Dehydration is a method of food preservation that involves removing water from food to prevent spoilage. It works by lowering the water activity level of foods, which inhibits the growth of microorganisms. There are several types of drying methods that work through evaporation of water, including direct convective drying, indirect drying, and radiation drying. The drying process can impact the physical, chemical, and sensory properties of foods through effects like protein denaturation, loss of structure, and browning reactions. By removing moisture, dehydration allows foods to be stored longer while retaining important qualities like flavor and nutrition.

1. DEHYDRATION-A method of
food preservation
Mamta Sahurkar

2. FOOD SPOILAGE
• Food spoilage can be defined as a
disagreeable change in a food's
normal state.
• NOT acceptable for human
consumption and for food industrial
usage.
• Such changes can be detected by
smell, taste, touch, or sight
(disgusting).
• Associated with Food safety and
quality

3. Classification of food
Semi perishable
foods
Foods handled and
stored properly
remain unspoiled for
long period
Eg: Potatoes, apples
Perishable foods
Spoil readily without
special
preservative methods
(fish, meat, eggs, milK)
Stable or non
perishable foods
Eg: Sugar, flour, dry
beans
Effect of food spoilage
• Changes in nutritional value : Decomposition of
proteins,carbohydrates,vitamins.
• Changes in organoleptic features: Colour, flavour, taste, consistency,
unpleasant odour.
• Unwholesome effects: Biogenic amines, toxins, Metabolites of

4. Physical changes
aw, temperature, mechanical
effects Caused by the
inappropriate transport,
and storage.
Chemical, biochemical
factors
non-microbial or
enzymatic changes
involving oxygen,
oxidation processes
rancidity of fats and
activity of endogenous
tissue enzymes.
Biological factors
Microbiological
bacteria, yeasts, moulds
(most common)
Macrobiological
rodents, insects, birds,
parasites
FOOD SPOILAGE: CAUSES

6. Food preservation involves the action taken to maintain foods with the desired
properties or nature for as long as possible.
 A number of new preservation techniques are being developed therefore
understanding the effects of each preservation method on food has become
critical.
 Preservation methods start with the complete analysis and understanding of the
whole food chain, including growing, harvesting, processing, packaging, and
distribution.
 It is important to identify the properties or characteristics of food that need to
be preserved.
FOOD PRESERVATION

7. To maintain product quality.
 To overcome inappropriate planning in agriculture, produce value-added
products, and provide variation in diet .
Value-added food products can give better-quality foods in terms of improved
nutritional, functional, convenience, and sensory properties.
 Eating should be pleasurable to the consumer.
 Variation in the diet is important to reduce reliance on a specific type of grain.
Why to Preserve Foods?

8. All foods contain at least some water.
 Free water
Held inside cells
Maintains properties of free water
May be removed by pressure.
 Bound water
Is part of molecule structure
Reduced mobility
Does not retain properties of free water
WATER IN FOOD

9. WATER ACTIVITY
• Water activity is the amount of free or available water in a product as opposed
to bound water.
• Water activity is the measure of energy status of the water in a system.
• Ratio of the vapor pressure of water in a food at a specified temperature to the
vapor pressure of pure water at the same temperature.
• Fish - 0.99
• Bread - 0.95
• Aged cheddar - 0.85
• Jams and jellies - 0.8
• Plum pudding - 0.8
• Dried fruit - 0.6
• Biscuits - 0.3
• Milk powder - 0.2
• Instant coffee- 0.2
Water activity of some foods

10. The level of water necessary to preserve food
is establish by:
• Water activity, aw defined as:
aw = p /p0 (0 - 1)
p -water vapor pressure over sample; p0 –water vapor
pressure over pure water
• Equilibrium relative humidity (%ERH)
%ERH = [p /po ]*100 = [x/x0]*100
x-mole fraction of water vapor over the sample and x 0
-mole fraction of water vapor over pure water

11. WHY WATER ACTIVITY IS IMPORTANT???
• Water activity (aw) is one of the most critical factors in determining
quality and safety of foods.
• Water activity affects the shelf life,safety,pH,texture,flavour and smell
of foods.
• While temperature pH and several other factors can influence it and
how fast organisms will grow in a product, water activity may be the
most important factor in controlling the spoilage.
• Most bacteria for example, do not grow at water activities below 0.91,
and most molds cease to grow at water activities below 0.80.
• By measuring water activity, it is possible to predict which
microorganisms will and will not be potential sources of spoilage.
• Water activity: not water content determines the lower limit of
available water for microbial growth.
• In addition to influencing microbial spoilage, water activity can play a
significant role in determining the activity of enzymes and vitamins in
food and can have a major impact their color, taste, and aroma

