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Food Preservation by Drying. Water is removed from foods:. Under natural field conditions Controlled dehydration processes Common operations such as cooking or baking. Foods dehydration refers to.

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Food Preservation by Drying

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Presentation Transcript
water is removed from foods
Water is removed from foods:
  • Under natural field conditions
  • Controlled dehydration processes
  • Common operations such as cooking or baking
foods dehydration refers to
Foods dehydration refers to
  • 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 may bring about changes in food
Drying may bring about changes in food:
  • Color
  • Flavor
  • Texture
  • Viscosity
  • Reconstitution Rate
  • Nutritional Value
  • Storage Stability
principles of preservation by drying
Principles of preservation by drying
  • Microbial growth and chemical reaction can occur only when enough water is present in food.
the level of water necessary to preserve food is establish by
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

water activity in some foods
Water activity in some foods

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 coffee0.2

water activity importance
Water activity importance

0.2-0.3 – monolayer of water –max shelf life for 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 reach max rate.

0.85 critical point for microbial spoilage and pathogens growth

in addition drying
In addition drying:
  • Reduces product weight and volume;
  • Improves transportation and storage.
  • Makes product may be more convenient in use by consumer (dried coffee, dry milk, dry soups, dried beverages etc)
food dehydration involves
Food dehydration involves:
  • Getting heat into the product
  • Getting moisture out of the product
  • Therefore, drying may be classified as heat-mass transfer process
factors controlling the rate of drying
Factors controlling the rate of drying:
  • Surface area
  • Temperature employed
  • Relative humidity of air
  • Velocity of air
  • Drying time
  • Atmospheric pressure vs vacuum
surface area
Surface area
  • Subdivision of food provides more surface area for contact with heating medium as well as the moisture escape
  • The smaller particles the shorter distance the heat must travel to center of food and the moisture must travel to reach surface.
temperature and humidity
Temperature and humidity
  • The greater the difference in temperature between the food and heating medium the greater the rate of drying
  • As water is driven from food it creates saturated atmosphere near the food surface
  • The dried and hotter the air the more moisture it may take up from the food surface- the more rapid is food drying
each food has its own equilibrium relative humidity
Each food has its own equilibrium relative humidity
  • Equilibrium relative humidity is the humidity at a given temperature at which food will neither lose moisture to atmosphere nor pick moisture from the atmosphere.
air velocity and pressure
Air velocity and pressure
  • Moisture must not only be taken up by the heating medium but also sweep away from the drying surface in order to prevent the moisture from formation of saturated atmosphere.
  • Pressure- evaporation of water depends on temperature. Boiling point is affected by the pressure. The lower the pressure the lower is the boiling point and the shorter is the drying time. Drying under vacuum is important for heat sensitive food products
drying curve consists of
Drying curve consists of
  • Constant Rate Period - As long the amount of water reaching the surface is equal to the amount of water evaporating from the surface - the evaporation is quite rapid and this cools the food surface
  • Falling Rate Period - When the amount of water reaching surface decreases - evaporation decreases and surface temperature increases
the falling rate of drying may be explained as follows
The falling rate of drying may be explained as follows:
  • Outside drier layer of food forms insulation barrier against rapid heat transfer (evaporated water leaves voids filled up with air).
  • As the water evaporates the concentration of solute in water increases. This elevates the boiling point.
  • The increase in the distance the water has to travel to reach surface.
the shape of drying curve depends on
The shape of drying curve depends on:
  • Food product
  • Type of drier
  • Temperature
  • Air velocity
  • Air humidity
  • Thickness of food
time vs temperature
Time vs. Temperature
  • Compromises must be made between the maximum possible drying rate and maintaining food quality
methods of drying
Methods of drying
  • Sun drying
  • Tray and tunnel drying
  • Spray drying
  • Drum drying
  • Freeze drying
  • Foam drying
  • Problems: no control
  • Slow process
  • Microorganisms and pests can attack
  • Rain
  • High nutrient loss
  • Inexpensive products: grains, acid fruits, spices
  • More efficient/control
  • Lower nutrient loss
  • More expensive
  • Products: dried vegetables, pasta, some fruits


  • More efficient than hot air
  • Lower nutrient loss
  • Cost about equal with air
  • Products: potato pastes & slurries
  • Low nutrient loss
  • More expensive than drum or air drying
  • Good control/efficiency
  • Use only for liquids
  • Products: milk, instant tea and coffee
  • Best nutrient quality
  • Best product quality (shape; rehydration)
  • Most expensive
  • Good control
  • Products: coffee, camping foods, military etc.