Drying of Food has an ancient method of preserving different Foods. During the olden days foods were sun dried but due to the advancement of science and technology, many techniques have been discovered for drying the food for the purpose of preservation.

1. Preservation by Drying and
Dehydration
Syeda Rubeena Fatima
This Photo by Unknown author is licensed under CC BY-SA-NC.

2. Introduction
• Preservation of food by drying has been practiced for
centuries some food for example creams or sufficiently dry
as harvested or with a little dry meat unspoiled for long
period and the proper storage conditions.
• Most food enough moisture to permit action of their own
enzymes and by microorganism so that to preserve them by
drivers the removal or binding of moisture is necessary.
• Drying usually is accomplished by removal of water but any
method that reduces the amount of available moisture i.e.,
lowers the aw in a food is a form of drying. Ex: 1. Dried fish
heavily salted to remove moisture so it is unavailable for
microorganisms.
• Sometimes the sugars are also added as in sweetened
condensed milk to reduce the amount of available moisture.
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3. Terms used in connection with drying of food
A Sun Dried food has moisture removed by exposure to Sun's rays without any
artificially produced heat and without controlled temperatures relative
humidity's or air velocities.
A Hydrated Or desiccated food has vein dried by artificially produced heat under
controlled condition of temperature relative humidity and air flow.
Condensed usually implies that moisture has been removed from liquid food and
Evaporated may have a similar meaning or maybe used synonymously with the
term dehydrated.

4. Methods of drying

5. Solar Drying
• Solar drying is limited to
climates with a hot sun and a dry
atmosphere certain
• fruits such as raisins, prunes,
figs, apricots, nectarines, pears
and peaches are dried by solar.
Fish,
• rice and other grains are also
dried.

6. Drying by mechanical
dryers
• Artificial drying involves the passage of heated air
with controlled relative humidity over the food to be dried.
• The simplest dryer is the evaporator or kiln, sometimes used
in the farm home.
• Forced – draft drying systems employ currents of heated air
that move across the food usually in tunnels.
• Alternative method is moving the food on conveyor belt or on
trays in carts through the heated air.
• Liquid foods like milk, juices and soups may be evaporated by
the use of low temperature and vacuum.
• Drum drying is the passage of food over a heated drum with
or without vacuum.
• Spray drying is spraying the liquid into a current of dry,
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7. Freeze Drying
• Freeze drying or sublimation of water from
a frozen fruit by means of vacuum plus
heat light at the drying shelf is being used
for a number of foods including meat
poultry seafood fruits and vegetables.
• Frozen layers of fruit of low sugar content
may be dried without vacuum by
sublimation of moisture during passage
of dry carrier gas.
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8. Smoke drying
• Certain foods are dried by
smoking.
• Smoking of foods usually has
two purposes
• Adding desired flavours and has
preservation effect. Ex: Meat.

9. Other
methods
Electronic heating
– Removal of still
more moisture
from food already
fairly well dried.
Foam mat drying –
liquid food is
whipped to foam
dried with warm
air and crushed to
a powder.
Pressure gun
puffing – Partially
dried foods to give
a porous structure
that facilitates
further drying.
Tower drying –
Drying by de
humified air at
30°C or lower used
for tomato
concentrate,
milk and potatoes.

10. Factors in the
control of Drying

11. Treatments of foods before drying
• 1. Selection and sorting for size, maturity and soundness
• 2. Washing especially fruits and vegetables
• 3. Peeling of fruits and vegetables by hand, machine, lye bath or abrasing
• 4. Subdivision into halves, slices, shreds or cubes
• 5. Alkali dipping such as raisins, grapes and prunes. It employs hot 0.1 to 1.5 % lye or
sodium carbonate.
• 6. Blanching or scalding of vegetables and some fruits (apricots, peaches)
• 7. Sulfuring of light coloured fruits and certain vegetables. Fruits are sulphured by exposure to
SO2 gas produced by the burning of sulphur 1000 – 3000 ppm depending on the fruit.
Vegetables may be sulphured after blanching in a similar manner or by dipping into or spraying
with sulfite solution. Sulfuring helps maintain an attractive light. Color, conserve vitamin C and
vitamin A and repel insects. It also kills many of the micro organisms present.

12. Procedures after drying
• 1. Sweating: Sweating is storage, usually in bins or boxes for equalization of
moisture or readdition of moisture to a desired level. Ex: Almonds, walnuts
• 2. Packaging: Most foods are packaged soon after drying for protection against
moisture, contamination with microorganisms and infestation with insects.
although some dried fruit may be held as long as a year before packing.
• 3. Pasteurization: bus station is limited for most of dried fruits and kills any
pathogens that may be present. The fruit usually is pasteurized in the package
and the treatment bearing with the fruit is from 30 to 70 minutes at 65.6 to 85
degrees Celsius.