12. Factors affecting water activity
• Drying: Water activity is decreased by physically removing water (eg: Beef
jerky)
• Solutes: Water activity is decreased by adding solutes such as salt or sugar( Eg:
Jams, cured meats)
• Freezing: Water activity id decreased by freezing
• Combination: One or more of the above can be combined for a greater
influence on water activity(Eg: Salting and drying fish)
Water content or moisture content is the
quantity of water contained in a material.
Water activity is related to water content in
a non linear relationship known as
moisture sorption isotherm curve
Moisture content

13. Water activity importance
0.2-0.3
Monolayer of water –max shelf life for dehydrated
dehydrated food products
0.35 -0.45
Loss of crispness, stickiness, recrystallization of
amorphous state of sugar
0.4-0.5
Soft food products like raisins become hard
0.6
Critical point above some microbes may grow
0.6-0.8
Intermediate moisture food products (chewy
granola bars. Many chemical deterioration reaction
reaction reach max rate.
0.85
Critical point for microbial spoilage and pathogens
pathogens growth

14. FOOD DEHYDRATION

15. Foods dehydration refers to
• Dehydration is one of the oldest methods of food
preservation, and many cultures still dry food in the
sun.
• The nearly complete removal of water that causes
minimum or ideally no change in food properties.
Foods are dried to the final moisture content 1-5%.
Stability over 1 year at room temperature.
• Concentration processes that remove only part of
water are not considered as dehydration processes.
• Drying is usually accomplished by the evaporation of
water, but any method that decreases the amount of
water activity in food is a form of drying.

16. • The drying process removes enough moisture from food to greatly
decrease these destructive effect.
• The moisture content of fresh foods ranges from 20% to 90%. Foods
require different levels of dryness for safe storage,
• The dryness of air is measured in terms of relative humidity (RH).
• If air is at 100% relative humidity, it has absorbed 100% of the
water it can hold at that temperature.
……..Continued…….

17. PURPOSES OF DRYING
• To avoid or eliminate moisture which may lead to
corrosion and decrease the product stability.
• To improve or keep the good properties of a material, eg:
flow ability, compressibility.
• To reduce the cost of transportation of large volume
materials(Liquids).
• To make the material easy or more suitable for handling
• Preservative
• The final step in: Evaporation, Filtration, Crystallization

18. Solar drying

19. TYPES OF DRYING
 Direct: Convective Drying
Drying is established through direct contact between the product and the
gas heating medium. Material reaches steady state temperature.
 Indirect Drying:
Established from heated surface in contact with the product. The heating
medium and product are separated by wall. Material reaches steady state
temperature near liquid boiling point for contact rate drying.
 Radiation:
Heat transfer established by radiation from energy source. There is no
contact from heated surface or medium and product

20. MECHANISM OF DRYING
 Liquid diffusion: If the wet solid is at a temperature below the boiling
point of the liquid
 Vapor diffusion: If the liquid vaporizes within material
 Condensation diffusion: If drying takes place at very low temperature
and pressure,Eg:Freeze drying
 Surface diffusion: (possible although not proven)
 Hydrostatic pressure differences: When internal vaporization rates
exceed the rate of vapor transport through the solid to the surroundings
 Combination

21. CHOICE OF METHODS OF DRYING
 Product is sensitive to heat or not
 Nature of solvent to be removed
 Process is to be carried out under asceptic conditions
 Quantity of products to be dried.
 Available source of heat
 Cost involved
Choice of methods of drying depends on:

22. THE EFFECTS OF DEHYDRATING ON FOODS
•The main principle of drying is the
evaporation of water, using higher than
normal temperatures. As the water
evaporates there is an increase in the
concentration of solutes in the product.
This increase can physically damage
the food. This damage can be seen in
the changed physical appearance of
dried apples.

23. DRYING CAUSES
•Denaturation of
proteins
•A general loss of
structure
•Browning reactions
•Discolouration
Changes occur in:
• Color
• Flavor
• Texture
• Viscosity
• Reconstitution Rate
• Nutritional Value
• Storage Stability