Abstract
Evaporation is a food preservation technique in which dilute liquid foods and solutions are concentrated by the evaporation of water, with the aim of increasing microbiological stability and shelf life. A second major reason for the concentration of liquids is the reduction in transport and storage costs which can be achieved by reducing the product bulk volume. In this way concentrated liquids can be transported at relatively low cost and water added later, closer to the point of sale. In addition, evaporation can be used to increase the concentration of solutions prior to the removal of the remaining water by drying, particularly by spray drying. This is an attractive option because high-efficiency evaporation is significantly less costly than drying and other methods of removing water as the data in Table 12.1 show.
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Abbreviations
- a W :
-
Water activity
- A :
-
Area
- A mean :
-
Average heat transfer surface area in a multiple effect evaporator
- b :
-
Constant
- C :
-
Mass flow rate of condensate
- D e :
-
Effective diffusivity of water vapour through the solid dried layer
- ERH:
-
Equilibrium relative humidity
- F :
-
Mass flow rate of feed
- h :
-
Enthalpy; heat transfer coefficient
- k :
-
Thermal conductivity
- k G :
-
Mass transfer coefficient
- L :
-
Mass flow rate of concentrated liquor; thickness of food block
- m :
-
Constant
- M w :
-
Molecular weight of water
- n :
-
Number of evaporator effects
- N w :
-
Molar flux of water vapour
- p w :
-
Partial pressure of water vapour
- p wf :
-
Partial pressure of water vapour in equilibrium with ice front
- \({p_{{{\textrm{w}}_{\textrm{1}}}}}\) :
-
Partial pressure of water vapour in the surrounding air
- \({p_{{{\textrm{w}}_{\textrm{2}}}}}\) :
-
Partial pressure of water vapour at the surface of the block
- p ′ w :
-
Pure component vapour pressure of water
- q :
-
Heat flux
- Q :
-
Rate of heat transfer
- R :
-
Rate of drying; universal gas constant
- R c :
-
Rate of drying in the constant rate period
- S :
-
Mass flow rate of steam
- t :
-
Time; freeze-drying time
- T :
-
Temperature; average temperature of dried layer
- T 1 :
-
Temperature of surrounding air
- T E :
-
Evaporator temperature
- T f :
-
Temperature of frozen layer
- U :
-
Overall heat transfer coefficient
- U av :
-
Average overall heat transfer coefficient
- U V :
-
Mass flow rate of vapour
- V S :
-
Volume of solids occupied by the initial unit mass of water present
- W :
-
Mass of dry solid
- x :
-
Mass fraction of solute or non-soluble solids; fraction of initial free moisture remaining
- X :
-
Moisture content (dry basis)
- X c :
-
Critical moisture content
- X e :
-
Equilibrium moisture content
- X 1 :
-
Initial moisture content
- X 2 :
-
Final moisture content
- y :
-
Mass fraction (vapour phase)
- γ :
-
Activity coefficient
- ΔL :
-
Thickness of dried layer
- ΔT :
-
Temperature difference
- λ S :
-
Latent heat of sublimation of ice
- C:
-
Condensate
- f:
-
Saturated liquid; ice front
- F:
-
Feed
- g:
-
Saturated vapour
- L:
-
Concentrated liquor
- S:
-
Steam
- V:
-
Vapour
- w:
-
Water; water vapour
Further Reading
Baker, C. G. J., (ed.). 1996. Industrial drying of foods. London: Chapman and Hall.
Hall, C. W., Farrall, A. W., and Rippen, A. L. 1986. Encyclopaedia of food engineering, 2nd ed. Westport, CT: AVI Publishing.
Heldman, D. R. and Hartel, R. W. 1997. Principles of food processing. London: Chapman and Hall.
Keey, R. B. 1978. Introduction to industrial drying operations. Oxford: Pergamon.
MacCarthy, D., (ed.). 1986. Concentration and drying of foods. London: Elsevier Applied Science.
Masters, K. 1991. Spray drying handbook. London: Longman.
Mellor, J. D. 1978. Fundamentals of freeze-drying. London: Academic.
Oetjen, G.-W. and Haseley, P. 2003. Freeze-drying. Weinheim: Wiley-VCH.
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Smith, P. (2011). Evaporation and Drying. In: Introduction to Food Process Engineering. Food Science Text Series. Springer, Boston, MA. https://doi.org/10.1007/978-1-4419-7662-8_12
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DOI: https://doi.org/10.1007/978-1-4419-7662-8_12
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