Moisture

Drying is one of the oldest methods of fruits and vegetables preservation. It is currently a
versatile and widespread technique in the food industry as well as a subject of continuous
interest in food research. Drying is a critical step in the processing of dehydrated products
because of the high energy requirement of the process (due to low thermal efficiency of dryers).
The main aim of drying fruits and vegetables is the removal of moisture up to certain level at
which microbial spoilage and deterioration chemical reactions are greatly minimized. In
addition to preservation, the reduced weight and bulk of dehydrated products decreases
packaging, handling, and transportation costs. Furthermore, most food products are dried for
improved milling or mixing characteristics in further processing. In contrast, with literally
hundreds of variants actually used in drying of particulates, solids, pastes, slurries, or solutions,
it provides the most diversity among food engineering unit operations.
In short, the main objectives of drying are:

➢ Extended Storage Life

➢ Quality Enhancement

➢ Ease of Handling

➢ Further Processing

Moisture content representation

The amount of moisture in a product is given on the basis of the weight of water present in the
product and is usually expressed in percent. Moisture content is designated by two methods,
(1) wet basis (w.b) and (2) dry basis (d.b).
Wet basis: The moisture content in this method is represented by the following expression,
weight of water in product
Percent moisture content = × 100
weight of product sample
Dry basis: In this method of representing moisture content; it is given on the basis of dry
weight of product. The dry basis moisture content is determined by the following expression
weight of water in product
Percent moisture content = × 100
weight of dry matter product sample
The value of dry basis moisture content is more than the wet basis moisture content. The
relationship between dry basis and wet basis moisture content is given by the following
expression
M. C(w. b)
M. C (db%) = × 100
1 − M. C(w. b)
If the moisture content is expressed in wet basis terms, and the moisture content of 100 kg
grains by drying has been brought down from 18% to 13%, it is incorrect to say that 5 kg of
water has been removed from the grains. In wet basis moisture representation, the denominator
of the equation changes in relation to the amount of moisture removed.
At 18% moisture content, the sample of grain contains 18 kg (100 x 0.18) of water and 82 kg
of dry matter. The same amount of grain is dried to 13% moisture content, the final weight of
water is calculated by the following relationship,
0.13 = weight of water/ weight of water+82
Weight of water = 0.13*82/0.87
= 12.25 kg
Thus, the actual amount of water removed from 100 kg of wet grain will be 18 – 12.25 kg =
5.75 kg.
Moisture measurements
Direct methods
1. Air oven method: When the moisture content of grains is up to 13%, 2-3 grams representative
ground samples of grains are placed in an air- oven. The temperature of the oven is set at 130°C
and the samples are kept in oven for 1-2 hours. Afterwards, the samples are taken out and
placed in a desiccator to cool down. The drop in the weight of grain is measured based on its
initial weight.
In other method, 25 to 30 grams of unground representative samples of grains are taken and
placed in an air-oven at 100°C temperature. The samples are kept in it for 72 to 96 hours.
Afterwards, the samples are taken out from oven placed in a desiccator to cool down to room
temperature. Moisture content of samples is measured based on drop in weight from initial
weight of sample.
2. Vacuum-oven method: In this method, 2-3 grams of representative sample of ground
material is placed in a vacuum-oven (25 mm vacuum) and dried at 100°C for 72-96 hours.
Afterwards, the samples are weighed. The drop in the weight of grain is measured based on its
initial weight.
The equipment generally used for determination of moisture content by air oven method are
given below.
(a) Moisture dishes for moisture measurement, the moisture dishes should be made of heavy
gauge aluminium sheet metal. The moisture dishes may not be easily damaged by dents. The
dishes should have tight covers. Same numbers should be marked on the dish and its cover.
Before use, the moisture dishes should be dried in an oven for one hour thereafter these should
be weighed.
(b) Desiccator: The desiccator should be properly air tight and have active absorbent inside.
c) Oven: Convective types of dryers are mostly used for moisture determination. These are
sufficiently heat resistant. These ovens maintain predetermined temperature. The oven should
have arrangements for proper air circulation, transferable perforated trays and a suitable
thermometer sensitive enough to show 0.5°C temperature difference. These ovens are equipped
with proper temperature control system. The oven should be run for few hours prior to their
use for moisture determination.
(d) Balance: An analytical balance should be used to determine the moisture content of product
correctly. The balance should be sensitive enough to weigh up to 0.01 grams.
 Seed Temperature, ̊C Heating period, hr
Wheat 130 19
Sorghum 130 18
Maize 130 72
Mustard 130 04
Sunflower 130 01
Beans 103 72
Barley 103 20

