dehydration - nc state university · outline •evaporation versus dehydration •purposes of...
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Dehydration
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Outline
• Evaporation versus dehydration• Purposes of dehydration• Moisture-Sorption Isotherm• Drying-rate curve• Types of dryers
– Tray/cabinet (with or without vacuum), tunnel, puff, fluidized-bed, spray, freeze, windhexe
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Evaporation versus Dehydration
• Evaporation– Concentration of a liquid product to a liquid product
• Involves partial removal of water
• Dehydration– Conversion of a liquid product to a dry (solid) product
• Involves (almost) complete removal of water
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Purposes of Dehydration
• Preservation– Reduction in rate of microbial growth and other
chemical reactions at low water activity• Reduction in transportation and storage costs• Possible savings in packaging costs (packaging a
solid product as opposed to a liquid product)
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Moisture-Sorption Isotherm with Hysteresis
5
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Drying-Rate Curve
Moisture content on dry basis (kg water / kg dry solids)
Dry
ing
rate
(kg
wat
er /
m2
s) A
BC
D
E
Constant rateFalling rate
A-B: Temp. increasesB-C: Temp. is constant
Below wc, rate of moisture removal dec.
wc: critical m.c.
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Types of Dryers• Based on design or process used
– Tray, cabinet, drum, spray, tunnel, fluidized bed, vacuum-freeze, atmospheric-freeze, filtermat, dielectric, ultrasonic, flash, through, vacuum, paddle, rotary, air impingement, spouted, contact-sorption, heat pump, pulse combustion, plasma torch, Carver-Greenfield process, organic solvent vapor, slush, shock wave
• Based on media– Air, superheated steam, flue gases
• Based on mode of operation– Batch, continuous
• Based on number of stages– Single, multiple
Emerging/alternative technologies: Microwave, infrared, windhexe
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Tray Dryer
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Tunnel Dryer
Air exhaust (may go to preheating incoming air)
Temperature control
Product outFeed in
Dry air
Air handling unit
Steam
Moist air
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Vacuum Drum Dryer
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Fluidized Bed Dryer
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Spray Dryer
Spraying (atomization) of product increases surface area of contact with hot air for drying
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Freeze DryerProduct is frozen and water is removed by sublimation at low pressure
Instant coffee powder is produced by freeze drying or spray drying
Door
ShelvesMushroom valve(open position)
Condenser
Vacuumpump
ConnectingDuct
ChamberCondenserProduct
Chamber
WaterVaporFlow
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Phase Diagram (For Freeze Drying)
Temperature (°C)
Pres
sure
(atm
)
0.006
1.0
0.01 100
Solid Liquid Gas
Melting point
Boiling pointTriple point
Water
Temperature (°C)
Pres
sure
(atm
)5.1
1.0
-56.6
Solid Liquid Gas
Triple point
CO2
-78.5
Phase changes: Melting/freezing, boiling/condensation, sublimation/deposition
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Freeze Dried Foods• Coffee• Meats• Soups• Vegetables, fruits• Ice-cream
– Apollo 7 (1968); sweet, sticky, crumbles easily
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Windhexe Drying
• One-step milling and drying using high pr. hot air– Eliminates additional equipment and process
• Low capital cost• Simple construction
– No moving parts– No issues with clogged spray nozzles
• Applicable to chunks of food– Does not have to be a pumpable slurry
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Windhexe Drying: Power of a TornadoWindhexe Dryer
High speed air pulverizes food, plastic, glass, metal, and removes any moisture in it
One step milling and drying
Drying purple sweetpotato
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Windhexe Dryer in Operation
Video
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Summary• While evaporation involves some removal of water,
dehydration involves almost complete removal of water from a product
• Dehydration is a preservation technique and it results in minimization of transportation costs
• Drying processes usually involve rise in temperature of the product followed by a constant rate of drying and finally a falling rate of drying
• Various types of dryers (batch & continuous) exist for drying foods
• Freeze drying usually results in best product quality– It is however slow and also expensive