AFS 2201 Principles of Food
Processing and Preservation
CH 5: Drying Unit Operation
Grace P. Okiror, PhD
Department of Biosystems Engineering
Introduction to Drying/Dehydration
❑ Dehydration (or drying) is defined as ‘the application of
heat under controlled conditions to remove the majority
of the water normally present in a food by evaporation’
(or in the case of freeze drying by sublimation).
❑ 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, but the
processing temperature is usually insufficient to cause
their inactivation.
❑ Therefore, any increase in moisture content during
storage, for example due to faulty packaging, will result
in rapid spoilage
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Introduction…
❑ The reduction in weight and bulk of food
reduces transport and storage costs
❑ For some types of food, dehydration provides a
convenient product for the consumer or more
easily handled ingredients for food processors
❑ Examples of commercially important dried
foods are coffee, powdered milk, raisins, and
other fruits, pasta, flours (including bakery
mixes), beans, nuts, breakfast cereals, tea and
spices.
❑ The final quality of the dried product is
determined by the drying rate
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Drying rate
There are a large number of factors that
control the rate at which foods dry, which can
be grouped into the following categories
❑ those related to the processing conditions
❑ those related to the nature of the food
❑ those related to the drier design.
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Methods of drying
1. Drying by heated air. The food is placed in contact
with a moving stream of heated air, heat being
supplied by convection
2. Drying by direct contact with a heated surface.
Heat is supplied by conduction
3. Microwave drying. Heat is supplied by radiation
4. Freeze drying. Heat may be supplied by
conduction or convection, but the drying process
is by sublimation
5. Spray drying. Used for drying liquids (milk/juice)
6. Sun or Solar drying
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Hot air drying
There are three inter-related factors that control the
capacity of air to remove moisture from a food:
1. The air temperature and velocity
2. The amount of air that passes over the food.
3. The amount of water vapour present in the air
▪ The amount of water vapour in air is expressed as either
absolute humidity or relative humidity (RH) (in per cent).
▪ Psychrometry is the study of inter-related properties of
air–water vapour systems.
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The Psychrometric Chart
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Hot air drying…
❑ Heat from drying air is absorbed by food and provides
the latent heat needed to evaporate water from the
surface. The temperature of the air, measured by a
thermometer bulb, is termed the dry-bulb temperature.
If the thermometer bulb is surrounded by a wet cloth,
heat is removed by evaporation of water from the cloth
and the temperature falls. This lower temperature is
called the wet-bulb temperature. The difference
between the two temperatures is used to find the
relative humidity of air on the psychrometric chart
❑ Adiabatic cooling lines are the parallel straight lines
sloping across the chart, which show how absolute
humidity decreases as the air temperature increases.
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Psychrometric problems
Using the psychometric chart, calculate the following:
1. the absolute humidity of air which has 50% RH and
a dry-bulb temperature of 60ºC;
2. the wet-bulb temperature under these conditions;
3. the RH of air having a wet-bulb temperature of
45ºC and a dry-bulb temperature of 75ºC;
4. the dew point of air cooled adiabatically from a
dry-bulb temperature of 55ºC and 30% RH;
5. the change in RH of air with a wet-bulb
temperature of 39ºC, heated from a drybulb
temperature of 50ºC to a dry-bulb temperature of
86ºC;
6. the change in RH of air with a wet-bulb
temperature of 35ºC, cooled adiabatically from a
dry-bulb temperature of 70ºC to 40ºC.
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Psychrometric problems…
Example 7:
The wet-bulb temperature in a particular room is
measured and found to be 20°C in air whose dry-bulb
temperature is 25°C. If this air is to be heated to a
dry-bulb temperature of 40°C, calculate the rate of
heat supply needed for a flow of 1000m3/h of this hot
air for a dryer, and the relative humidity of the heated
air.
Ans: Reading from the chart at 40°C and humidity 0.0
125kg/kg , the enthalpy is 73 kJ/kg, specific volume
is 0.906m3kg-1 and RH 27%. Ms = 1104kg, Qs = 5kW
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Hot air drying mechanism
❑ In addition to air temperature and humidity,
another important factor is the air velocity. When
hot air is blown over a wet food, water vapor
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.
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Hot air drying mechanism…
❑ The boundary film acts as a barrier to both heat
transfer and water vapor removal during drying.