3. Brown-Duvel fractional distillation method: The moisture content of products is measured

by fractional distillation method. The method is recognised as an official method for
determination of moisture content. Hundred-gram whole grains along with 150 ml. of mineral
oil is taken in a flask. The sample is boiled as shown in fig. Moisture from the sample is thus
evaporated, collected and condensed in a graduated cylinder. The millilitre of moisture
collected shows the percentage moisture content. The determined moisture content is on wet
basis. In this method the temperature of mineral oil in flask should reach to 200°C within 26
minutes. The time required for moisture determination is about 30 minutes. If the temperature
of mineral oil reaches to desired temperature within time, the moisture content determination
is found to be fairly accurate. When samples of grains are taken from bulk, a variation in
moisture content to the tune of 1 to 1.5% is possible.

4. Infra-red method: In this method, grain moisture content is directly measured by evaporation
of the water from a sample of grain with an infra-red heating lamp. A commercial infra-red
moisture meter is also available in market. The instrument consists of a balance, a pan counter
balanced by a fixed weight and a variable length of weighing chain. An infra-red lamp is
mounted on an arm above the pan with a provision to change its height. A scale calibrated in
percentage moisture content is incorporated in the stem of the instrument as shown in fig. At
the end of the test, when the balance is zeroed, a direct reading of moisture content is obtained.
The sample of grains may be unground, but when ground sample is used, the time needed to
evaporate the water is reduced.
Indirect methods
1. Electrical resistance method: The electrical conductivity or resistance of a product depends
upon its moisture content. This principle is employed in resistance measuring devices. The
universal moisture meter measures the electrical resistance of the grain at a given compaction.
The electrical resistance apart from compaction is also affected by grain temperature and
impurities present in the sample. Such moisture meters are calibrated for grain/seed types,
degree of compaction and temperature. Universal moisture meter gives fairly accurate readings
of moisture content on wet basis.
2. Dielectric method: Such devices measure the dielectric constant of grains. The grains are
filled in chamber. The sides of the chamber is formed by the plates of a condenser between
which a high frequency current is passed to measure the capacitance of the sample. The
capacitance varies as per the water present in sample, the degree of compaction and the grain
temperature. The electrical properties of grain are temperature dependent. Since the
measurement of grain temperature is difficult, therefore, ambient air temperature is often used.
This may result a source of error because of the difference between air and grain temperatures.
3. Chemical method: The removal of water by strong desiccants (CaCl₂) is caused by the vapour
pressure gradients. The moisture moves from the wet grain to the drying agent, since the vapour
pressure of the grain is higher than that of the desiccant. The hydration of salt is accompanied
by evolution of heat. This heat of evolution helps in driving the water out of the grains. Calcium
chloride, when heated to redness reacts with superheated steam to form HCI and calcium
hydroxide.
CaCl₂ + 2 H₂O → Ca (OH)₂ + 2 HCI
In some other method, calcium chloride is used to measure the grain moisture content. Calcium
carbide reacts with water present in the sample. As a result of this acetylene gas is produced.
This acetylene gas can be measured volumetrically to calculate the loss of moisture. The
chemical action can be given as under,
CaC2+2H2O→Ca (OH)2 + C2H2

Dryer types
Fluidized Bed Dryer
Fluidized Bed Dryer works on the principle of fluidization. When hot air is passed through a
granular bed (i.e., non-sticky wet granules), friction occurs between the granular surface and
hot air, that leads to pressure drop (decrease in pressure). On increasing the velocity of air, the
pressure increases and a point is attained at which the pressure is equal to the total weight of
the granules. At this point, the granules get separated and further increase in velocity of hot air
increases the motion of the granules and finally they get fluidized. Fluidization is a stage in
which the granules are suspended in an air stream without any adhesion.
Construction of Fluidized Bed Dryer
It consists of a fluidization chamber made up of stainless steel. A pump is fitted at the bottom
along with heaters such that the air entering the chamber gets heated. Feed inlet and product
outlet are provided on either side. A pre-filter is fitted at the bottom (of chamber) below which
an air inlet is present. Equipment also contains a separator (granule and air separator) which is
used for collection of dried material from time chamber. Fluidized bed dryers without separator
are also available which are mainly useful for batch type operations.

