Chapter 8 DEHYDRATION

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Chapter 8 DEHYDRATION

• STATE OF WAER IN FOODS
• EFFECTS OF DRYING ON PRODUCT QUALITY
• MOISTURE SORPTION AND DESORPTION
• RATE OF DEHYDRATION
• FACTORS THAT INFLUENCE DRYING
• DRYING METHODS
• SPRAY DRYING
• FREEZE DRYING

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Vocabulary

• Drying, dehydrate, rehydrate, equilibrium
  relative humidity, water activity, isotherms
  sorption desorption hysteresis behavior
  hypothesis capillary semiempirical empirical,
  critical moisture content

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DEHYDRATION

• Drying of foods is an important food processing
  operation used to preserve foods. The
  distinguishing features between drying and
  concentration are the final level of water and
  nature of the product. Concentration leaves a
  liquid food, whereas drying typically produces
  product with water content sufficiently low to
  give solid food.

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Reasons for drying foods

• Historically, there was a need to preserve foods
  for longer times so that food was available
  during times of limited food production or
  availability. Hunters needed a technique to
  preserve meat for more than a few days to ensure
  a continuous food supply In the same manner, we
  have techniques that allow us to preserve foods
  as they are harvested, so that we can enjoy them
  at later times.

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Reasons for drying foods

• One of the easiest ways to preserve foods is to
  remove water, since microorganisms need water to
  survive and grow, and many chemical reactions
  require water to proceed. Early hunters dried
  their meat to help maintain a more continuous
  food supply. Nowadays, we dry foods for the same
  reason to provide a continuous supply of foods
  that we can enjoy at any time.

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Other reasons for drying foods

• Removal of water leaves a product reduced in
  weight and often in bulk. This reduces shipping
  costs and makes the food supply more economical.
  Dried foods also provide convenience. Dried
  convenience foods may be used for special
  expedition--type (military) foods where weight is
  a major concern.

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• There are many methods and technologies by which
  we can dehydrate foods. We must first understand
  the nature of water in food products to
  appreciate the difficulties in producing
  high-quality dried products. Removal of water
  from foods is not a difficult task. However,
  removing the water in such a way that the product
  regains its initial form when rehydrated is not
  so easy.

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STATE OF WAER IN FOODS

• In dehydration, it is important to understand the
  behavior of water so that it can be removed most
  effectively and still leave a high-quality
  product. Food technologists often use the
  thermodynamic measure of water activity to
  describe how water interacts in food products.

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Water activity

• Water activity (aw)is defined as the ratio of the
  vapor pressure of water measured at the food
  surface (Pw)to the saturation vapor pressure of
  pure water at the same temperature (Pwº)

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Water activity

• For a cup of water, the vapor pressure over the
  surface is measured as the saturation vapor
  pressure, and aw is 1. When there are solutes in
  the water such as sugars, salts, etc. the vapor
  pressure over the water surface is lower than the
  saturation vapor pressure, and aw is reduced to
  some value less than 1. The reduction in water
  activity depends on the type of solutes present
  and their levels.

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Water Activity

• For food products, the water activity is
  generally less than 1. aw is related to the
  moisture content of the food, the types and
  concentrations of different solutes, and the
  structure or physical characteristics of the
  food.

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Relationship between RH aw

• The water activity of a food can be related to an
  equilibrium relative humidity in the air around
  the product. That is, at only one relative
  humidity will the air be in moisture equilibrium
  with the food product where the food neither
  gives up or adsorbs water. This relative humidity
  is the "equilibrium relative humidity" or ERH.

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Free" water "bound" water

• In the past people simplified the state of water
  in foods by denoting two types "free" water or
  "bound" water. "free" water or "bound" water. The
  working definition for these terms is Free water
  is that which gives water activity of 1, bound
  water gives water activity less than 1.

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• Free water is relatively easy to remove from a
  food product while bound water takes more energy
  to release from the food. Thus, the latent heat
  required to remove a molecule of water from a
  food increases as the water activity decreases.
  This is important to those who design drying
  operations, since the energy requirement to
  provide sufficient driving force for drying is
  related to the latent energy of vaporization.

