Drying or dehydration is accomplished by the removal of water from the fruits
and vegetables below a certain level at which enzyme activity and growth of
microorganisms is affected adversely. The dried fruits and vegetable are called
as high sugar high acid foods or high value low volume foods.
These dried or
concentrated products save energy, money and space in shipping, packaging,
storing and transportation. Dehydration or drying process usually involves
heating, in which water is removed from solid or near solid substances. The
term drying is generally used for drying of the produce under the influence of
non-conventional energy sources like sun and wind.
Dehydration on the other hand refers to the process of removal of moisture by the application of
artificial heat under controlled conditions of temperature, relative humidity
and air flow. The sun drying is a slow process and thus, not suitable for many
high quality products. Generally, it lowers the moisture contents below about
15% which is too high for storage stability of numerous products.
PRINCIPLES
The basic principle in the process of drying or dehydration is the removal of
sufficient moisture to protect the product from spoilage. The process reduces
the amount of available moisture i.e. the water activity (aw ) and hence, product
becomes shelf-stable and is preserved for quite a long period.
sufficient moisture to protect the product from spoilage. The process reduces
the amount of available moisture i.e. the water activity (aw ) and hence, product
becomes shelf-stable and is preserved for quite a long period.
Moisture-solid,
relationship in fruits and vegetables are more complex than in inorganic
materials as the matter in fruits and vegetables exhibits an energetic retention
of moisture and the moisture is bound to the solid. The solid skeleton consists
essentially of numerous cells joined together to provide a network of
capillaries, some of them are very fine.
relationship in fruits and vegetables are more complex than in inorganic
materials as the matter in fruits and vegetables exhibits an energetic retention
of moisture and the moisture is bound to the solid. The solid skeleton consists
essentially of numerous cells joined together to provide a network of
capillaries, some of them are very fine.
First the moisture in the larger
capillaries has to be evaporated then only the moisture in the finer capillaries
can be removed. The cell walls act as semi-permeable membranes for the
diffusion of moisture which is mainly held osmotic ally.
capillaries has to be evaporated then only the moisture in the finer capillaries
can be removed. The cell walls act as semi-permeable membranes for the
diffusion of moisture which is mainly held osmotic ally.
Finally, there is a
small amount of moisture adsorbed on the skeletal frame in multi molecular
layers. In order to dehydrate any product specific requirements need to be
fulfilled so that the product retains as much as possible, its original
characteristics.
small amount of moisture adsorbed on the skeletal frame in multi molecular
layers. In order to dehydrate any product specific requirements need to be
fulfilled so that the product retains as much as possible, its original
characteristics.
MECHANISM OF DEHYDRATION
The changes during dehydration can be largely explained in terms of heat and
mass transfer phenomena. A cue of food in the course of dehydration loses
moisture from its surface and develops dried layer with remaining moisture
confined to its centre.
mass transfer phenomena. A cue of food in the course of dehydration loses
moisture from its surface and develops dried layer with remaining moisture
confined to its centre.
From the centre to the surface a moisture gradient will
be stabilized. The outside dried layer acts as an insulation barrier against rapid
heat transfer into the food pieces, this is further decreased by air voids formed
by evaporating water.
be stabilized. The outside dried layer acts as an insulation barrier against rapid
heat transfer into the food pieces, this is further decreased by air voids formed
by evaporating water.
In addition to less driving force from decreased heat
transfer, the centrally remaining water also now has further to travel to get out
of the food piece than did surface moisture at the start of drying. In addition,
as the food dries it approaches its normal equilibrium relative humidity, as it
does; it begins to pick up molecules of water vapour from the drying
atmosphere as fast as it loses them. When these rates are equal drying ceases.
transfer, the centrally remaining water also now has further to travel to get out
of the food piece than did surface moisture at the start of drying. In addition,
as the food dries it approaches its normal equilibrium relative humidity, as it
does; it begins to pick up molecules of water vapour from the drying
atmosphere as fast as it loses them. When these rates are equal drying ceases.
THEORY OF DRYING
There are two steps involved in drying & dehydration.