Working of Fluidized Bed Dryer

The material to be dried is placed into the fluidizing chamber through the feed inlet. Pump and
the heaters are switched on. Hot air enters the chamber from the bottom through the pre-filter.
The air flows from the bottom to the top with a high velocity such that the particles (granules)
get suspended in the air stream and the fluidization is attained. Each particle is surrounded
(entirely) by air, due to which effective drying is achieved. The particles remain fluidized for
a period of about 1 to 2 minutes (if only surface liquid is to be dried) or 16 to 30 minutes (if
water from interior of porous material is to be removed).
The dried particles move from the chamber to separator through a connecting pipe. Here, the
air along with dust is eliminated and the dried material is collected from the product outlet (at
the bottom). The presence of separator facilitates continuous operation.
In some fluidized bed dryers, separate (rectangular) compartments are present for fluidization.
in which sequential flow of solids from inlet to outlet takes place, which is called as plug flow
system. Cold air is circulated in the last compartment, due to which the material gets cooled
before it is discharged.
Advantages of Fluidized Bed Dryer
1. It is easy to handle and requires less time when compared to tray dryer
2. Low labour cost
 3. High heat transfer coefficient
4. Drying can be done either batch-wise or continuous
5. Apart from drying, it can also be employed for coating, mixing and granulation etc
6. Thermal efficiency is much greater (2 to 6 times) when compared to tray dryer
7. It is suitable for both small and large scale drying
8. Drying capacity is more than tray dryer
9. Useful for thermolabile materials
Disadvantages of Fluidized Bed Dryer
1. Electrostatic charges may develop due to collision of particles, hence earthing of dryer
is compulsory
2. Due to collision, granules may break, thereby forming fine particles
3. Not suitable for sticky materials
Rotary dryer
Rotary dryers are one of the most common types of industrial dryer, utilised for large quantities
of material with particles of size 10 mm or larger. Typical rotary dryer consists of cylindrical
shell made of steel plates slightly inclined (1O to 5O) to horizontal to assist the transportation
of the wet material fed for the processing, shell is typically 0.3-5 m in diameter, 5-90 m in
length and rotating at 1-5. Rotary dryer shell tube acts as body to transfer the wet feedstock
and number of steam heated tubes are placed symmetrically around the perimeter and rotate
with it. Wet material is fed into the upper end of dryer and the material travels through it by
virtue of rotation and slope of the shell and dried product is picked as the lower end. The feed
rate, speed of rotation of shell, the volume of heated air or gases, and their temperature are so
regulated such that by the time material reaches to discharge point of rotary dryer it’s accurately
dried. Rotary dryer performs due role in complete drying process, 1) as a conveyor, carrying/
moving material from feed end to discharge end 2) as heating/ drying device. Movement of
material within the dryer is influenced by the lifting, cascade action, sliding and bouncing.
Louisiana State University Dryer
This is a continuous flow-mixing type of grain dryer which is popular in India and the U.S.A.
It consists of 1) a rectangular drying chamber fitted with air ports and the holding bin, 2) an air
blower with duct, 3) grain discharging mechanism with a hopper bottom, and 4) an air eating
system.
1) Rectangular bin: Usually the following top square sections of the bin are used for the design
of LSU dryer. i) 1.2m x 1.2m, ii) 1.5m x 1.5m, iii) 1.8m x 1.8m and iv) 2.1m x 2.1m the
rectangular bin can be divided into two sections, namely top holding bin and bottom drying
chamber.
2) Air distribution system
Layers of inverted V-shaped channels (called inverted V ports) are installed in the drying
chamber. Heated air is introduced at many points through the descending grain bulk through
these channels. One end of each air channel has an opening and the other end is sealed.
Alternate layers are air inlet and air outlet channels. In the inlet layers, the channel openings
face the air inlet plenum chamber but they are sealed at the opposite wall, where as in the outlet
layers, the channel openings face the exhaust but are sealed other side. The inlet and outlet
ports are arranged one below the other in an offset pattern. Thus, air is forced through the
descending grain while moving from the feed end to the discharge end. The inlet ports consist
of a few full-size ports and two half size ports at two sides. All these ports of same size are
arranged in equal spacing between them. The number of ports containing a dryer varies widely
depending on the size of the dryer. Each layer is offset so that the top of the inverted V ports
helps in splitting the stream of grain and flowing the grains between these ports taking a zigzag
path. In most models, the heated air is supplied by a blower.
3) Grain discharging mechanism
Three or more ribbed rollers are provided at the bottom of the drying chamber which can be
rotated at different low speeds for different discharge rates of grains. The grain is discharged
through a hopper fixed at the bottom of the drying chamber. Causing some mixing of grain and
air the discharge system at the base of the dryer also regulates the rate of fall of the grain.
4) Air heating system
The air is heated by burning gaseous fuels such as natural gas, butane gas, etc, or liquid fuels
such as kerosene, furnace oil, fuel oil etc, or solid fuels like coal, husk, etc. Heat can be supplied
directly by the use of gas burner or oil burner or husk fired furnace and indirectly by the use of
heat exchangers. Indirect heating is always less efficient than direct firing system. However,
oil fired burner or gas burners should be immediately replaced by husk fired furnace for
economy of grain drying. The heated air is introduced at many points in the drier so as to be
distributed uniformly through the inlet ports and the descending grain bulk. It escapes through
the outlet ports. This type of dryer is sometimes equipped with a special fan to blow ambient
air from the bottom cooling section in which the dried or partially dried warm grain comes in
contact with the ambient air. In general, the capacity of the dryer varies from 2 to 12 tonnes of
grain, but sometimes dryers of higher capacities are also installed. Accordingly, power
requirement varies widely. Recommended air flow rate is 60-70 m3/min/tonne of parboiled
paddy and optimum air temperatures are 60°C and 85°C for raw and parboiled paddy
respectively. A series of dryers can also be installed.
Flatbed dryer
The flatbed batch type dryer is similar to deep bed dryers except that the surface area of the
dryer is more and the depth of the drying layer is less. These dryers are of usually 1-2 tonne
capacity. These are designed for farm level operation (Fig). Grains are spread 0.6 to 1.2 m deep
over the perforated floor and dried. The main advantages of this type of dryer are, (1) the whole
batch is dried quickly and there are less chances of spoilage due to moulding, (2) there is less
likelihood of over-drying of the grain, and (3) a lower air pressure is required to force the drying
air.
Solar dryer
Conventional method of drying is to spread the material in a thin layer on ground and let it
exposed to the sun. Such a method has various disadvantages like,

