1. ERT 426 Food Engineering Semester 1 Academic Session 2017/18
Drying / Dehydration
ERT Food Engineering
Semester 1 Academic Session 2017/18

2. Subtopics Drying / Dehydration Drying using heated air
Mechanism of drying
Drying rate
Spray drying
Freeze drying

3. Subtopics
Equipment
Effect of drying on food quality
Rehydration

4. 1. Drying / Dehydration
Drying (or Dehydration) - ‘the application of heat under controlled conditions to remove the majority of the water present in a food by evaporation’ or sublimation (freeze drying).
The main purpose of dehydration is to extend the shelf life of foods by a reduction in water activity.
This inhibits microbial growth and enzyme activity.
The reduction in weight and bulk of food reduces transport and storage costs.

5. Drying / Dehydration
For some types of food, dehydration provides a convenient product for the consumer or more easily handled ingredients for food processors.
Drying causes deterioration of both the eating quality and the nutritional value of the food.
The design and operation of dehydration equipment aim to minimise these changes by selection of appropriate drying conditions for individual foods.

6. 2. Drying using heated air
Dehydration involves the simultaneous application of heat and removal of moisture from foods.
There are three inter-related factors that control the capacity of air to remove moisture from a food:
the amount of water vapour already carried by the air
the air temperature
the amount of air that passes over the food

7. Drying using heated air
The amount of water vapour in air is expressed as either absolute humidity (termed moisture content) or relative humidity (RH) (in per cent).
Psychrometry is the study of inter-related properties of air–water vapour systems.
These properties are conveniently represented on a psychrometric chart.

8. Drying using heated air
Moisture content equals the mass of water vapour per unit mass of dry air (in kilograms per kilogram).
Relative humidity defined as ‘the ratio of the partial pressure of water vapour in the air to the pressure of saturated water vapour at the same temperature, multiplied by 100.

9. 3. Mechanism of drying
The rate of drying is controlled by air temperature, humidity, & air velocity.
When hot air is blown over a wet food, water vapour diffuses through a boundary film of air surrounding the food and is carried away by the moving air.
A water vapour pressure gradient is established from the moist interior of the food to the dry air.
This gradient provides the ‘driving force’ for water removal from the food.

10. Figure 1: Movement of moisture during drying.
Mechanism of drying
Figure 1: Movement of moisture during drying.
The boundary film acts as a barrier to both heat transfer and water vapour removal (mass transfer) during drying.

11. Mechanism of drying
The thickness of the film is determined primarily by the air velocity; if the velocity is low, the boundary film is thicker
this reduces the heat transfer coefficient & the rate of removal of water vapour.
Water vapour leaves the surface of the food and increases the humidity of the surrounding air, to cause a reduction in the water vapour pressure gradient and hence the rate of drying.
the faster the air, the thinner the boundary film & hence the faster the rate of drying.

12. Mechanism of drying
Three (3) characteristics of air that are necessary for successful drying when the food is moist are:
a moderately high dry-bulb temperature
a low RH
a high air velocity.

13. Mechanism of drying Figure 2: Drying curves.
The temperature and humidity of the drying air are constant and all heat is supplied to the food surface by convection.

14. Mechanism of drying
Water moves from the interior of the food to the surface by the following mechanisms:
liquid movement by capillary forces, particularly in porous foods
diffusion of liquids, caused by differences in the concentration of solutes at the surface and in the interior of the food
diffusion of liquids which are adsorbed in layers at the surfaces of solid components of the food

15. Mechanism of drying
water vapour diffusion in air spaces within the food caused by vapour pressure gradients.
Other factors which influence the rate of drying include:
The composition and structure of the food
The amount of food placed into a drier in relation to its capacity.

16. 4. Drying rate
When simple drying behaviour is known and data on critical and equilibrium moisture contents or thermal properties of foods are known, drying times can be estimated by calculation.
However, this data is not available for many foods and results of pilot scale drying trials are used to estimate drying times.

17. Drying rate The rate of heat transfer is found using:
The moisture content of a food may be expressed on a wet weight basis (Mwater /Mwet food) or a dry weight basis (Mwater /Mdry solid).
The rate of mass transfer is found using:
Rate of heat transfer (J/s)
Surface heat transfer coefficient for convective heating (Wm-2K-1)
Dry bulb temperature of drying air (0C)
Wet bulb temperature of drying air (0C)
Surface area (m2)
Humidity of air (kg/kg)
Change of mass with time (drying rate) (kg/s)
Mass transfer coefficient (kg m-2 s-1)
Humidity at the surface of the food (kg/kg)

18. for perpendicular air flow:
Drying rate
During the constant-rate period, an equilibrium exists between the rate of heat transfer to the food and the rate of mass transfer in the form of moisture loss from the food, these rates are related by
for parallel air flow:
for perpendicular air flow:
G (kgm-2s-1) = mass flow rate of air / area.
latent heat of vaporisation at the wet bulb temperature (J/kg)

