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Dehydration

Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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Page 1: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Dehydration

Page 2: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 3: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 4: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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)

Page 5: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Moisture-Sorption Isotherm with Hysteresis

5

Page 6: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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.

Page 7: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 8: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Tray Dryer

Page 9: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Tunnel Dryer

Air exhaust (may go to preheating incoming air)

Temperature control

Product outFeed in

Dry air

Air handling unit

Steam

Moist air

Page 10: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Vacuum Drum Dryer

Page 11: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Fluidized Bed Dryer

Page 12: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Spray Dryer

Spraying (atomization) of product increases surface area of contact with hot air for drying

Page 13: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 14: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 15: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Freeze Dried Foods• Coffee• Meats• Soups• Vegetables, fruits• Ice-cream

– Apollo 7 (1968); sweet, sticky, crumbles easily

Page 16: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 17: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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

Page 18: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

Windhexe Dryer in Operation

Video

Page 19: Dehydration - Nc State University · Outline •Evaporation versus dehydration •Purposes of dehydration •Moisture-Sorption Isotherm •Drying-rate curve •Types of dryers –Tray/cabinet

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