The thickness of the film is determined primarily by
the air velocity; if the velocity is low, the boundary
film is thicker and this reduces both the heat
transfer coefficient and the rate of removal of water
vapor. 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. Therefore
the faster the air, the thinner the boundary film and
hence the faster the rate of drying.
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Hot air drying mechanism…
❑ The best conditions for hot air drying are therefore:
1. a moderately high dry-bulb temperature
2. a low RH
3. a high air velocity.
❑ 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 food321kgf`
▪ water vapour diffusion in air spaces within the food caused by
vapour pressure gradients.
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Hot air drying mechanism…
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Hot air drying mechanism…
Drying curves. The temperature and humidity of the drying air are
constant and all heat is supplied to the food surface by convection.
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Hot air drying mechanism…
Drying curves. The temperature and humidity of the drying air are
constant and all heat is supplied to the food surface by convection.
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Factors affecting the drying rate
❑ The composition and structure of the food has an
influence on the mechanism of moisture removal.
For example, the orientation of fibres in vegetables
(e.g. celery) and protein strands in meat allow more
rapid moisture movement along their length than
across the structure.
❑ The amount of food placed into a drier in relation
to its capacity (in a given drier, faster drying is
achieved with smaller quantities of food).
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Heated surfaces drying
❑ Slurries of food are deposited on a heated steel
drum. Heat is conducted from the hot surface,
through the food, and moisture is evaporated from
the exposed surface. The main resistance to heat
transfer is the thermal conductivity of the food
❑ Additional resistance arises if the partly dried food
lifts off the hot surface, forming a barrier layer of
air between the food and the drum. Knowledge of
the rheological properties of the food is therefore
necessary to determine the thickness of the layer
and the way in which it is applied to the heated
surface
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Types of driers
Hot-air driers
Bin driers
Bin driers are large, cylindrical or rectangular
containers fitted with a mesh base. Hot air passes
up through a bed of food at relatively low velocities
Cabinet driers (tray driers)
These consist of an insulated cabinet fitted with
shallow mesh or perforated trays, each of which
contains a thin (2–6 cm deep) layer of food. Hot air
is blown at 0.5–5ms1 through a system of ducts
and baffles to promote uniform air distribution over
and/or through each tray.
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Types of dryers…
Tunnel driers
❑ Layers of food are dried on trays, which are stacked
on trucks programmed to move semi continuously
through an insulated tunnel, having one or more
types of air flow
❑ Typically a 20m tunnel contains 12–15 trucks with
a total capacity of 5000 kg of food.
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Types of dryers…
Conveyor driers (belt driers)
❑ Continuous conveyor driers are up to 20m long and
3m wide. Food is dried on a mesh belt in beds 5–15
cm deep. The air flow is initially directed upwards
through the bed of food and then downwards in
later stages to prevent dried food from blowing out
of the bed.
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(a) Conveyor
drier and
(b)
three-stage
conveyor
drier.
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Fluidized-bed driers
❑ The main features of a fluidised-bed drier are a
distributor to evenly distribute the air at a uniform
velocity around the bed of material; a plenum chamber
below the distributor to produce an homogenous region
of air and prevent localised high velocities; and a
disengagement or ‘freeboard’ region above the bed to
allow disentrainment of particles thrown up by the air.
❑ Air from the fluidised bed is usually fed into cyclones to
separate out fine particles, which are then added back
to the product or agglomerated. Above the distributor,
mesh trays contain a bed of particulate foods up to 15
cdeep. Hot air is blown through the bed, causing the
food to become suspended and vigorously agitated
(fluidised), exposing the maximum surface area of food
for drying
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Drying Effect on foods
❑ 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 minimize these changes while
maximizing process efficiency.
❑ The main changes to dried foods are to the texture
and loss of flavor or aroma, but changes in color
and nutritional value are also significant in some
foods.
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Texture
❑ Changes to the texture of solid foods are an
important cause of quality deterioration, The loss
of texture in these products is caused by
gelatinization of starch, crystallization of cellulose,
and localized 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. On rehydration the
product absorbs water more slowly and does not
regain the firm texture of the fresh material. There
are substantial variations in the degree of
shrinkage and rehydration with different foods
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Texture…
❑ In general, rapid drying and high temperatures cause
greater changes to the texture of foods than do
moderate rates of drying and lower temperatures. As
water is removed during drying, solutes move from the
interior of the food to the surface. The mechanism and
rate of movement are specific for each solute and
depend on the type of food and the drying conditions
used
❑ Evaporation of water causes concentration of solutes at
the surface. High air temperatures (particularly with
fruits, fish and meats), cause complex chemical and
physical changes to solutes at the surface, and the
formation of a hard impermeable skin. This is termed
case hardening and it reduces the rate of drying to
produce a food with a dry surface and a moist interior It
is minimised by controlling the drying conditions to
prevent excessively high moisture gradients between
the interior and the surface of the food.
