The document provides an overview of dehydration principles and technologies. It discusses:
- The history of food dehydration dating back 5000 years and developments through World Wars I and II.
- Key concepts in dehydration including moisture transport mechanisms, drying curves, kinetics, equilibrium moisture content, and factors that influence the process.
- Common drying technologies classified by operation mode, pressure, heat transfer mode, and product state. Advantages and disadvantages of different methods are described.
- Quality aspects of dehydrated foods regarding physical properties, microbiological and chemical changes, and nutritional impacts. Drying methods and pretreatments can influence quality.
This document summarizes various drying techniques. It discusses the purpose of dehydration as removing water from foods to extend shelf life through inhibiting microbial growth and enzyme activity. Various drying methods are then outlined, including hot air drying, tray drying, tunnel drying, conveyor drying, fluidized bed drying, kiln drying, and pneumatic drying. Factors that influence the drying rate like food composition, size, and psychrometric properties of air are also summarized.
The PPT includes importance of grain drying, moisture content determination methods, equilibrium moisture content, different mode of heat transfer, types of different drying methods, different dryers etc. The presentation is best suitable for graduation level students.
This document discusses various aspects of drying grains, including:
- Drying removes moisture from grains to allow for safe long-term storage and preservation. It is one of the oldest food preservation methods.
- Different drying methods include tray drying, solar drying, and other methods that use conduction, convection, or radiation to transfer heat and evaporate moisture from grains.
- The moisture content, equilibrium moisture content, and heat transfer during drying are also examined.
Dehydration is a process that removes water from foods through evaporation or sublimation under controlled conditions. This preserves foods by reducing water activity and microbial growth while also lowering storage and transportation costs. There are various methods of dehydration like sun drying, hot air drying, and freeze drying. Proper design of dehydration systems requires understanding moisture content calculations, sorption isotherms, heat and mass transfer principles, and predicting drying times and rates.
Drying of fruit & vegetables- An approach for entrepreneurship DevelopmentTh Bidyalakshmi Devi
Drying is one of the easiest and commonly used food processing unit operations that can provide entrepreneurship development to a large extent. Drying of agricultural produce and selling of dried products can generate a huge amount of income.
This document provides an overview of drying in pharmaceutical engineering. It discusses the objectives, applications and mechanisms of drying. The key types of dryers covered include tray dryers, drum dryers, spray dryers and fluidized bed dryers. Tray dryers are described as using forced convection to remove moisture from solids placed on trays inside a heated and insulated chamber. The document also examines the rate of drying curve and factors that influence drying, such as material properties and humidity.
The document discusses drying as a food preservation method that involves removing water through evaporation or sublimation. It describes the drying process as involving heat and mass transfer to remove moisture from food. There are typically three periods in a drying rate curve: a constant rate period where the surface remains saturated, a falling rate period where the surface starts drying out, and a critical moisture content point where the drying rate begins to decrease. Key parameters that influence drying include moisture diffusion, water activity, and moisture content expressed on wet or dry bases.
Refrigeration is a technique used for preserving food in low temperatures. This procedure slow down or stop most bacteria from dividing and thereby multiplying, but do not kill them.
This document summarizes various drying techniques. It discusses the purpose of dehydration as removing water from foods to extend shelf life through inhibiting microbial growth and enzyme activity. Various drying methods are then outlined, including hot air drying, tray drying, tunnel drying, conveyor drying, fluidized bed drying, kiln drying, and pneumatic drying. Factors that influence the drying rate like food composition, size, and psychrometric properties of air are also summarized.
The PPT includes importance of grain drying, moisture content determination methods, equilibrium moisture content, different mode of heat transfer, types of different drying methods, different dryers etc. The presentation is best suitable for graduation level students.
This document discusses various aspects of drying grains, including:
- Drying removes moisture from grains to allow for safe long-term storage and preservation. It is one of the oldest food preservation methods.
- Different drying methods include tray drying, solar drying, and other methods that use conduction, convection, or radiation to transfer heat and evaporate moisture from grains.
- The moisture content, equilibrium moisture content, and heat transfer during drying are also examined.
Dehydration is a process that removes water from foods through evaporation or sublimation under controlled conditions. This preserves foods by reducing water activity and microbial growth while also lowering storage and transportation costs. There are various methods of dehydration like sun drying, hot air drying, and freeze drying. Proper design of dehydration systems requires understanding moisture content calculations, sorption isotherms, heat and mass transfer principles, and predicting drying times and rates.
