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Lecture 1.1 - General concepts in dehydration.pdf

  1. Chapter 1: General Concepts in Dehydration
  2. Introduction • Dehydration or Drying is a processing method or preservation method? • What the purposes of food dehydration?
  3. Introduction • Persian and Chinese people dried fruits and vegetables in the sun as long as 5000 years ago. • The 18th century: used artificial energy with controlled drying conditions: Vegetables were soaked in hot water and conduction drying. • Continuing to develop in World War I and II. • During World War I and II: continue to develop other drying methods such as vacuum drying (fish meat), drum drying (butter, soup, tomato paste), spray drying (milk and eggs). • 1960s: Development of sublimation drying.
  4. Introduction • Due to high demands of customers (high quality, low price, etc.), many novel drying technologies (microwave, ultrasound, heat pump, etc.) have been researched and developed.
  5. Theoretical considerations of air- water mixtures • The dehydration process depends on understanding the relationship between the water contained in a foodstuff and the water present in the drying medium, which is usually air. • Ideal gas equations for air and water are as follows: PV = n RT P: the partial pressures; n: the number of moles; V: the total volume; R: the gas constant; T: the absolute temperature. • If the mixture consists only of air and water, the total pressure (Pt ) is the sum of the water and air partial pressures (Dalton law): Pt = Pa + Pw Pa và Pw: are the partial pressures of air and water in the mixture, respectively
  6. Absolute humidity • The mass ratio of water to dry air is known as the absolute humidity, which can be defined as the amount of moisture in the air at any condition (Y ): Mw và Ma: are the molecular weights of water and air, respectively. • Dry air (78% nitrogen, 21% oxygen, and 1% other gases) has an average Ma of 29 Da and water has an Mw of 18 Da.
  7. Saturation Absolute Humidity • The mass ratio of water to air is called the saturation absolute humidity. Pw0: is the saturation vapor pressure of water.
  8. Relative humidity • is defined as the ratio of Pw to Pw0 at the same temperature. • It is a relative measure of the amount of moisture that wet air can hold at a given temperature:
  9. Psychrometric or humidity chart • The psychrometric chart is very useful for heat and mass balance determinations involving air–water mixtures. • Psychrometric consists of: o Dry bulk temperature o Wet bulk temperature or saturation temperature o Dew point temperature o Absolute and relative humidities o Enthalpy (total heat) o Specific volume
  11. Terms • Dry bulb temperature: ambient temperature, not affected by moisture content • Wet bulb temperature: temperature of adiabatic saturation
  12. • Dew point temperature: the temperature at which water vapor starts to condense out of the air. • Relative humidity: measure of the amount of water air can hold at a certain temperature. • Humidity ratio of moist air: weight of the water contained in the air per unit of dry air. • Enthalpy: heat energy content of moist air. • Specific volume: the space occupied by air.
  13. Drying • Heated air at 50oC and 10% RH is used to dry rice in a bin dryer. The air exits the bin under saturation conditions. Determine the amount of water removed per kg of dry air.
  14. Dry bulb and wet bulb temperature
  15. Relative humidity
  16. Dew point temperature
  17. Enthalpy
  18. Specific volume
  19. Questions • Question 1: Given the ambient temperature is 70°F measured by a dry bulb thermometer and 60°F measured by a wet bulb thermometer, what is the relative humidity? • Question 2: An air conditioning system is not working well. The temperature of the evaporator coil is 53°F. The air in the room is at 76°F and 40% relative humidity. Will the air conditioner remove moisture from this air? • Question 3: A house is 4500 ft2 and has 12 ft ceilings. For comfort, the home owner specifies 0.3 changes of air per hour. The outside air temperature is 90°F dry bulb and 73.5° wet bulb. The air indoors is 75°F dry bulb 50% relative humidity. What is the amount of cooling required to provide the fresh air?
  20. DEHYDRATION PRINCIPLES • A wet material placed in a medium having lower water partial pressure (at the same temperature) will dehydrate until equilibrium is reached. • The medium is usually air at pressures ranging from a high vacuum to atmospheric (or higher), although superheated steam, hot oil, solvents, and solutions may also be used. • Food dehydration is a complex phenomenon involving simultaneous mass and energy transport in a hygroscopic and shrinking system.