• Accumulation of dust and harms due to insects

• Wastage of material due to birds

• Non uniform drying due to varying intensity of sun

• Larger area required for drying

All these difficulties are removed by using solar drier. There are two types of solar driers.

Natural convection solar drier

Natural air-drying is an in bin drying system with the following typical characteristics:

• Drying process is slow, generally requiring 4 to 8 weeks

• Initial moisture content is normally limited to 22 to 24%

• Drying results from forcing unheated air through grain at airflow rates of 1 to 2
cfm/bu

• Drying and storage occur in the same bin, minimizing grain handling

• Bin is equipped with a full-perforated floor, one or more high-capacity fans, a grain
distributor and stairs

• Cleaning equipment is used to remove broken kernels and fines

Description of Cabinet drier

It can be of fixed type and also of portable type. Generally, it has an area of about 3 x 5 m2 glass
sheet fixed at the top at an angle of about 0 to 300. Holes are provided at the bottom and at the
topsides for airflow by natural convection. Wire meshed black tray is provided to the material
to be dried.

Forced convection solar dryer (Hot air system)

In these, the collectors are provided with duct. Generally, a duct of 2.5 cm depth is provided. It
is made out of two plates welded together lengthwise. Cold air is blown through a blower into
the collectors, which gets heated during the passage through it. The hot air thus available is
then used for drying the products kept on the shelves of driers. This hot air takes away the
moisture of the products and is let out through a properly located outlet.

1. Absorber with ducting

2. Blower with motor and

3. Drying bin
Description

This drier has three main components viz., flat plate collector, blower and drying bin. The area
of the collector is 8m2. It is divided into 4 bays each having 2m x 1 m absorber area. The
absorber is made out of 20 g. corrugated G.I. sheet and is painted with dull black
colour. Another plain G.I. sheet placed 5 cm below the absorber plate creates air space for
heating. This sheet is insulated at the bottom with glass wool and is supported at the bottom
with another plain G.I. sheet. The absorber is covered at the top with two layers of 3 mm thick
plain glass. The unit is supported on all sides with wooden scantling and is placed at 110 to the
horizontal facing south. Baffle plates are provided in the air space. The air space is open at
the bottom to suck atmospheric air and at the top it is connected to a duct leading to suction
side of the blower. The blower is of 80 m3 / min, capacity run by 3HP electric motor. The
delivery side of the blower is connected to the plenum chamber of a circular grain holding bin.