Drying of fruit & vegetables- An approach for entrepreneurship DevelopmentTh Bidyalakshmi Devi
Drying is one of the easiest and commonly used food processing unit operations that can provide entrepreneurship development to a large extent. Drying of agricultural produce and selling of dried products can generate a huge amount of income.
This document provides an overview of drying in pharmaceutical engineering. It discusses the objectives, applications and mechanisms of drying. The key types of dryers covered include tray dryers, drum dryers, spray dryers and fluidized bed dryers. Tray dryers are described as using forced convection to remove moisture from solids placed on trays inside a heated and insulated chamber. The document also examines the rate of drying curve and factors that influence drying, such as material properties and humidity.
The document discusses drying as a food preservation method that involves removing water through evaporation or sublimation. It describes the drying process as involving heat and mass transfer to remove moisture from food. There are typically three periods in a drying rate curve: a constant rate period where the surface remains saturated, a falling rate period where the surface starts drying out, and a critical moisture content point where the drying rate begins to decrease. Key parameters that influence drying include moisture diffusion, water activity, and moisture content expressed on wet or dry bases.
Refrigeration is a technique used for preserving food in low temperatures. This procedure slow down or stop most bacteria from dividing and thereby multiplying, but do not kill them.
An Innovative Approach for Humidity Control by Using Deliquescent Materials i...IJMER
This document summarizes a paper that proposes using deliquescent materials to control humidity in test chambers. It begins with an introduction explaining the need for humidity control in various industries. It then reviews literature on traditional humidity control methods and their limitations. The proposed method uses deliquescent materials like zinc chloride, which absorb and release moisture based on temperature to maintain a constant relative humidity. The document outlines objectives to design and test a chamber using this approach. It concludes that deliquescent materials can effectively control humidity by utilizing their moisture absorption properties.
Drying is commonly the last stage of manufacturing to remove water. There are thermal and non-thermal drying methods. Drying is done to avoid moisture issues, improve material properties, reduce transportation costs, and make materials easier to handle. The rate of drying has distinct phases from initial adjustment to equilibrium. Efficient drying requires large surface area, efficient heat transfer, mass transfer of evaporated water, and efficient vapor removal. Common industrial dryers include pan dryers, fluidized bed dryers, and spray dryers. Freeze drying uses sublimation to dry heat-sensitive materials below their freezing point.
The document discusses drying processes and different types of dryers. It defines drying as the removal of small amounts of liquid like water from solids through the application of heat. Drying involves both heat and mass transfer operations. Several factors influence drying like material properties, moisture content, temperature, and surface area. Different types of dryers are discussed like tray dryers, drum dryers, fluidized bed dryers, and spray dryers. Selection of a suitable dryer depends on material properties, drying characteristics, desired product quality and available facilities.
it is diverse topic and with content not evenly distributed.
it covers all guidelines, short comings, and recommendations from latest journals and standard text book.
The document discusses different aspects of drying processes. It defines drying as the removal of small amounts of liquid like water or volatile liquids from materials through the application of heat. Drying involves both heat and mass transfer operations. There are key differences between drying and evaporation processes. Some common types of industrial dryers are drum dryers, spray dryers, fluidized bed dryers, tray dryers, tunnel dryers, and vacuum dryers. Drying is necessary to improve product characteristics, handling, and preservation. The rate and efficiency of drying depends on factors like material properties, moisture content, temperature, and dryer design.
This document provides an overview of different drying techniques used in pharmaceutical manufacturing. It discusses the objectives and mechanisms of the drying process. Several types of dryers are described, including tray dryers, drum dryers, spray dryers, fluidized bed dryers, vacuum dryers, and freeze dryers. For each dryer type, the document outlines the basic principles, construction, working, uses, advantages, and disadvantages. The document provides a high-level summary of key drying concepts and various dryer technologies commonly employed in the pharmaceutical industry.
Lecture 1.3 - General concept - Rehydration (3).pdfssuser972a6c
The rehydration of dried foods is a fundamental unit operation in the food industry.
0 The quality of rehydrated and reconstituted products is affected by the drying conditions and rehydration processes utilized, ultimately influencing consumer acceptance.