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Flavor 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 extent of
volatile loss depends on the temperature and final
moisture content of the food and on the vapour
pressure of the volatiles and their solubility in water
vapour.
❑ Volatiles which have a high relative volatility and
diffusivity are lost at an early stage in drying. Foods
that have a high economic value due to their
characteristic flavours (for example herbs and
spices) are dried at low temperatures
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Flavour and aroma…
❑ Flavour changes, due to oxidative or hydrolytic
enzymes are prevented in fruits by the use of
sulphur dioxide, ascorbic acid or citric acid, by
pasteurisation of milk or fruit juices and by
blanching of vegetables. Other methods which are
used to retain flavours in dried foods include:
recovery of volatiles and their return to the product
during drying
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Flavour and aroma…
mixing recovered volatiles with flavour fixing
compounds, which are then granulated and added
back to the dried product (for example dried meat
powders)
addition of enzymes, or activation of naturally
occurring enzymes, to produce flavours from
flavour precursors in the food (for example onion
and garlic are dried under conditions that protect
the enzymes that release characteristic flavours).
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Colour changes
❑ There are a number of causes of colour loss or
change in dried foods; 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.
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Colour changes…
❑ This is prevented by blanching or treatment of
fruits with ascorbic acid or sulphur dioxide. 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.
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Colour changes…
❑ The rate of Maillard browning in stored milk and
fruit products depends on the water activity of the
food and the temperature of storage. The rate of
darkening increases markedly at high drying
temperatures, when the moisture content of the
product exceeds 4–5%, and at storage temperatures
above 38ºC
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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. In fruits and
vegetables, losses during preparation usually
exceed those caused by the drying operation
❑ For example Escher and Neukom (1970) showed
that losses of vitamin C during preparation of apple
flakes were 8% during slicing, 62% from blanching,
10% from pureeing and 5% from drum drying
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Nutritional value…
❑ Vitamins have different solubilities in water and as
drying proceeds, some (for example riboflavin)
become supersaturated and precipitate from
solution, so losses are small. Others, for example
ascorbic acid, are soluble until the moisture content
of the food falls to very low levels and these react
with solutes at higher rates as drying proceeds.
❑ Vitamin C is also sensitive to heat and oxidation
and short drying times, low temperatures, low
moisture and oxygen levels during storage are
therefore necessary to avoid large losses.
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Vitamin losses in selected dried foods
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Rehydration
❑ Water that is removed from a food during
dehydration cannot be replaced in the same way
when the food is rehydrated (that is, rehydration is
not the reverse of drying); loss of cellular osmotic
pressure, changes in cell membrane permeability,
solute migration, crystallisation of polysaccharides
and coagulation of cellular proteins all contribute to
texture changes and volatile losses and are each
irreversible
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Practice Problems
1. A single-drum drier 0.7m in diameter and
0.85m long operates at 150ºC and is fitted
with a doctor blade to remove food after 34
rev. It is used to dry a 0.6mm layer of 20%
w/w solution of gelatin, pre-heated to 100ºC,
at atmospheric pressure. Calculate the speed
of the drum required to produce a product
with a moisture content of 4 kg of solids per
kilogram of water. (Additional data: the
density of gelatin feed is 1020 kgm-3 and the
overall heat transfer coefficient 1200W/m2/K;
assume that the critical moisture content of
the gelatin is 450% (dry weight basis).)
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Practice problems…
2. Air at 60°C and 8%RH is blown through a
continuous dryer from which it emerges at a
temperature of 35°C. Estimate the quantity of
water removed per kg of air passing, and the
volume of drying air required to remove 20 kg
water per hour.
3. A flow of 1800m3/h of air initially at a
temperature of 18°C and 50% RH is to be used in
an air dryer. It is heated to 140°C and passed over
a set of trays in a shelf dryer, which it leaves at
60% RH. It is then reheated to 140°C and passed
over another set of trays which it leaves at 60% RH
again. Estimate the energy necessary to heat the
air and the quantity of water removed per hour.
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