526 13 Food preservation through dehydration

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Information about 526 13 Food preservation through dehydration

Published on January 8, 2008

Author: Dora



Food preservation through dehydration:  Food preservation through dehydration Slide2:  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. Slide5:  Main methods of drying used in food processing Drying with heated air Drying by direct contact with a heated surface, Drying through radiation, microwave or dielectric source, Freeze drying Basic concepts of drying:  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. Slide7:  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: Studying mechanisms of drying and designing dehydration processes, Predicting storage stability of dried foods. Slide8:  Typical shape of a sorption isotherm Slide10:  Sorption isotherms for some selected dried fruits Estimation of drying times in hot air drying:  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. 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 ) Slide12:  changes in the moisture content rate of changes in the moisture content Drying curves Slide13:  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. Slide14:  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. 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. Slide15:  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 = - Kg1 A ( Hs - Ha ) , where Kg1 = 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. Slide16:  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 )   Slide17:  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: (W0Wc), t: (0tc). Slide18:  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)   Slide19:  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 Slide20:  Complexities in specific applications of food dehydration: Food constituents; proteins, carbohydrates, fats, vitamins, enzymes, inorganic salts, have different hydration properties (aw, We, drying mechanism). During drying solubles move with water (hc, s ) The food shrinks (hc, s, d) Case hardening (hc) Slide21:  Drying behaviour of single fillet piece. TD=30oC Drying methods:  Drying methods Hot-air drying Kiln drier Cabinet, tray or compartment drier Tunnel drier Conveyor drier Bin drier Fluidized bed drier Pneumatic drier Rotary drier Spray drier Drying by contact with a heated surface Drum drier Vacuum shelf drier Vacuum band drier Slide23:  Drying by the application of energy from radiating, microwave or dielectric source. Radiant heating drying Continuous infra-red drier Microwave and dielectric heating drying Freeze drying (sublimation drying, lyophilisation) Batch freeze-driers Multicabinet freeze-driers Tunnel freeze-driers Dehydro freezing Freeze drying:  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. Freeze drying:  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 Slide26:  Phase diagram of water Slide27:  Basic elements of a freeze-drying system

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