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.
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.
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.
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.
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
MECHANISM OF DEHYDRATION
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.
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.
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
Heat Transfer Theory
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.
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
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.
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 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.
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.
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
IMPORTANCE OF WATER ACTIVITY (aw)
solution to that of pure water at the same temperature i.e.
The aw has a major role to play in microbiological spoilage and chemical
changes produced in the food. The principles of water and microorganisms
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.
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.
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).
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.
chemicals. In general organisms are most sensitive at high water
activities (i.e. in dilute solution) and minimum sensitivity occur in an
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.
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).
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.
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
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.
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