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physical changes

• As a food product dries out and the water
  molecules become less mobile, physical changes
  also occur in the food. As water is removed, the
  remaining product generally becomes increasingly
  viscous. The product may go through several
  regions of properties, where viscosities are
  intermediate between a pumpable liquid and a
  stationary solid.

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The state diagram

• For a simple system of solute and solvent The
  glass transition curve represents a metastable
  transition where viscosity is so high that the
  product does not "flow". Below this curve, the
  food is stable to diffusion-limited processes for
  extremely long times

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• For example, powdered milk products remain dry
  and stable when maintained below the glass
  transition temperature. However, if the powder
  picks up moisture from the air or experiences
  elevated storage temperature, it may exceed the
  glass transition curve and be less stable. In
  this case, powdered milk would be likely to get
  sticky, and the powder would cake together.

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EFFECTS OF DRYING ON PRODUCT QUALITY

• After rehydrating the food cannot reach the
  original quality. There is always some change
  that gives a loss of quality in the product. The
  goal is to minimize these changes, while
  optimizing process efficiency and minimizing
  costs. Several types of changes can occur during
  drying. Two main problems are loss and change of
  flavors, and change in physical qualities of
  dried products.

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Effect on flavor

• One problem with dried foods is that the flavor
  of the rehydrated product is not the same as that
  of the original. During drying, flavor compounds
  that are typically more volatile than water are
  removed in the drying process. The physical
  forces that cause water molecules to be removed
  from the food during drying also cause volatile
  compounds (alcohols, aldehydes, ketones, etc.) to
  be removed.

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burnt flavor

• Dried products have less of these volatile
  flavoring compounds than the original starting
  material. In addition, the rates of chemical
  reactions are enhanced at the elevated temp., and
  many of these reactions generate undesired flavor
  compounds. For example, the browning reaction
  (between reducing sugars and proteins) is
  enhanced and generates a burnt flavor.
  (reconstituted milk from a dried powder)

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Browning

• Other chemical reactions may also take place
  during drying. Browning occurs in many foods
  which results in color changes. Protein
  denaturation can occur during drying, which
  causes increasd viscosity, Thermal degradation of
  vitamins and proteins may also influence the
  nutritional status of dried products.

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• The extent of these changes depends on the nature
  of the drying process. Some types of dryers
  produce products having superior properties on
  reconstitution. The instant coffee spray-dried
  and freeze-dried is different. Since
  freeze-drying does not involve a vapor-liquid
  interface, the volatile flavor and aroma
  compounds are not lost during drying, and
  freeze-dried products have higher quality

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MOISTURE SORPTION AND DESORPTION

• During drying, both moisture content and water
  activity change. At any given relative humidity
  of air used for drying, there is an equilibrium
  water content with the product, At this point the
  activity of water in the air is the same as that
  in the product. This relationship specifies the
  water content in a food product that can be
  reached for any condition of drying air.

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Isotherms

• By holding a food product in air at different
  relative humidities and measuring the equilibrium
  water content, the curve of water content and
  water activity can be obtained. Its nature
  depends on whether the food product is being
  dried or allowed to pick up moisture from the
  air. The direction of the experimental
  measurement affects the relationship between
  water content and water activity. Isotherms

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MOISTURE SORPTION AND DESORPTION

• Moisture sorption (picking up water) curves
  typically are slightly lower in water contents
  than moisture desorption (drying) curves. Several
  mechanisms have been proposed for this hysteresis
  behavior.
• capillary forces
• volume expansion

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RAT E OF DEHYDRATION

• In drying, water molecules must make their way
  through the food to the surface (internal
  resistance to drying) in contact with drying air.
  Once at the surface, water molecules are
  transfered into the air (external resistance to
  drying) based on the difference in vapor pressure
  between the air and the surface. When the vapor
  pressure in the air reaches the same value as the
  vapor pressure of water at the surface of the
  food, drying ceases.