Forced Convection Solar Drier for Drying of Grains

For drying high moisture paddy, the solar drier can be used. The different components of the
drier are air heater, air ducts and blower and grain drying chamber. The flat plate collector used
for heating the air has an efficiency of 60% and rise in ambient air temperature is 13oC. Freshly
harvested paddy can be dried and it may take about 7-8 hours to bring the moisture content
from 30% to 16% (d.b). After drying the grains, the milling quality can be tested. The use of
solar air heater for drying of grains indicates that 10-15oC rise in the temperature of the air is
enough to reduce the relative humidity of the air to 60% or less which is quite useful for drying
of cereal grains.

To the level consists of safe moisture content for storage 500 kg of paddy could be dried from
30 to 40 % moisture content in a period of 6 hours on bright sunny day by using air flow rate
of 4 m3/min with temperature rise 8-10oC.

Solar drier consists of air heater, blower drying chamber, air distribution system and thermal
storage system. The heated air is blown to drying chamber by blowers of the centrifugal type
to handle large quantity of air. Batch type or continuous flow type drying chamber artificially
creates the necessary radiation to reduce moisture. Hot air from the collector is sucked by a
blower through the inlet pipe and is being forced into the drying chamber. An auxiliary heating
system to supplement heat requirement may be arranged. This type of auxiliary systems and
thermal storage systems for collecting extra energy during daytime, take care of the night
operations.

The heat required Q in kcal/hr

Q = V x ρ x Cp x ΔT -------------- (1)

Where V = air flow rate, m3/hr

ρ = density of air, kg/ m3

Cp = the specific heat of air, and

ΔT = temperature rise.
Moisture content assessed per tonne of paddy (m) for drying pre-boiled paddy, yield the volume
of air to be handled V from

m x latent heat = V x ρ x Cp x ΔT x efficiency ------------- (2)

The volume of rock pile required V’ for thermal storage of heat energy Q is
𝑄
𝑉 = 𝜌.𝐶 --------------- (3)
𝑝 .∆𝑇

Where, ρ = density of rock, Cp = specific heat of rock, ΔT = temperature increase in rock

Equilibrium moisture content (EMC)