0 During the dryi
Drying is the process of removing small amounts of liquid, like water or volatile liquids, from a material by applying heat. It involves two operations - heat transfer and mass transfer. There are a few key differences between drying and evaporation, such as drying typically deals with removing small amounts of water from solids while evaporation removes larger amounts of water from liquids. Some common methods of drying include drum drying, spray drying, fluidized bed drying, tray drying, tunnel drying, and vacuum drying. Drying has several purposes like preservation, improving handling and characteristics, reducing costs, and purification of products.
Drying removes moisture from agricultural products to allow for safe storage. There are two main drying periods - a constant rate period where drying is dependent on conditions like temperature/humidity differences, and a falling rate period where moisture moves more slowly within the product. Drying curves plot moisture content or drying rate over time/moisture content. Thermal properties like specific heat and thermal conductivity also impact drying. Drying can occur in thin layers, with full air exposure, or in deep beds with changing internal conditions.
Pharmaceutical Engineering Unit -3.pptxNikita Gupta
This document provides an overview of various drying techniques used in pharmaceutical engineering. It discusses the objectives, mechanisms, and principles of operation for common dryers like tray dryers, drum dryers, spray dryers, fluidized bed dryers, vacuum dryers, and freeze dryers. The document also covers topics like drying applications, measurements of equilibrium moisture content, and rate of drying curves. Overall, it serves as a comprehensive introduction to drying as a crucial process in pharmaceutical manufacturing.
Drying grains is important to allow for safe long-term storage by reducing moisture content to prevent spoilage. There are various drying methods like sun drying, convection drying using hot air or gases, and radiation drying using electromagnetic waves. Common dryers include flat bed dryers, mechanical tray dryers, and solar dryers which utilize the sun's energy. Proper drying is necessary to retain nutrients while sufficiently lowering moisture for storage.
Dehydration is the oldest method of food preservation that involves reducing the water activity in food below the threshold for microbial growth through drying. The key objectives of dehydration are to minimize chemical degradation, selectively remove water over other components, maintain product structure, and allow for long term storage stability with less refrigeration. Dehydration works by transferring heat through convection and conduction to evaporate water from the food in a drying curve that typically shows an initial heating period, constant drying rate period, and falling rate period as moisture becomes bound within the material.
1) Drying is an important process for producing stable and consistent pharmaceutical materials. It involves the transfer of energy, phase transformation of water or solvent from liquid to vapor, and removal of vapor.
2) Dryers can be classified based on heat transfer methods and processing type. Common dryers include tray dryers, rotary dryers, fluidized bed dryers, spray dryers, freeze dryers, and microwave/RF dryers.
3) Fluidized bed dryers suspend materials in an upward-moving stream of heated air or gas, allowing for uniform and rapid drying. They are useful for heat-sensitive materials and granule production.
This document discusses humidification during mechanical ventilation. It begins by outlining the objectives and introducing humidification as a way to artificially condition inspired gas. It then covers the indications for humidification when the upper airway is bypassed and describes how inadequate humidification can cause clinical signs. The document delves into the physiology of normal heat and moisture exchange and how this is disrupted during mechanical ventilation. It describes different types of humidifiers including passive heat and moisture exchangers and active bubble through, passover and nebulizer humidifiers. It outlines principles of humidifier function and discusses indications, contraindications and hazards of humidification. It concludes by assessing the need for different humidifiers and common problems.
This document provides an overview of drying processes. It discusses that drying involves removing moisture from solids, solutions, slurries, and pastes. There are various modes of heat transfer used in drying, including convection, conduction, and radiation. Common industrial drying equipment includes batch dryers like tray and agitated dryers, as well as continuous dryers like tunnel dryers, belt dryers, and turbo-tray tower dryers. Drying rates can be constant, falling, or exponentially decreasing depending on the moisture content and properties of the material being dried.
Drying & dehydration of horticultural cropsKrishiCareer
This document discusses the importance of fruits and vegetables and their production in India. It notes that fruits and vegetables are key sources of nutrients but that a significant percentage are wasted each year in India due to lack of infrastructure. Drying is presented as an effective method to increase shelf life and reduce waste. The document then covers various drying methods and considerations, including factors that affect the drying process and potential effects on foods. It provides examples of commonly dried fruits and vegetables.