  21. DEHYDRATION PRINCIPLES • Heat transfer from the drying medium to the wet solid can be a result of: • convection, • conduction, • radiation effects, • a combination of them (in some cases) • Internal heat transfer is usually very rapid compared to external transfer; • Mass transfer depends on either the movement of moisture within the solid or the movement of water vapor from the solid surface to the bulk medium. • Internal mass transfer is generally recognized to be the principal rate- limiting step during drying. • The structure of food material being dried plays an important role in the mechanism of water movement within a product
  22. DEHYDRATION PRINCIPLES • Liquid materials and gels: water transport is by molecular diffusion from the interior to the surface of the product, where it is removed by evaporation. • Capillary-porous materials: the possible physical mechanisms are numerous and can be classified as 1) liquid movement caused by capillary and gravity forces, 2) liquid diffusion caused by a difference in concentration, 3) surface diffusion, 4) water vapor diffusion caused by partial pressure gradients, 5) water vapor flow under differences in total pressure, 6) flow caused by an evaporation–condensation sequence
  23. DEHYDRATION KINETICS • The drying kinetics of the product are the most important data required for the design and simulation of dryers. • Drying kinetics are affected by: v the external conditions of the medium v the chemical and physical structure of the food • In the most general case, drying a food under constant conditions is considered in order to obtain the kinetics curve.
  24. Drying curves as a function of drying time: (a)drying kinetics (b)(b) drying rate.
  25. • Free moisture content, Xf: Xf = X – Xe (X: Moisture content in db, Xe: equilibrium moisture content). • Drying rate, Nw, kg water/m2/h): ms: the mass of the dry solids A: the area available for drying.
  26. DEHYDRATION KINETICS • The drying rate depends on üthe heat and mass transfer coefficients, üdiffusion coefficients, üthe nature of the food, üand the external drying conditions. • Optimization of any or all of these factors would result in increased drying rates.
  27. DRYING TECHNOLOGIES • Selection criteria for classification of dryers: 1. Mode of operation (batch or continuous) 2. Operating pressure (vacuum, atmospheric, and high pressure) 3. Mode of heat transfer (conduction, convection, radiation, dielectric heating, and combination of different modes) 4. State of product being dried (stationary, moving, agitated, fluidized, and atomized) 5. Residence time (short: below 1 min, medium: 1–60 min, long: higher than 60 min). 6. Cost 7. Quality of end product 8. Safety 9. Ease of design and operation
  28. Classification of dryers according to heat mode transfer
  31. Quality aspects of dehydrated foods
  32. Water sorption isotherms Equilibrium moisture content, Xe
  33. Water sorption isotherms • Sigmoidal • hysteresis
  34. Hysteresis Gas-liquid absorption (a) and liquid-solid adsorption (b) mechanism. Blue spheres are solute molecules (a) (b)
  35. Hysteresis • Many theories and hypotheses have been formulated to explain the phenomenon of hysteresis. 1. the nature of the food, 2. temperature, storage time, 3. adsorption - desorption cycles.
  36. Temperature effect on sorption isotherms • Water activity changes with temperature . Thus, water sorption isotherms must also exhibit temperature dependence. • A temperature increase results in a decrease of the amount of water adsorbed. • The effect of temperature on water sorption isotherms is often represented by the Clausius–Clapeyron equation: Qs is the net isosteric heat or heat of sorption, which is defined as the difference between the total molar enthalpy change and the molar enthalpy of vaporization. R is the universal gas constant.
  37. Effect of temperature on water sorption isotherm for carrot
  38. Relationship between Aw and Moisture content X: moisture content (% dry basis); Xm: monolayer value (% dry basis); T: temperature (K); C, K, A, B, K1, K2, n1, n2: constants.
  39. Physical Properties • Physical properties such as color, texture, density, porosity, rehydration capacity are affected by drying methods.
  40. Microbiological and chemical quality
  41. Nutritional Quality • Discussion ???
  42. Influence of Drying Methods and Pretreatments