food preservation through dehydration

28
Food preservation through dehydration

Upload: revandifitro

Post on 02-May-2017

226 views

Category:

Documents


3 download

TRANSCRIPT

Page 1: Food Preservation through Dehydration

Food preservation through dehydration

Page 2: Food Preservation through Dehydration

Dehydration is an operation in which nearly all the water present in a food is removed by evaporation or sublimation under controlled conditions.

Preservation of foods by large reductions in water activity (many reactions retarded, microbial activity inhibited). Added advantage is the large savings in packaging, storage and transportation costs.

Page 3: Food Preservation through Dehydration
Page 4: Food Preservation through Dehydration
Page 5: Food Preservation through Dehydration

Main methods of drying used in food processing

1. Drying with heated air2. Drying by direct contact with a

heated surface,3. Drying through radiation, microwave

or dielectric source,4. Freeze drying

Page 6: Food Preservation through Dehydration

Basic concepts of drying

Moisture content = mass of water / unit mass of dry solids

Equil. moisture content = f ( temperature, humidity )

Sorption isotherms : curves relating the m.c. of the material and the humidity of the atmosphere with which it is at equilibrium at different temperatures.

Sorption isotherms of foods are also expressed as moisture content vs. water activity.

Page 7: Food Preservation through Dehydration

Water activity aw = P/Po (vapor pressure of food moisture/ saturation vapor pressure of pure water)T

Relative humidity of atmosphere = pw / ps (partial pressure of water vapor/ saturation partial pressure of pure water)T

At equilibrium: pw = P , ps = Po

Sorption behaviour of foods is important in:1. Studying mechanisms of drying and

designing dehydration processes,2. Predicting storage stability of dried foods.

Page 8: Food Preservation through Dehydration

Typical shape of a sorption isotherm

Page 9: Food Preservation through Dehydration
Page 10: Food Preservation through Dehydration

Sorption isothermsfor some selecteddried fruits

Page 11: Food Preservation through Dehydration

Estimation of drying times in hot air drying A wet, solid food being dried using hot air at constant temperature and humidity. The hot, dry air supplies both the sensible and the latent heat and carries away water vapor. All heat

is supplied by convection.

FOOD d

hot, dry air wet, cold air

Page 12: Food Preservation through Dehydration

According to the mechanism of drying, the typical drying cycle can be divided into three stages:

Settling down stage ( A-B )Constant drying rate stage ( B-C )Falling drying rate stage ( C-D )

Page 13: Food Preservation through Dehydration

changes in themoisture content

rate of changes in themoisture content

Drying curves

Page 14: Food Preservation through Dehydration

Settling down stage (A-B) The solid surface conditions come into

equilibrium with the drying air. The temperature of the food surface increases to the wet bulb temperature of the drying air.

Page 15: Food Preservation through Dehydration

Constant drying rate stage (B-C)

The surface of the food remains saturated with water. Rate of movement of water to the surface is larger than the rate of water removal from surface. T of surface remains constant at wet bulb T of air. Rate of mass transfer from food surface:

( dw/dt )c = - Kg A ( ps-pa)  (dw/dt)c is the drying rate, Kg is the mass transfer

coefficient, A is the drying surface area, ps is the water vapor pressure at surface and pa is the partial pressure of water vapor in air.

Page 16: Food Preservation through Dehydration

This equation can be expressed in terms of absolute humidities considering:Absolute humidity = (wt.of moisture/unit wt.of air) H = Mw pv / Ma (P-pv)  since pv << P , P - pv P . H = ( Mw/Ma) ( pv/P )  ( dw/dt )c = - Kg

1 A ( Hs - Ha ) , where Kg

1 = Kg ( Ma P / Mw )Hs is the humidity at surface (saturation humidity of air at surface temp.), Ha is the humidity of the air stream.

Page 17: Food Preservation through Dehydration

The rate of heat transfer to the drying surface:

( dQ/dt )c = hc A ( a - s )   ( dQ/dt )c is the rate of heat transfer, hc is the heat

transfer coefficient for convective heating, a is the temperature of drying air (dry bulb temp.) and s is the temperature at surface (wet bulb temp.).