This document discusses various methods for analyzing the macro and micronutrients in foods. It describes techniques for determining moisture, total solids, ash, crude fat, and crude protein content. Moisture content is measured by drying samples and comparing weights before and after. Ash content is measured by burning off organic compounds and weighing the remaining minerals. Crude fat is extracted using Soxhlet extraction and crude protein is measured via the Kjeldahl method. The appropriate sample preparation, equipment used, calculations, and advantages/disadvantages of each technique are outlined.
An Innovative Approach for Humidity Control by Using Deliquescent Materials i...IJMER
This document summarizes a paper that proposes using deliquescent materials to control humidity in test chambers. It begins with an introduction explaining the need for humidity control in various industries. It then reviews literature on traditional humidity control methods and their limitations. The proposed method uses deliquescent materials like zinc chloride, which absorb and release moisture based on temperature to maintain a constant relative humidity. The document outlines objectives to design and test a chamber using this approach. It concludes that deliquescent materials can effectively control humidity by utilizing their moisture absorption properties.
Drying is commonly the last stage of manufacturing to remove water. There are thermal and non-thermal drying methods. Drying is done to avoid moisture issues, improve material properties, reduce transportation costs, and make materials easier to handle. The rate of drying has distinct phases from initial adjustment to equilibrium. Efficient drying requires large surface area, efficient heat transfer, mass transfer of evaporated water, and efficient vapor removal. Common industrial dryers include pan dryers, fluidized bed dryers, and spray dryers. Freeze drying uses sublimation to dry heat-sensitive materials below their freezing point.
The document discusses drying processes and different types of dryers. It defines drying as the removal of small amounts of liquid like water from solids through the application of heat. Drying involves both heat and mass transfer operations. Several factors influence drying like material properties, moisture content, temperature, and surface area. Different types of dryers are discussed like tray dryers, drum dryers, fluidized bed dryers, and spray dryers. Selection of a suitable dryer depends on material properties, drying characteristics, desired product quality and available facilities.
it is diverse topic and with content not evenly distributed.
it covers all guidelines, short comings, and recommendations from latest journals and standard text book.
The document discusses different aspects of drying processes. It defines drying as the removal of small amounts of liquid like water or volatile liquids from materials through the application of heat. Drying involves both heat and mass transfer operations. There are key differences between drying and evaporation processes. Some common types of industrial dryers are drum dryers, spray dryers, fluidized bed dryers, tray dryers, tunnel dryers, and vacuum dryers. Drying is necessary to improve product characteristics, handling, and preservation. The rate and efficiency of drying depends on factors like material properties, moisture content, temperature, and dryer design.
This document provides an overview of different drying techniques used in pharmaceutical manufacturing. It discusses the objectives and mechanisms of the drying process. Several types of dryers are described, including tray dryers, drum dryers, spray dryers, fluidized bed dryers, vacuum dryers, and freeze dryers. For each dryer type, the document outlines the basic principles, construction, working, uses, advantages, and disadvantages. The document provides a high-level summary of key drying concepts and various dryer technologies commonly employed in the pharmaceutical industry.
Lecture 1.3 - General concept - Rehydration (3).pdfssuser972a6c
The rehydration of dried foods is a fundamental unit operation in the food industry.
0 The quality of rehydrated and reconstituted products is affected by the drying conditions and rehydration processes utilized, ultimately influencing consumer acceptance.
0 During the dryi
Drying is the process of removing small amounts of liquid, like water or volatile liquids, from a material by applying heat. It involves two operations - heat transfer and mass transfer. There are a few key differences between drying and evaporation, such as drying typically deals with removing small amounts of water from solids while evaporation removes larger amounts of water from liquids. Some common methods of drying include drum drying, spray drying, fluidized bed drying, tray drying, tunnel drying, and vacuum drying. Drying has several purposes like preservation, improving handling and characteristics, reducing costs, and purification of products.
Drying removes moisture from agricultural products to allow for safe storage. There are two main drying periods - a constant rate period where drying is dependent on conditions like temperature/humidity differences, and a falling rate period where moisture moves more slowly within the product. Drying curves plot moisture content or drying rate over time/moisture content. Thermal properties like specific heat and thermal conductivity also impact drying. Drying can occur in thin layers, with full air exposure, or in deep beds with changing internal conditions.