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• The rate of drying may be limited by either the
  rate of internal migration of water molecules to
  the surface or the rate of evaporation of water
  molecules from the surface into the air,
  depending on the conditions of drying. In fact
  most foods switch from an external drying process
  during initial stages to an internal drying
  process as the product dries out.

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RAT E OF DEHYDRATION

• Drying Curves
• Constant Rate Period
• Falling Rate Period

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Drying Curves

• A curve of loss of moisture during drying of a
  food product are typically generated by weighing
  a sample of food undergoing drying and relating
  weight loss to moisture content. Moisture content
  is most often expressed as kg of water per kg of
  dry product (or matter).

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• The kg of dry matter (initial product weight
  minus weight from water) are always constant
  during drying, so a constant reference point is
  used when referring to drying in kg water/kg dry
  matter.

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• The shape of the drying curve is similar for many
  food products. After short initial equilibration
  period (for thermal equilibration), the moisture
  content decreases rapidly, and almost linearly,
  with time. This initial drying period is followed
  by a much slower rate of drying as the moisture
  content of the product decreases. The rate of
  drying is the slope of the moisture content
  change with time, expressed in kg water/kg dry
  matter-minute.

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constant rate period

• drying rate is plotted against the moisture
  content (instead of time). Since moisture content
  goes from high to low during drying, the initial
  drying condition is given by the point at the
  right of the graph. Initially, the rate of drying
  may be nearly constant until some critical
  moisture content Xc, is reached. Xc represents
  the moisture content where drying changes from
  constant rate to falling rate. This initial
  period of constant rate drying is called the
  "constant rate period," or CRP.

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falling rate period

• After the product is dried below Xc, the rate of
  drying decreases. This is called the "falling
  rate period," or FRP. Here, drying rate depends
  on the moisture content remaining in the product.
  If the product is dried extensively, the product
  eventually equilibrates with the drying air. The
  equilibration point depends on temperature and
  relative humidity of the air used in the dryer.

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Constant Rate Period

• The initial rate of drying-- The rate at which
  water molecules arrive at the surface by
  migration from the interior is greater than (or
  equal to) the rate at which water molecules are
  lost from the surface to the drying air. So there
  is sufficient water to be evaporated, the thermal
  energy to the food is used as latent heat, the
  temp. of food is not elevated.

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Energy equation

• For the simplest case, where only convective heat
  transfer occurs, all of the heat energy goes into
  vaporizing moisture during the constant rate
  period. That is, the rate of heat transfer into
  the product is balanced by the rate of energy
  removal due to the vaporizing moisture. The rate
  of energy removal with vaporized water can be
  found as the product of the rate of drying and
  the latent heat of vaporization. That is, for
  each molecule of water vaporized at the surface
  (liquid to vapor), an amount of energy equivalent
  to the latent heat of vaporization is required.

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• The constant rate drying period lasts as long as
  the rate of moisture migration from the interior
  of the product to the surface is sufficiently
  rapid that the moisture content at the surface is
  constant. At the point where moisture migration
  from the interior is slower than the surface
  vaporization, the constant rate period ends and
  the time for constant rate drying, tCRP can be
  found as

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Falling Rate Period

• After the critical moisture point, the rate at
  which moisture migrates to the surface limits
  drying. That is, the rate of moisture loss from
  the surface to the drying air is faster than the
  rate at which that moisture is replenished at the
  surface.

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Mechanisms of internal mass transfer

• 1. Liquid diffusion.
• 2. Vapor diffusion.
• 3. Capillary flow.
• 4. Pressure flow.
• 5. Thermal flow.

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FACTORS THAT INFLUENCE DRYING

• 1.Process Conditions
• Temperature/Air Velocity/Relative
  Humidity/Pressure
• 2.Food Properties
• Surface Area/Constituent /Orientation/Cellular
  Structure/ Type and Concentration of Solutes.