Most of the agricultural products, especially the food grains absorb moisture from environment
or loose moisture. When the ambient temperature rises and humidity of air decreases, the water
present in food grains vaporises, consequently the grains loose moisture which results in
drying. Thus, we find that at particular condition the moisture content of grains depends upon
the temperature and relative humidity of the environment. If the vapour pressure of the water
present in grains is more than the vapour pressure of water vapours in air, the water present in
grain vaporises and diffuses in the atmosphere. Alternatively, if the vapour pressure of water
present in grain is less than the atmospheric vapour pressure, grain will absorb moisture from
atmosphere. This property of gaining or loosing of moisture as per the atmospheric conditions
is known as hygroscopicity.
The moisture content attained by a grain with respect to a set of atmospheric temperature and
relative humidity is called the equilibrium moisture content (EMC) of the grain. In such
condition, the grain moisture is in equilibrium with surrounding air.
Methods for determination of EMC
The methods for determination of EMC of agricultural products can be categorised into two;
(1) static method and (2) dynamic method. In the static method, grains are left in humid and
still air until they attain equilibrium, while in the dynamic method humid air is agitated or
moved by mechanical means and the grains attain equilibrium condition.
Static method: In static methods, to bring the atmospheric air to desired relative humidity
levels different concentrations of sulphuric and hydrochloric acids are used. Static methods are
generally too much time consuming, and to bring the grain to equilibrium condition by acids,
3-4 weeks are required. Thus, in case of higher humidity and high temperature conditions,
chances of attack of molds are high. Decomposition and change in grain structure is also
possible. It is essential to maintain required humidity and temperature conditions of air
throughout the test period. Temperature of air is generally maintained by an air-oven, whereas
the relative humidity of air is maintained by acid solution.
Various concentrations of sulphuric and hydrochloric acids give relative humidity of air
between 0 to 100 percent. The vapour pressure above the acid solution depends upon the
concentration of chemical solution and the temperature. In below table values of relative
humidity attainable by different concentration of sulphuric acid are given.
Handling of concentrated acid solution is not easy. Determination of EMC with salt solutions
is relatively safer, but to maintain a wide range of relative humidity values a number of salts
are needed. Considering this point, use of salts for EMC determination is cumbersome as
compared to use of acids. Hall (1957) and Lal (1969) have collected information on use of salts
for EMC determination. In below table the values of relative humidities with respect to salt
solutions and various temperature conditions are given. for EMC determination. In below table
Values of relative humidity attainable by use of saturated solution of some salts at
different temperatures
Temperature, °C Amount of salt needed to
Salt 20 25 30 35 40 50 dissolve in 100 g water, g (as
Average relative humidity per temperature)
Potassium 97.2 96.9 96.6 96.4 96.2 95.8 11-17
Sulphate
Potassium 93.2 92 90.7 89.3 87.9 85 32-35
Nitrate
Potassium - 84.7 84.5 83 - - 34-33
Chloride
Ammonium 80.6 - 80 79.6 79.1 -
Sulphate
Ammonium - 77 78.2 76.6 - - -
Chloride
Sodium 75.5 75.8 75.6 75.5 75.4 74.7 36-37
Chloride
Sodium Nitrate 65.3 64.3 63.3 62 61.7 - 90.2-118
Sodium 59.2 57.8 56.3 54.6 - - -
Bromide
Potassium - 43.8 43.5 43.4 - - -
Carbonate
Magnesium 33.6 33.2 32.8 32.5 32.1 31.4 408-600
Chloride
Potassium 23.2 22.6 22 21 - - -
Acetate
Lithium 12.4 12 11.8 11.7 11.6 11.4 79-98
Chloride
Vacuum Oven 0 0 0 0 0 0 -
the values of relative humidities with respect to salt solutions and various temperature
conditions are given.
Dynamic methods
The following methods are being followed in determination of EMC by dynamic means.
(i) Desorption method: In this method, the property of dry air to absorb moisture from moist
grains is employed. Moist grains are put in an airtight container. When the air comes in
equilibrium to grain its relative humidity is measured by an electric hygrometer or by a hair
hygrometer. Since the container has little quantity of air, it reaches in equilibrium with grain in
short period.
(ii) Isotenoscopie method: This method also employs absorption of moisture by dry air to
determine grain EMC. But in this method arrangement is available to measure directly the
vapour pressure exerted by the moist grains. In Fig schematic diagram of isotenoscope is
shown. In a conical flask, grain sample is kept.

Schematic diagram of isotenoscope 1. Vacuum storage, 2. Constant temperature water bath,

3. Sample flask, 4. Vacuum pump
Isotenoscope is a U tube filled with the liquid of negligible vapour pressure. The arms of the
tube have an enlarged section above the level of liquid to prevent drawing of the liquid out of
the tube while evacuating or readmitting air to the flask. The isotenoscope is connected to a
vacuum pump through a vacuum storage jar. Atmospheric pressure can be brought back into
this jar by means of a valve 'V. The 'V' is a shut off valve connecting closed end of mercury
manometer to the vacuum system. In operation, valve 'V' is closed while all air is evacuated
from the flask, the vacuum storage jar, and from the system. Under this condition, vapour
pressure builds up in the flask which forces the liquid in the two arms of the isotenoscope to
dissimilar level. The level of the liquid is then equalised by bleeding a small amount of air into
the vacuum storage jar. This equalization pressure is continued until vapour pressure built up
in the flask has reached a maximum for the temperature of water bath. Valve 'V2' is then closed
and the absolute pressure indicated in the manometer is read. The isotenoscope is removed
from the flask and the flask is closed by a properly weighed stopper. The weight of flask with
sample is recorded to determine sample moisture content.
EMC models
For determination of EMC of agricultural products many theoretical, semi-theoretical and
experimental models have been proposed. But, none of the theoretical EMC equation is found
to predict or to provide the EMC values correctly in the entire range of temperature and relative
humidity values. Some of EMC models are described below.
(i) Kelvin equation: Kelvin in 1871 has given the model of moisture adsorption by solid
material. For evolving the model, the phenomenon of capillary condensation in pores of solid
materials was considered. The relationship between the vapour of water present in capillaries
and saturated vapour pressure at same temperature is the basis of capillary condensation theory.
The kelvin equation is as under
𝑃𝑣 2𝜎𝑉𝑐𝑜𝑠𝛼
ln [ ]=
𝑃𝑣𝑠 𝑟𝑅𝑇𝑎
were, Pv = vapour pressure of grain, Pvs = saturated vapour pressure at temperature in
equilibrium with the system, σ = moisture surface tension, 𝛼 = angle between moisture and
capillary wall, V = Volume of moisture, r = radius of cylindrical capillary, R = universal gas
constant, Ta = absolute temperature
The utility of above equation for grain EMC determination is limited in conditions of relative
humidities above 95% when the action of capillary condensation takes place.
(ii) Harkins-Jura equation: This model is based on the theory of existence of a potential field
above surfaces of solid materials. In this concept, the work required to adsorb or desorb a water
molecule is the sum of work required to overcome vapour molecule to come on surface and the
work necessary for condensation. Considering the above potential field theory, Harkins-Jura
have proposed in 1944 the following equation
𝑃𝑣 𝑒
ln [ ]= 𝑑− 2
𝑃𝑣𝑠 𝑣
where, d and e= product constant depend on temperature
The Harkins-Jura equation does not predict satisfactorily accurate EMC values when the
relative is more than 30%.
(iii) Chung-Pfost equation: Chung-Pfost has proposed an equation for determination of EMC
on the basis of potential field theory. The equation is given below
𝑃𝑣 𝐴
ln [ ]= − 𝑒𝑥𝑝(−𝐵𝑀)
𝑃𝑣𝑠 𝑅𝑇