Pharmaceutical Engineering Unit -3.pptxNikita Gupta
This document provides an overview of various drying techniques used in pharmaceutical engineering. It discusses the objectives, mechanisms, and principles of operation for common dryers like tray dryers, drum dryers, spray dryers, fluidized bed dryers, vacuum dryers, and freeze dryers. The document also covers topics like drying applications, measurements of equilibrium moisture content, and rate of drying curves. Overall, it serves as a comprehensive introduction to drying as a crucial process in pharmaceutical manufacturing.
Drying grains is important to allow for safe long-term storage by reducing moisture content to prevent spoilage. There are various drying methods like sun drying, convection drying using hot air or gases, and radiation drying using electromagnetic waves. Common dryers include flat bed dryers, mechanical tray dryers, and solar dryers which utilize the sun's energy. Proper drying is necessary to retain nutrients while sufficiently lowering moisture for storage.
Dehydration is the oldest method of food preservation that involves reducing the water activity in food below the threshold for microbial growth through drying. The key objectives of dehydration are to minimize chemical degradation, selectively remove water over other components, maintain product structure, and allow for long term storage stability with less refrigeration. Dehydration works by transferring heat through convection and conduction to evaporate water from the food in a drying curve that typically shows an initial heating period, constant drying rate period, and falling rate period as moisture becomes bound within the material.
1) Drying is an important process for producing stable and consistent pharmaceutical materials. It involves the transfer of energy, phase transformation of water or solvent from liquid to vapor, and removal of vapor.
2) Dryers can be classified based on heat transfer methods and processing type. Common dryers include tray dryers, rotary dryers, fluidized bed dryers, spray dryers, freeze dryers, and microwave/RF dryers.
3) Fluidized bed dryers suspend materials in an upward-moving stream of heated air or gas, allowing for uniform and rapid drying. They are useful for heat-sensitive materials and granule production.
This document discusses humidification during mechanical ventilation. It begins by outlining the objectives and introducing humidification as a way to artificially condition inspired gas. It then covers the indications for humidification when the upper airway is bypassed and describes how inadequate humidification can cause clinical signs. The document delves into the physiology of normal heat and moisture exchange and how this is disrupted during mechanical ventilation. It describes different types of humidifiers including passive heat and moisture exchangers and active bubble through, passover and nebulizer humidifiers. It outlines principles of humidifier function and discusses indications, contraindications and hazards of humidification. It concludes by assessing the need for different humidifiers and common problems.
This document provides an overview of drying processes. It discusses that drying involves removing moisture from solids, solutions, slurries, and pastes. There are various modes of heat transfer used in drying, including convection, conduction, and radiation. Common industrial drying equipment includes batch dryers like tray and agitated dryers, as well as continuous dryers like tunnel dryers, belt dryers, and turbo-tray tower dryers. Drying rates can be constant, falling, or exponentially decreasing depending on the moisture content and properties of the material being dried.
Drying & dehydration of horticultural cropsKrishiCareer
This document discusses the importance of fruits and vegetables and their production in India. It notes that fruits and vegetables are key sources of nutrients but that a significant percentage are wasted each year in India due to lack of infrastructure. Drying is presented as an effective method to increase shelf life and reduce waste. The document then covers various drying methods and considerations, including factors that affect the drying process and potential effects on foods. It provides examples of commonly dried fruits and vegetables.
This document discusses various methods for analyzing the macro and micronutrients in foods. It describes techniques for determining moisture, total solids, ash, crude fat, and crude protein content. Moisture content is measured by drying samples and comparing weights before and after. Ash content is measured by burning off organic compounds and weighing the remaining minerals. Crude fat is extracted using Soxhlet extraction and crude protein is measured via the Kjeldahl method. The appropriate sample preparation, equipment used, calculations, and advantages/disadvantages of each technique are outlined.
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.
10.
11. 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
13. Terms
• Dry bulb temperature: ambient temperature, not affected by
moisture content
• Wet bulb temperature: temperature of adiabatic saturation
14. • 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.
15. 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.
24. 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?
25. 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.
26. 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
27. 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
28.
29. 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.
30. Drying curves as a function
of drying time:
(a)drying kinetics
(b)(b) drying rate.
31. • 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.
32. 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.
33. 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
44. 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.
45. 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.