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DRYING METHODS

• 1.Direct Contact Dryers
• Sun Dryer/Bin Dryer/Kiln Dryer/ Tray or Cabinet
  Dryer/Tunnel Dryer/Belt or Conveyor Dryer/
  Fluidized Bed Dryer/Rotary Air Dryer/Spray Dryer.
  
• 2.Indirect Contact Dryers
• Drum Dryer
• 3.Infrared or Dielectric Dryers
• Infrared Dryers/Microwave Dryers.

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Tray or Cabinet Dryer

• The food product placed in a pan is placed inside
  a drying chamber with hot air blowing across the
  product until drying is complete. Some of the hot
  air used for drying may be recirculated through
  the dryer to conserve energy However, increased
  relative humidity of the recirculated air
  decreases dryer efficiency.

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Tunnel Dryer

• The food product is loaded onto trays that are
  placed into carts. The carts are input at one end
  of the tunnel dryer and move through to the
  outlet. Air blowing within the tunnel causes
  drying at a specified rate, so that the food
  product reaches the exit on completion of drying.
  (1) cocurrent (2) countercurrent or (3) mixed
  flow,

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Belt or Conveyor Dryer

• Product may also be moved through a dryer by
  placing it on a belt or conveyor. In order to
  extend the time within a conveyor dryer, a series
  of conveyors may be arranged one above the other.
  In this case, product drops from an upper
  conveyor to a lower conveyor. air flow can be
  through the conveyor and through the bed of food
  product laying on the conveyor.

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Fluidized Bed Dryer

• Air flow through a bed of product is sufficient
  to lift the product. Since there is intimate
  contact between air and product drying rates in
  this type of dryer are quite good. This type of
  dryer is limited to granulated powders, or small
  pieces of product. The air velocity depends on
  particle size, density.

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Rotary Air Dryer

• These dryers are typically arranged as horizontal
  cylinders that rotate along their main axis. Wet
  product enters one end of the dryer and moves
  towards the other end by a combination of gravity
  and the baffle arrangement within the cylinder.
  As the cylinder rotates, air forced into the
  cylinder blows across the product as it tumbles,
  to provide effective contact between air and
  product.

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Vocabulary

• Spray drying, high pressure nozzle, droplet
  atomization, centrifugal atomizer, spinning disk,
  dryer chamber, ambient air, accelerate, cyclone
  separator, tangentially, segregate, conical,
  stickiness and agglomeration, reconstitution,
  dispersability

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SPRAY DRYING

• Atomization
• Air Handling
• Dryer Chamber
• Powder Separation

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Atomization

• Atomization produces a cloud of droplets with
  very large surface area for drying.
• High pressure nozzle droplet size is controlled
  by pressure of the fluid food against the nozzle.
• Centrifugal atomizer Liquid food is pumped into
  a spinning disk, where it is accelerated by
  centrifugal force and expelled from the ends of
  the disk-shaped atomizer, become a cloud of
  droplets .

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Air Handling

• Ambient air is taken in through a vent and heated
  prior to circulation into the drying chamber.
  Heating can be accomplished in several ways. Air
  can be passed either through steam coils or an
  electric heater to attain elevated temperatures,
  typically between l50 and 500?.

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Dryer Chamber

• In the residence time of droplets in the spray
  drying chamber, the droplets go from a moisture
  content in the range of about 40 to only about
  5-10.
• The food droplets are sprayed at the top of the
  chamber and fall down to the bottom by gravity.
  Both air and food droplets enter the chamber at
  the top and fall to the bottom of it, where air
  is separated from dry powder and the product is
  removed from the dryer.

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Powder Separation

• Primary separation of powder is accomplished by
  gravitational setting of the heavier powder
  particles. Separation of air and finer powder
  particles is usually accomplished in a cyclone
  device. The stream is circulated tangentially
  into the cyclone separator. Centrifugal force
  causes the particles to segregate from the air
  and settle to the bottom of the conical
  separator.
• A textile or bag filter is sometimes used.