13. Microbiology of
dried foods

14. 1.Before reception at the
processing plant
• Fruits and vegetables have soil and water organisms on them when
harvested and also contain their own natural surface flora.
• Growth of some of these organisms may take place before the foods reach
the processing plant.
• Piled vegetables may heat and support the surface growth of slime
forming, flavour – harming, or even rot producing organisms.
• Meats and poultry are contaminated by soil, intestinal contents, handlers
and equipment.
• Fish are contaminated by water and by their own slime and intestinal
contents as well as by handlers and equipments.
• Eggs are dirtied by hen, nests and the handlers.
• Milk is subject to contamination from the cow and may support the
growth of some bacteria.
This Photo by Unknown author is licensed under CC BY-NC-ND.

15. 2. In the plant before drying
• Growth of microorganisms present on foods may continue in the plant up
to time of drying.
• Equipment and workers may contaminate the food.
• Grading, selection and sorting of foods like fruits, vegetables, eggs and milk
will influence the kinds and number of microorganisms present.
• Elimination of spoiled fruits and vegetables or of spoiled parts will reduce
numbers of organisms in the product to be dried.
• Washing fruits and vegetables removes soil and other adhering materials
and serve to removes soil and other adhering materials and serve to
remove microorganisms.
• Water used for washing should not add contamination.
• Washing eggs may prove more harmful than helpful unless they are used
promptly, for the moisture helps bacteria penetrate the shell.
• Peeling fruits or vegetables with steam or lye reduce number of
microorganisms. Slicing or cutting should not increase numbers
of organisms.
• Dipping in alkali before sun drying may reduce the microbial population.
• Blanching or scalding vegetables reduces bacterial numbers greatly as
much as 99%.
• Sulfuring of fruits and vegetables also caused a great reduction in numbers

16. 3. During the drying process
• Heat applied during drying process causes a reduction in total numbers of
microorganisms but the effectiveness varies with the kinds and numbers of
organisms originally present and the drying process employed.
• Spores of bacteria, molds and vegetative cells of certain bacteria may
survive.
• Improper conditions during drying may even permit the growth of
microorganisms.
• More microorganisms are killed by freezing than by dehydration during the
freeze drying process.
• Any method that involves abrupt and considerable changes in temperature
either an increase during drying by heat or a decrease during freeze drying
is likely to cause metabolic damage to some organisms.

17. 4. After drying
• If the drying process and storage conditions are adequate, there will be no growth
of microorganisms in the dried food.
• During storage there is a slow decrease in numbers of organisms, more rapid at first and
slower thereafter.
• The microorganisms that are resistant to drying will survive best, therefore the percentages
of such organisms will increase. Ex: Spores of bacteria and molds.
• Sometimes there may be contamination of the dried food during packaging and
other handling subsequent to drying.
• Sweating of dry fruits to equalize moisture may permit some microbial growth.
• Microbial content and the temperature of water used to rehydrate dried foods will
affect the keeping quality of the rehydrated product.
• Bacteria in freeze dried chicken meat are further reduced in numbers by rehydration with
water at 50°C.
• Growth of bacteria in the rehydration meat will occur at favourable temperatures, but there
is good shelf at 4°C.
• Staphylococcus aureus has been found to survive freeze drying and rehydration at 60°C.

18. Microbiology of specific Dried foods

19. Dried fruits
• The numbers of microorganisms
on most fresh fruits range from
comparatively few to many
depending on pretreatments.
• Most dried fruits may; vary from a
few hundred per gram of fruit to
thousands.
• They are mostly on the outer
surfaces. Spores of bacteria and
molds are likely to be the most
numerous
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20. Dried vegetables
• Range from negligible numbers to millions
per gram.
• If drying trays are improperly loaded, souring
of such vegetables as onions and potatoes by
lactic acid bacteria with a marked increases in
numbers of bacteria may take place during
the drying process.
• Risk is greater with onions because they are
not blanched.
• Ex: Chiefly bacteria are Escherichia,
Enterobacter, Bacillus, Clostridium,
Micrococcus, Pseudomonas, Streptococcus.
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21. Dried Eggs
• Dried eggs may contain from a few hundred micro organisms,
mostly bacteria per gram up to over 100 million, depending on
the eggs broken and the methods employed.
• A variety of kinds of organisms have been found in dried eggs
includes Microcci, Streptococci, Coliforms, Spore formers and
molds.
• The drying process may reduce numbers tend to 100 fold but
still comment large number of microorganisms to survive.
• Egg white has been pretreated by fermentation process the
count on the right product may be high eggnog is better
culture medium than the white and is likely to have higher
content at breaking to support growth better before drying.

22. Dried Milk
• No. of microorganisms in dry
milk may vary from a few
hundred per gram to millions.
• Rulers all drum drying kills more
organisms than does the spray
process.
• Predominant kinds of organisms
in dry milk are Streptococci,
Micrococci and spore formers.