19. Drying rate
For a tray of food, in which water evaporates only from the upper surface, the drying time is found using :
The drying time in the constant rate period is found using:
Bulk density of food (kg m-3)
Thickness of the bed of food (m)
Initial moisture content (kg/kg)
Thickness of the bed of food (m)
Critical moisture content (kg/kg)
Bulk density of food (kg m-3)
Drying time (s)

20. Drying rate
For water evaporating from a spherical droplet in a spray drier, the drying time is found using:
Density of the liquid (kg m-3)
Final moisture content (kg/kg)
Radius of the droplet (m)

21. Drying rate
To calculate the drying time from the start of the falling-rate period to the equilibrium moisture content (Assume the nature of moisture movement and the absence of shrinkage of the food).
Equilibrium moisture content (kg/kg)
Mass transfer coefficient (kg m-2 s-1)
Moisture content at time t from the start of the falling-rate period (kg/kg)
Partial water
vapour pressure (Torr)
Saturated vapour pressure at the wet bulb temperature (Torr)

22. 5. Spray Drying
The drying of liquid food products is often accomplished in a spray dryer.
Moisture removal from a liquid food occurs after the liquid is atomized or sprayed into heated air within a drying chamber.
Although various configurations of the chamber are used, liquid droplets are generally introduced into a heated air stream.
While liquid food droplets are moving with the heated air, the water evaporates and is carried away by the air.

23. Schematic illustration of a spray-drying system.
Much of the drying occurs during a constant-rate period and is limited by mass transfer at the droplet surface.
After reaching the critical moisture content, the dry food particle structure influences the falling-rate drying period.
During this portion of the process, moisture diffusion within the particle becomes the rate-limiting parameter.
Schematic illustration of a spray-drying system.

24. Spray Drying
After the dry food particles leave the drying chamber, the product is separated from air in a cyclone separator.
The dried product is then placed in a sealed container at moisture contents that are usually below 5%.
Product quality is considered excellent due to the protection of product solids by evaporative cooling in the spray dryer.
The small particle size of dried solids promotes easy reconstitution when mixed with water.

25. and falling-rate drying periods.
Spray Drying
During the constant-rate period of drying, a spray drying process may be described by heat transfer from the heated air to the droplet surface or by mass transfer from the droplet surface to the heated air.
Illustration of constant-rate
and falling-rate drying periods.

26. for 1< NRe <70,000 & 0.6 < NPr <400
Spray Drying
When the process is described in terms of heat transfer, the convective heat transfer coefficient may be estimated by:
for 1< NRe <70,000 & 0.6 < NPr <400
where NNu is Nusselt number [hdc/k]; h is convective heat-transfer coefficient (W/[m 2 C]); dc is the characteristic dimension (m); k is thermal conductivity of fluid (W/[m C])

27. Spray Drying
For descriptions in terms of mass transfer, the mass transfer coefficient is estimated by:
The similarity between the expressions used to estimate the surface coefficients is obvious.
Sherwood number
Schmidt
number

28. Spray Drying
By recognizing that the surface area of a sphere is 4πR2, a specific expression for drying time during spray drying:
The convective heat transfer coefficient at the surface of a droplet during spray drying can be estimated by the ratio of the thermal conductivity of air to the droplet radius.
Initial moisture content (kg water/kg dry solids)
Critical moisture content
(kg water/kg dry solids)
Latent heat of vaporization for water at the wet bulb temperature of heated air (kJ/kg water)
Product surface temperature (°C)
Radius of the liquid food droplet
Heated air temperature (°C)

29. Spray Drying
Latent heat of vaporization for water at the wet bulb temperature of heated air (kJ/kg water)
Initial moisture content
(kg water/kg dry solids)
Critical moisture content
(kg water/kg dry solids)
Radius of the liquid food droplet
Heated air temperature (°C)
Product surface temperature (°C)

30. Spray Drying
The prediction expression for total drying time during spray drying of a liquid food droplet will be:
Critical moisture content
Radius of the product particle at the critical moisture content
Effective mass diffusivity
(m 2 /s)
Thermal conductivity (W/(m °C)
Equilibrium moisture content (kg water/kg dry solids)

31. 6. Freeze Drying
Freeze-drying is accomplished by reducing the product temperature so that most of the product moisture is in a solid state, and by decreasing the pressure around the product, sublimation of ice can be achieved.
When product quality is an important factor for consumer acceptance, freeze-drying provides an alternative approach for moisture removal.

32. Schematic illustration of a freeze-drying system.
The heat- and mass-transfer processes during freeze-drying are unique.
Depending on the configuration of the drying system, heat transfer can occur through a frozen product layer or through a dry product layer.
Obviously, heat transfer through the frozen layer will be rapid & not rate-limiting.
Schematic illustration of a freeze-drying system.