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Drying in the Spray Dryer

• Constant Rate Drying.
• Falling Rate Drying.
• Stickiness and Agglomeration
• Product Quality

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Stickiness and Agglomeration

• During spray atomization, the particles are
  sprayed outwards towards the wall of the drying
  chamber. If these droplets have not sufficiently
  dried when they come in contact with the wall,
  they stick and form a scale on the inside of the
  drying chamber or stick to one another to form
  agglomerated particles. The chamber must be
  designed to ensure that the droplets have dried
  sufficiently so that they are no longer sticky as
  they approach the chamber wall.

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Product Quality

• The ability to be wetted by water during
  reconstitution, dispersability of the powder into
  water and solubility in water. Changes in product
  attributes, particularly at the case- hardened
  surface of the droplets, decreases the ability of
  a powder to be wetted and dispersed into water.
  Agglomeration of particles may also influence the
  amount of surface available for wetting.

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Agglomeration Process, or Instantizating

• In some cases, wettability and dispersability of
  a powder are enhanced by an agglomeration
  process, or instantizing, immediately following
  the spray dryer. In an instantizer, the surface
  of powder particles is slightly wetted by a fine
  spray of steam. These particles are fluidized in
  air to cause contacts between individual
  particles, forming agglomerated powders, with
  enhanced wettability and dispersability.

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Vocabulary

• Freeze drying, sublimation, sublimation front,
  primary and secondary drying stages, radiation
  collapse, ice crystals, pore, porous channels,
  diffuse, stress cracking, rehydrate

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FREEZE DRYING

• Moisture is removed from the solid state (ice)
  directly to the vapor state by sublimation.
  Drying actually occurs in two steps, primary and
  secondary drying stages. It is in the primary
  stage that water is removed by sublimation,
  whereas vaporization of unfrozen liquid water
  molecules occurs in the secondary stage of
  drying.

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Product quality of Freeze Drying

• The original structure of the food is maintained
  and flavor retention in is excellent.
• The cost of freeze drying is very high.

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Steps in Freeze Drying

• Freezing.
• Primary Drying.
• Secondary Drying.
• Heat and Mass Transfer in Freeze Drying

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Primary Drying

• Sublimation of ice is accomplished by controlling
  the vacuum level in the freeze dryer and through
  careful heat input. A high vacuum is desired to
  enhance sublimation rate.
• Introduction of heat is to supply energy to a
  plate on which the food is sitting (conduction
  heating), while also providing a radiation source
  above the product.

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Sublimation Front

• The ice recedes into the food product as drying
  occurs. This boundary between frozen and dried
  product is called the sublimation front. Heat
  must be transfered into the product to this front
  to promote sublimation, and water vapor must then
  be removed by mass transfer through the dried
  product

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Secondary Drying

• Once all the ice is sublimed out of the frozen
  food, the secondary drying process begins. Heat
  is continually added, but at a slower rate since
  moisture loss occurs only by diffusion of water
  molecules out of the freeze--dried matrix.

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Collapse Behavior

• Rapid heat addition causes the temperature of the
  product to exceed its collapse temperature.
  product becomes suffiiently flowable that it
  "collapses!' During collapse, the pockets where
  ice crystals have sublimed disappear as the food
  slowly flows into these regions. This causes
  product to have higher density and reduces its
  ability to be rehydrate.

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Vapor Condensor

• A condensor collects the vapors as they exit the
  freeze dryer to enhance efficiency and prevent
  fouling of the vacuum pump.

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Front View of Freeze Dryer
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Heating Plate in the Dryer
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Trays
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Moving Shelves for Trays
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Condenser Vacuum System
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Vacuum System
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Bird View of Workshop
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Problems

• Describe the definition of water activity.
• Describe the relationship between water activity
  of foods and the relative humidity of environment
  .
• What are the CRP FRP? Describe the cause of
  them and the differences between them.
• Describe principles of tunnel dryer and spray
  dryer.

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Problems

• Describe the principles of freeze drying.
• How to understand the concept of collapse during
  freeze drying.
• What are the purposes of each step in freeze
  drying.