing, preservation, packaging, handling, and storage of foods.
2. Engineering Principles Used in Food Processing
Major engineering principles used in food industries include heat transfer, mass transfer, fluid mechanics, thermodynamics, material and energy balances, and reaction kinetics.
3. Rheology
Rheology is the study of the flow and deformation of materials under applied forces. In food engineering, rheology is used to characterize liquids, suspensions, pastes, gels, doughs, and other food materials.
4. Rheological Parameters
Important rheological parameters include shear stress, shear rate, strain, viscosity, storage modulus (G′), and loss modulus (G″). G′ represents elastic energy storage, whereas G″ represents viscous energy dissipation.
5. Newton’s Law of Viscosity
For a Newtonian fluid, shear stress is directly proportional to shear rate:
τ = μγ̇
where τ is shear stress, μ is dynamic viscosity, and γ̇ is shear rate. This relationship does not apply to all food materials because many foods are non-Newtonian.
6. Rheological Tests
Important rheological tests include stress-relaxation, creep and recovery, and small-amplitude oscillatory tests. These tests are used to characterize the viscous, elastic, and viscoelastic behavior of foods.
7. Rheological Models
Common models used to describe food flow behavior include the Newtonian, Power-law, Bingham plastic, and Herschel–Bulkley models. Model selection depends on the measured flow behavior of the food.
8. Types of Viscosity
Important viscosity concepts include:
- Dynamic viscosity: resistance of a fluid to deformation under shear.
- Apparent viscosity: effective viscosity of a non-Newtonian material at a specified shear rate.
- Relative viscosity: ratio of the viscosity of a solution or dispersion to that of a reference fluid.
- Intrinsic viscosity: a solution-property parameter related to the hydrodynamic volume of dissolved macromolecules.
B. GLASS TRANSITION AND HEAT TRANSFER
9. Glass Transition
Glass transition is a temperature-dependent transition in an amorphous material between a glassy, rigid state and a more mobile rubbery or viscous state. It is not equivalent to ordinary melting of a crystalline solid.
10. Differential Scanning Calorimetry (DSC)
DSC is a thermal-analysis technique that measures differences in heat flow between a sample and reference during controlled heating or cooling. It can be used to study glass transition, melting, crystallization, and other thermal transitions.
11. Modes of Heat Transfer
Heat transfer occurs through three fundamental mechanisms:
- Conduction: transfer through molecular interactions within a material.
- Convection: heat transfer associated with fluid motion and conduction.
- Radiation: transfer of thermal energy through electromagnetic radiation.
These mechanisms are not restricted to solids, liquids, or gases. More than one mechanism can occur simultaneously.
12. Heat Exchangers
Heat exchangers transfer thermal energy between a food product and a heating or cooling medium. Common food-processing designs include plate, tubular, and scraped-surface heat exchangers.
13. Important Thermal Properties of Foods
Important thermal properties include specific heat capacity, thermal conductivity, thermal diffusivity, density, thermal expansion coefficient, and phase-transition properties such as freezing and glass transition.
14. Measurement of Thermal Conductivity
Thermal conductivity of foods can be measured using methods such as heated-probe or transient hot-wire techniques. The appropriate method depends on the physical characteristics of the food.
15. Enthalpy of Frozen Foods
The enthalpy changes associated with freezing and thawing can be measured using calorimetric techniques such as differential scanning calorimetry. These data are useful for estimating refrigeration and freezing energy requirements.
16. Radiative Heat-Transfer Properties
Important radiation properties are:
- Emissivity: ability of a surface to emit radiation.
- Reflectivity: fraction of incident radiation reflected.
- Absorptivity: fraction of incident radiation absorbed.
- Transmissivity: fraction of radiation transmitted through a material.
C. MOISTURE AND MASS TRANSFER
17. Moisture Sorption Isotherm
A moisture sorption isotherm represents the relationship between the equilibrium moisture content of a food and its water activity at a specified temperature. It is important in drying, packaging, storage, and shelf-life studies.
18. Equilibrium Moisture Content
Equilibrium moisture content is the moisture content of a food material when it is in equilibrium with the surrounding environment at specified temperature and relative humidity. At equilibrium, there is no net moisture transfer between the food and its surroundings.
19. Molecular Diffusion
Molecular diffusion is the net movement of molecules caused by random molecular motion along a chemical-potential or concentration gradient.
20. Applications of Mass Transfer in Foods
Mass-transfer principles are involved in moisture migration, solute diffusion, diffusion through porous foods, aroma transfer, drying, extraction, membrane separation, and packaging-related mass transfer.
21. Food Processes Involving Mass Transfer
Important food processes involving mass transfer include extraction, distillation, gas absorption, crystallization, drying, membrane separation, and migration through packaging materials.
22. Electrical Conductivity of Foods
The electrical conductivity of many aqueous food systems increases with increasing temperature because ionic mobility generally increases. The exact relationship depends on food composition, ionic concentration, moisture, and temperature.
23. Thermal Death-Time Parameters
Thermal processing of foods is commonly described using D-values, z-values, and F-values. These parameters quantify microbial resistance and the lethality delivered by a thermal process.
24. D₂₅₀ Value
A D₂₅₀ value is the decimal reduction time at 250°F (121.1°C). It is the time required under specified conditions at that temperature to reduce the population of a specified microorganism by one log cycle, or 90%. The value depends on the microorganism and food matrix.
25. Sorption Hysteresis
The difference between adsorption and desorption isotherms at the same water activity is called sorption hysteresis. Consequently, the equilibrium moisture content can differ depending on whether moisture is being gained or lost.
26. Hysteresis in Hygroscopic Foods
Sorption hysteresis is commonly observed in hygroscopic food materials. The extent of hysteresis depends on food composition, structure, temperature, and moisture history.