Heat Transfer Theory
Transfer of heat consists of transferring of molecular or atomic motion from
one region to another. There are three broad mechanisms by which such
transfer can occur, conduction, convection, and radiation. In conduction, the
energy is transmitted from particle to particle by a process of direct contact.
one region to another. There are three broad mechanisms by which such
transfer can occur, conduction, convection, and radiation. In conduction, the
energy is transmitted from particle to particle by a process of direct contact.
Transfer of heat by convection involves bulk mixing of fluids of different
temperatures. Radiation is the transfer of energy from a radiating source
through space which may or may not be occupied by matter. It is by radiation
that we receive all our energy from the sun.
temperatures. Radiation is the transfer of energy from a radiating source
through space which may or may not be occupied by matter. It is by radiation
that we receive all our energy from the sun.
Mass Transfer Theory
The removal of moisture from a food product involves simultaneous heat and
mass transfer. Heat transfer occurs within the product structure and is related
to the temperature gradient between the product surface and the water surface
at some location within the product.
mass transfer. Heat transfer occurs within the product structure and is related
to the temperature gradient between the product surface and the water surface
at some location within the product.
As sufficient thermal energy is added to
the water to cause evaporation, the vapours are transported from the water surface within the product to the product surface. The gradient causing
moisture –vapour diffusion is vapour pressure at the liquid water surface, as
compared with the vapour presser of air at the product surface.
the water to cause evaporation, the vapours are transported from the water surface within the product to the product surface. The gradient causing
moisture –vapour diffusion is vapour pressure at the liquid water surface, as
compared with the vapour presser of air at the product surface.
The heat and
the mass transfer within the product structure occurs at the molecular level,
with heat transfer being limited by thermal conductivity of the product
structure, while mass transfer is proportional to the molecular diffusion of
water vapour in air. The rate of moisture diffusion can be estimated by the
expression for molecular diffusion.
the mass transfer within the product structure occurs at the molecular level,
with heat transfer being limited by thermal conductivity of the product
structure, while mass transfer is proportional to the molecular diffusion of
water vapour in air. The rate of moisture diffusion can be estimated by the
expression for molecular diffusion.
The mass flux for moisture movement is a
function of the vapour pressure gradient as well as the mass diffusion for
water vapour in air, the distance for water vapour movement within the
product structure and temperature.
function of the vapour pressure gradient as well as the mass diffusion for
water vapour in air, the distance for water vapour movement within the
product structure and temperature.
The transport of vapour from the product
surface to the air and the transfer of heat from the air to the product surface is
a function of the existing vapour pressure and temperature gradients,
respectively, and the magnitude of the convective coefficient at the product
surface.
surface to the air and the transfer of heat from the air to the product surface is
a function of the existing vapour pressure and temperature gradients,
respectively, and the magnitude of the convective coefficient at the product
surface.
IMPORTANCE OF WATER ACTIVITY (aw)
Water activity (aw) is defined as the ratio of the vapour on the aqueous
solution to that of pure water at the same temperature i.e.
solution to that of pure water at the same temperature i.e.
aw = Vapour pressure of solution at To
C
C
Vapour pressure of pure water at To
C.
C.
Water activity is also equal to the equilibrium relative humidity (ERH);
aw =
The aw has a major role to play in microbiological spoilage and chemical
changes produced in the food. The principles of water and microorganisms
relation includes:
The aw has a major role to play in microbiological spoilage and chemical
changes produced in the food. The principles of water and microorganisms
relation includes:
1) Water activity, rather than water content, determines the lower limit of
available water for microbial growth. Most bacteria do not grow below aw
0.91 and most molds cease to grow at water activity of 0.80. Some
xerophylic fungi have been reported to grow at water activities of 0.65, but
the range of 0.70 – 0.75 is generally considered their lower limit.
available water for microbial growth. Most bacteria do not grow below aw
0.91 and most molds cease to grow at water activity of 0.80. Some
xerophylic fungi have been reported to grow at water activities of 0.65, but
the range of 0.70 – 0.75 is generally considered their lower limit.
2) Environmental factors affect the level of water activity required for
microbial growth. The less favourable the other environmental factors
(nutrients, pH, oxygen pressure, temperature) the higher becomes the
minimum a w at which microorganisms can grow.
microbial growth. The less favourable the other environmental factors
(nutrients, pH, oxygen pressure, temperature) the higher becomes the
minimum a w at which microorganisms can grow.