33. Freeze Drying
Heat transfer through the dry product layer will be at a slow rate due to the low thermal conductivity of the highly porous structure in a vacuum.
In both situations, the mass transfer will occur in the dry product layer.
The diffusion of water vapor would be expected to be the rate-limiting process because of the low rates of molecular diffusion in a vacuum.

34. Freeze Drying
The advantages of the freeze-drying process are superior product quality resulting from low temperature during sublimation and the maintenance of product structure.
These advantages are balanced against the energy-intensive aspects of the product freezing and vacuum requirements.

35. Freeze Drying
Freeze drying is a drying process where the drying rate is limited by internal mass transfer.
Since heat transfer occurs from the hot plate to the drying front through the frozen product layer of dry product, and moisture diffusion becomes the rate-limiting process.

36. The drying time: Freeze Drying Thickness of product layer (m)
Absolute temperature
(K)
Diffusion coefficient (m 2/s)
Universal gas constant
( m3 Pa/[kgmol K])
mass transfer coefficient
(kg mol/[s m 2 Pa])
Molecular weight (kg/kg mol)
Specific volume of water
(m 3/kg water)
Vapor pressure of ice (Pa)
Vapor pressure of air at the condenser surface (Pa)

37. Freeze Drying
The equation is limited to situations where the drying time is based on a process where rate is limited by moisture diffusion within the structure of the dry product layer.
The calculation of drying time requires knowledge of the moisture diffusivity (D) and mass transfer coefficient (k m), and the magnitude of both is likely to be product-dependent.
Often, these property values must be measured for individual situations.

38. Spray drying vs Freeze drying

39. 7. Equipment
The cost of fuel for heating air is the main economic factor affecting drying operations and commercial driers have a number of features that are designed to reduce heat losses or save energy.
The criteria for selection of drying equipment and potential applications are described in Table 1.

40. Equipment
Table 1: Characteristics of driers

41. Equipment
Table 1: Characteristics of driers

42. Equipment
Table 2: Advantages and limitations of parallel flow, counter-current flow, centre-exhaust & cross-flow drying

43. Equipment
Table 3: Types of contact drier

44. 8. Effect of drying on food quality
All products undergo changes during drying and storage that reduce their quality compared to the fresh material and the aim of improved drying technologies is to minimise these changes while maximising process efficiency.
The main changes to dried foods are to the texture and loss of flavour or aroma, but changes in colour and nutritional value are also significant in some foods.

45. 8.1 Texture
The loss of texture in fruit & vegetable products is caused by gelatinisation of starch, crystallisation of cellulose(glass transition), and localised variations in the moisture content during drying, which set up internal stresses.
These rupture, crack, compress and permanently distort the relatively rigid cells, to give the food a shrunken shrivelled appearance.

46. 8.2 Flavour and aroma
Heat not only vaporises water during drying but also causes loss of volatile components from the food and as a result most dried foods have less flavour than the original material.
The open porous structure of dried food allows access of oxygen, which is a second important cause of aroma loss due to oxidation of volatile components and lipids during storage.

47. Flavour and aroma The changes can be reduced by: vacuum or gas packing
low storage temperatures
exclusion of ultraviolet or visible light
maintenance of low moisture contents
addition of synthetic antioxidants
preservation of natural anti-oxidants

48. 8.3 Colour
Drying changes the surface characteristics of a food and hence alters its reflectivity and colour.
In fruits and vegetables, chemical changes to carotenoid and chlorophyll pigments are caused by heat and oxidation during drying and residual polyphenoloxidase enzyme activity causes browning during storage.
This is prevented by blanching or treatment of fruits with ascorbic acid or sulphur dioxide.

49. Colour
For moderately sulphured fruits and vegetables the rate of darkening during storage is inversely proportional to the residual sulphur dioxide content.
However, sulphur dioxide bleaches anthocyanins, and residual sulphur dioxide is also linked to health concerns.
Its use in dried products is now restricted in many countries.

50. 8.4 Nutritional value
Large differences in reported data on the nutritional value of dried foods are due to wide variations in the preparation procedures, the drying temperature and time, and the storage conditions.
Riboflavin – loss (become supersaturated and precipitate from solution).
Ascorbic acid (Vitamin C) & Thiamin– loss (sensitive to heat and oxidation).
Oil-soluble nutrients - mostly contained (not concentrated) during drying.

51. 9. Rehydration
Water that is removed from a food during dehydration cannot be replaced in the same way when the food is rehydrated (not reversible).
loss of cellular osmotic pressure, changes in cell membrane permeability, solute migration, crystallisation of polysaccharides & coagulation of cellular proteins
all contribute to texture changes and volatile losses and are each irreversible.

52. Rehydration
The rate and extent of rehydration may be used as an indicator of food quality.
those foods that are dried under optimum conditions suffer less damage and rehydrate more rapidly and completely than poorly dried foods.