27. Water Activity
Water activity (aᵥ) is the ratio of the vapor pressure of water in a food to the vapor pressure of pure water at the same temperature:
aᵥ = p / p₀
Water activity indicates the thermodynamic availability of water, not simply the total amount of water present.
28. Water Activity at Equilibrium
When food is in equilibrium with surrounding air, its water activity is approximately equal to the equilibrium relative humidity expressed as a decimal:
aᵥ ≈ ERH / 100
29. Adsorption and Desorption
- Adsorption: uptake of water by a food material from its surroundings.
- Desorption: removal or loss of water from the food material to its surroundings.
30. Effect of Temperature on Moisture Sorption
At a fixed water activity, the equilibrium moisture content of many food materials decreases as temperature increases. However, the magnitude and exact behavior depend on the composition and structure of the food.
D. GLASS TRANSITION, STRUCTURE AND DRYING
31. Dilatometry
Dilatometry is a technique that measures changes in dimensions or volume of a material as a function of temperature. It can be used to study thermal expansion and glass-transition behavior.
32. Methods for Measuring Glass Transition
Glass-transition behavior can be investigated using dilatometry, differential scanning calorimetry (DSC), and dynamic mechanical or thermomechanical analysis. The measured transition may depend on the technique and experimental conditions.
33. Porosity
Porosity is the fraction of the total volume of a material occupied by pores or void spaces:
Porosity = Pore volume / Total volume
Porosity strongly influences density, heat transfer, mass transfer, and drying behavior.
34. Shrinkage
Shrinkage is the reduction or change in the dimensions or volume of a food material during processing, particularly during drying. It is commonly expressed relative to the original dimensions or volume.
35. Fick’s Law of Diffusion
Fick’s laws describe mass diffusion. Fick’s first law relates diffusion flux to a concentration gradient under steady-state conditions, whereas Fick’s second law describes changes in concentration with time during transient diffusion.
36. Spray-Dryer Design
Droplet size is a critical parameter in spray drying because it affects drying rate, residence time, particle size, final moisture content, and powder properties. Determining the required droplet-size distribution is therefore an important design consideration.
37. Freeze Drying
Freeze drying, or lyophilization, involves freezing the food and removing ice primarily by sublimation under reduced pressure during primary drying. A subsequent secondary-drying stage removes additional water by desorption.
38. Industrial Freeze Dryers
Industrial freeze-drying systems include tray or shelf freeze dryers and tunnel freeze dryers. Equipment selection depends on product characteristics and production requirements.
E. MEMBRANE PROCESSING AND EVAPORATION
39. Membrane Processing
Important membrane processes used in food engineering include:
- Reverse osmosis (RO)
- Nanofiltration (NF)
- Ultrafiltration (UF)
- Microfiltration (MF)
These processes separate components according to membrane properties and operating conditions.
40. Membrane Fouling
Membrane fouling is the accumulation of food components or other materials on or within a membrane. It reduces permeate flux, increases membrane resistance, can alter separation performance, and increases cleaning requirements.
41. Applications of Membrane Technology
Membrane processes are widely used in food and beverage industries for clarification, concentration, fractionation, water removal, protein processing, and separation of selected food components.
42. Types of Evaporators
Food-processing evaporators include:
- Short-tube evaporators
- Long-tube rising-film evaporators
- Long-tube falling-film evaporators
- Forced-circulation evaporators
- Scraped-surface evaporators
- Plate evaporators
- Thin-film evaporators
The choice depends on viscosity, heat sensitivity, fouling tendency, solids content, and desired concentration.
F. MVR, FREEZING AND FLOW OF SOLIDS
43. Mechanical Vapor Recompression (MVR)
In an MVR evaporator, vapor produced during evaporation is mechanically compressed to increase its pressure and saturation temperature. The compressed vapor is then reused as a heating medium. The energy input is mainly mechanical/electrical energy supplied to the compressor or vapor fan.
44. Freezing Time
Freezing time is an important design parameter in food freezing. It depends on product dimensions, initial and final temperatures, freezing-point depression, latent heat, thermal properties, heat-transfer coefficient, and refrigerant temperature.
45. Plank’s Equation
Plank’s equation is a classical method for estimating the freezing time of food materials under simplified assumptions. It considers heat transfer associated with the phase-change region but may require correction or more advanced models for real foods.
46. Freezing Systems
Freezing systems can be classified according to the method of heat transfer. Common categories include:
- Direct-contact freezing: the product directly contacts the refrigerating medium or cold surface.
- Indirect-contact freezing: heat is removed through a separating heat-transfer surface.
47. Types of Heat Exchangers
Common food-processing heat exchangers include:
- Tubular heat exchangers
- Multi-tube or triple-tube heat exchangers
- Plate heat exchangers
- Scraped-surface heat exchangers
The design is selected according to viscosity, particle size, fouling tendency, heat sensitivity, and required heat-transfer rate.
48. Flow of Granular Solids
The flow of granular food materials is influenced by gravity, interparticle friction, cohesion, adhesion, particle size, particle shape, moisture content, bulk density, and consolidation.
49. Friction Factor
The friction factor is a dimensionless parameter representing resistance to fluid flow in a conduit. It is used with flow equations to calculate pressure losses caused by wall friction. The Darcy and Fanning friction factors have different definitions and should not be interchanged.
50. Polymorphism
Polymorphism is the ability of the same chemical substance to exist in more than one crystalline structure. Different polymorphs may differ in melting point, stability, solubility, density, and other physical properties. Techniques such as X-ray diffraction, DSC, NMR, and dilatometry can be used to characterize polymorphic forms and phase transitions.