where, R = universal gas constant, T = absolute temperature, A and B = constant dependent

upon grain temperature, M = moisture content, % (db)
The above equation provides fairly accurate EMC values of grains between 20 to 90% relative
humidity values.
(iv) Henderson equation: Handerson in 1952 has proposed an equation for EMC
determination. This EMC equation is very much popular and based on the Gibbs' adsorption
equation. The following equation for the EMC curves (Fig) has been derived by Henderson.
𝑛
1 − 𝑟ℎ = 𝑒 −𝐶𝑇𝑀𝑒
were, rh = relative humidity, T = absolute temperature, M=EMC, % (db), Cand n = constant,
dependent on crop type and temperature.
Out of the above described theoretical, semi-theoretical or empirical equation, no single
relationship can predict grain's EMC values in the full range of relative humidities and
temperatures generally encountered.
Importance of EMC
EMC is of particular importance for drying and storage of agricultural materials. The
usefulness of EMC is: -
(i) EMC gives us the idea whether the material will gain or lose moisture at a particular
atmospheric condition
(ii) It also gives an idea about rate of moisture removal
(iii) EMC helps to determine drying characteristics
(iv) With the help of EMC, it can be predicted that to which moisture level product can be dried
with heated
Hysteresis effect
When some agricultural products in the pro- cess of losing moisture attains equilibrium
moisture content with the surroundings, the EMC is known as desorption EMC. But when a
dry product gains moisture from the surroundings and attains EMC, that value of EMC is said
to be adsorption EMC. At same relative humidity and temperature level there is a meaningful
difference between the desorption and adsorption EMC values. The desorption EMC values
are higher than the adsorption EMC values. The difference between desorption and adsorption
curves is known as hysteresis effect.
Bound moisture
It is the moisture content in a material which exerts a vapour pressure less than that of pure
liquid at the same temperature.
Unbound moisture
It is the moisture held by material in excess of the equilibrium moisture content corresponding
to saturation humidity. This type of moisture generally present in the void spaces. Water also
may be entrapped in foods such as pectin gels, fruits, vegetables, and so on. Entrapped water
is immobilized in capillaries or cells, but if released during cutting or damage, it flows freely.
Entrapped water has properties of free water and no properties of bound water.
Free moisture
It is the moisture content of material that can be removed by drying. It may include bound and
unbound moisture. Free moisture content can be obtained from the total average of moisture
content minus the EMC for the prevailing conditions of drying. Water that can be extracted
easily from foods by squeezing or cutting or pressing.
Thin-layer drying
The thin-layer drying process shows the condition of nearly complete exposure of grains to
heated air. The thickness of grains in thin-layer drying is normally up to 15 cm. The drying
action can be represented on the basis of Newton's law (equation) by replacing moisture content
in place of temperature.
𝒅𝑴
= − 𝑲(𝑴 − 𝑴𝒆 )
𝒅𝜽
𝑴 − 𝑴𝒆
= 𝒆−𝒌𝜽
𝑴𝟎 − 𝑴𝒆
M = moisture content at any time 𝜃, % db; Me = equilibrium moisture content, % db; 𝑀0 =
initial moisture content, % db; K = drying constant; 𝜃 = time, hour