The heat transferred to the drying surface will be used to evaporate water from surface:

  ( dw/dt )c L = - (dQ/dt ), L = latent heat of vaporization.  ( dw/dt )c = - (hc A/L ) ( a - s )  

Page 18: Food Preservation through Dehydration

In terms of the rate of change of moisture content W :

( dW/dt )c = - (hc A'/L ) ( a - s ) A' is the effective drying surface area per unit mass of dry

solids in the food. For a tray of material of depth d evaporating only from its

upper surface, assuming no shrinkage during drying, the rate of change in the moisture content of the dried solid can be expressed as:

  ( dW/dt )c = - (hc /s L d ) ( a - s )  s is the bulk density of the dry material. The drying time

necessary in the constant rate period can be estimated by integrating the expression subject to boundary conditions of W: (W0Wc), t: (0tc).

Page 19: Food Preservation through Dehydration

Falling drying rate stage (C-D)

Movement of moisture within the material is not sufficient to keep the surface saturated, the surface starts to dry out. The moisture content at this point is called the critical moisture content Wc. After this point the surface temperature begins to rise and approaches the dry-bulb temperature of air as material approaches dryness.

The rate of drying in this phase can be expressed by: ( dW/dt )f = - K ( W - We )

We is the equilibrium moisture content at air temperature and humidity (sorption isotherm)  

Page 20: Food Preservation through Dehydration

At the start of the falling rate period:   K = (-dW/dt )c / ( Wc - We )

Substituting: ( dW/dt )f = -hc ( a - s ) ( W - We ) / s L d ( Wc- We ) The drying time in the falling rate period can be found by

integrating this expression: at t=0, W=Wc and at t=tf, W=Wf

When drying takes place from both surfaces then d is equal to the 1/2 thickness of the plate.

Total drying time = tc + tf

Page 21: Food Preservation through Dehydration

Complexities in specific applications of food dehydration:

1. Food constituents; proteins, carbohydrates, fats, vitamins, enzymes, inorganic salts, have different hydration properties (aw, We, drying mechanism).

2. During drying solubles move with water (hc, s ) 3. The food shrinks (hc, s, d)4. Case hardening (hc)

Page 22: Food Preservation through Dehydration

Drying behaviour of single fillet piece. TD=30oC

Page 23: Food Preservation through Dehydration

Drying methods

Hot-air drying1. Kiln drier2. Cabinet, tray or compartment drier3. Tunnel drier4. Conveyor drier5. Bin drier6. Fluidized bed drier7. Pneumatic drier8. Rotary drier9. Spray drier

Drying by contact with a heated surface1. Drum drier2. Vacuum shelf drier3. Vacuum band drier

Page 24: Food Preservation through Dehydration

Drying by the application of energy from radiating, microwave or dielectric source.

1. Radiant heating drying2. Continuous infra-red drier3. Microwave and dielectric heating drying Freeze drying (sublimation drying, lyophilisation)1. Batch freeze-driers2. Multicabinet freeze-driers3. Tunnel freeze-driers

Dehydro freezing

Page 25: Food Preservation through Dehydration

Freeze drying

Product has a light, porous structure retaining the original shape and size.

Minimal shrinkage, reconstitution characteristics are good both in the rate and extent.

Reconstitutability of dried foods is the rate and extent to which dried foods absorb water and revert to a condition resembling the undried material when put in contact with sufficient water.

Page 26: Food Preservation through Dehydration

Freeze drying

Drying is carried out under very high vacuum, (below 300 N/m2 0.003atm.). Processing pressures between 13.5-270 N/m2, at a range of temperatures. Quality of final product is sensitive to the processing pressure-temperature combination.

Commercial applications: instant tea, coffee, shrimps, prawns, some rare fruits (berries) and vegetables; products for which flavor and reconstitutability are important quality factors

Page 27: Food Preservation through Dehydration

Phase diagram of water

Page 28: Food Preservation through Dehydration

Basic elements of a freeze-drying system