3) Some adaptation to low water activities occurs, particularly when aw is
depressed by addition of water soluble substances (principle of IMF –
Intermediate Moisture Food), rather than by water crystallization (frozen
foods) or water removal (dehydrated foods).
depressed by addition of water soluble substances (principle of IMF –
Intermediate Moisture Food), rather than by water crystallization (frozen
foods) or water removal (dehydrated foods).
4) When water activity is depressed by solutes. The solutes themselves may
have effects which complicate the effect of aw per se. For instance, at a
given aW microbial growth is less effectively depressed by glycerol than by
sodium chloride. More recent (IMF – Intermediate Moisture Food) have
resulted in the following additional findings.
have effects which complicate the effect of aw per se. For instance, at a
given aW microbial growth is less effectively depressed by glycerol than by
sodium chloride. More recent (IMF – Intermediate Moisture Food) have
resulted in the following additional findings.
a) Water activity modifies sensitivity of microorganisms to heat, light and
chemicals. In general organisms are most sensitive at high water
activities (i.e. in dilute solution) and minimum sensitivity occur in an
intermediate moisture.
chemicals. In general organisms are most sensitive at high water
activities (i.e. in dilute solution) and minimum sensitivity occur in an
intermediate moisture.
Minimum water activities for production of
toxins are often higher than those for microbial growth. The
phenomenon may represent an important safety factor in the
distribution of dehydrated and intermediate moisture foods.
toxins are often higher than those for microbial growth. The
phenomenon may represent an important safety factor in the
distribution of dehydrated and intermediate moisture foods.
b) The effect of water on chemical reactions in foods are more
complicated than are its effect on microbial growth. It plays one or
more of the following roles; a) as a solvent for reactant and for
products, b) as a reactant (e.g. in hydrolysis reactions) c) as a product
of reactions and d) as a modifier of the catalytic or inhibiting activities
of other substances (e.g. water in activities some metallic catalysts of
lipid per oxidation).
complicated than are its effect on microbial growth. It plays one or
more of the following roles; a) as a solvent for reactant and for
products, b) as a reactant (e.g. in hydrolysis reactions) c) as a product
of reactions and d) as a modifier of the catalytic or inhibiting activities
of other substances (e.g. water in activities some metallic catalysts of
lipid per oxidation).
All microorganisms have an optimal and minimal water activity for growth.
Adjusting the aw of a product by addition of solutes or the removal of water,
to a point below the minimal aw of the normal spoilage flora results in a
microbiological stable product.
Adjusting the aw of a product by addition of solutes or the removal of water,
to a point below the minimal aw of the normal spoilage flora results in a
microbiological stable product.
Many of the products contain viable
microorganisms and spores, which are not able to germinate because of the
restrictative aw. In fabrication of a product with a reduced aw other factors
which would affect the growth of microorganisms present need to be
considered, since the aw on microorganisms is influenced by pH, oxygen level,
temperature, nutrient content, and possibly food preservative, either natural or
added.
microorganisms and spores, which are not able to germinate because of the
restrictative aw. In fabrication of a product with a reduced aw other factors
which would affect the growth of microorganisms present need to be
considered, since the aw on microorganisms is influenced by pH, oxygen level,
temperature, nutrient content, and possibly food preservative, either natural or
added.
Water activity (aw) influences the physical, chemical and microbiological
properties of many substances. The shelf life of foods, their colour, stability,
taste, texture, vitamin content, aroma, mold formation and microbiological
growth properties are influenced directly by the aw value.
properties of many substances. The shelf life of foods, their colour, stability,
taste, texture, vitamin content, aroma, mold formation and microbiological
growth properties are influenced directly by the aw value.
aw measurement is
required to meet standards like FDA – Food Drug Act, USDA – United State
Department of Agriculture, GMP – General Manufacturing Practices,
HACCP – Hazard Analysis and Critical Control Points, and BIS 15000 –
Bureau of Indian Standards: The foods types and range of aw is discussed as
given below.
required to meet standards like FDA – Food Drug Act, USDA – United State
Department of Agriculture, GMP – General Manufacturing Practices,
HACCP – Hazard Analysis and Critical Control Points, and BIS 15000 –
Bureau of Indian Standards: The foods types and range of aw is discussed as
given below.