The drying rate in thin-layer drying is equal to the ratio of water vapour pressure driving force
and resistance to drying. Therefore, the drying rate can be shown by the following equation

𝒅𝑴 (𝒑𝒈 − 𝒑𝒂 )
= = 𝒌𝒈 . 𝒂𝒎 . (𝒑𝒈 − 𝒑𝒂 )
𝒅𝜽 𝟏
( )
𝒌𝒈 . 𝒂𝒎

M = moisture content, % db
𝜃 = time, hour
𝑘𝑔 = mass transfer coefficient, kg water/hr-m2

am = effective area, m2
Pg = grain vapour pressure, Hg mercury
Pa = air vapor pressure, Hg mercury

Deep bed drying

In deep bed dryers, the drying takes place in a drying zone and the layer of grains is more than
15 cm. A typical deep bed dryer is shown in Fig. 3.21. At heated or drying air entry point,
drying of products starts. A drying zone is formed above the bottom layer which comes in
contact of heated air. Bulk of the drying takes place in the drying zone and this moves along
the direction of drying air. In deep bed system at a particular air flow rate, if the layer of product
is more then the layer which comes in contact with heated air first, may over dry. Therefore, it
has been recommended that if the drying air is 43°C, the thickness of grain layer may be limited
to 45 cm. For analysis of deep bed drying method, it is assumed that the deep bed is formed by
several thin layers. In each thin layer, the incoming and outgoing air humidity and temperature
change with time and drying stages. This method is used to determine the drying period and
the amount of water-vapour removed from the products. Additional amount of moisture may
be removed from dry layer till it attains the equilibrium moisture content.

Deep bed dryers’ schematic diagram

1. Exist air 2. Wet grain 3. Drying zone 4. Dry grain 5. blower
Deep bed drying system has been analysed by Hukill. The Hukill analysis is suitable for
correlating the drying period, product moisture and product depth factors for the solution of
drying problems.

Constant rate period

In the initial period of drying, the behaviour of food materials follows constant rate period and
followed by falling rate period. In constant rate period, the rate of evaporation under any given
set of air conditions is independent of the solid and is essentially the same as the rate of
evaporation from a free liquid surface under the same conditions.
The rate of drying during constant rate period is based on the
1. Difference between the temperature of air and temperature of the wetted surface at constant
air velocity and relative humidity
2. Difference in humidity between air stream and wet surface at constant air velocity and
temperature
3. Air velocity at constant air temperature and humidity.
In constant rate period drying takes place by surface evaporation and moisture moves by vapour
pressure difference. The moisture content at which the drying rate ceases to be constant is
known as critical moisture content. The critical moisture constant of a product also depends
upon the characteristics of a food material such as shape, size and drying conditions.
Falling rate period
Cereal grains are usually dried entirely under falling rate period. The falling rate period enters
after the constant drying rate period and corresponds to the drying cycle where all surface is no
longer wetted and wetted surface continuously decreases until at the end of this period. The
cause of falling off in the rate of drying is due to the inability of the moisture to be conveyed
from the centre of the body to the surface at a rate comparable with the moisture evaporation
from its surface to the surroundings. The falling rate period is characterized by gradual increase
in temperature both at the surface and within the food materials. The changes in the air velocity
have much smaller effect during the constant rate period. The movement of moisture within
the food material ie., from the centre to the surface by liquid diffusion and the removal of water
content from the surface of the food material to the atmosphere are the two important
controlling factor in drying of foods. The falling rate period consists of two stages (Fig).

First falling rate

The moisture is transported from the inside of the product to the surface and the critical
moisture content is reached. However, dry spots appear on the surface and surface area
evaporation decreases.
Second falling rate
In the second falling rate period, there is a slow diffusion of water to the inner surface leading
to desorption and diffusion through pores to the surface. In conventional heat treatment,
products are dried at higher temperature and for a longer drying time, creating thermal damage.
The mechanism of drying process is as follows
1. Liquid diffusion surface: Water movement due to moisture concentration difference
2. Capillary theory of drying: Water movement due to surface forces
3. Surface diffusion: Water movement due to moisture diffusion in the pores present in the food
materials
4. Vapour diffusion: Vapour movement due to difference in vapour pressure
5. Thermal diffusion: Vapour movement due to temperature difference
6. Hydrodynamic flow: Water and vapour movement due to total pressure differences