This document discusses concentration and dehydration of foods. It provides methods for concentrating foods like fruit jelly and candied fruits by partially removing moisture. Dehydration removes almost all water from foods down to 5% moisture. Methods for dehydrating foods include sun drying, hot air drying, microwave vacuum drying and osmotic dehydration. Water activity is defined as the ratio of water vapor pressure in a food to pure water vapor pressure and influences microbial spoilage and texture. Moisture sorption isotherms relate water content to water activity and may exhibit hysteresis.

1. Topics:
Importance of concentration and dehydration
Various methods
Concept of water activity and sorption
isotherms
Fundamentals of air-water vapour mixtures
Physico-chemical changes caused by
concentration and dehydration
Introduction
Group members:
SIBO BORO (M.Tech)

2. Concentration of food
• Concentration of food means
partial removal of moisture from
liquid foods to increase their final
total solids up to 70%
• For example – Jelly, Candied
fruits

3. Importance of Concentration
• Product can be stored without hermetic sealing.
• Concentrated foods are more attractive
• Can be preserved for longer period of time
• Prevents microbial spoilage

4. Methods of Concentrating Food
• Fruit Jelly
Washing
Cutting
Boiling(till fruit is soft)
Staining(through cheese cloth or jelly bag)
Boil the stained juice and add sugar
Boil till 220⁰F(jellying point)

5. • Candied Fruits
Pick perfectly ripe fruit
Blanching
Soak fruit in variety of different syrups
increasing the concentration
Once candied drain and sprinkle with
icing sugar

6. Dehydration of Food
• Removal of almost all the water
from the solid and liquid foods as
low as 5% moisture content.
• For example – Dried Fruits, Milk
Powder, Instant Coffee

7. Importance of Dehydration
• Increased concentration of nutrition
• Increased digestibility of protein(when dehydrated in low temperature)
• Improves storage stability
• Protection against browning
• Retention of ascorbic acid(when packaging is done at temperature up to 49ºC)

8. Methods of Dehydrating Food
• Grain Drying:
Hundreds of millions of tones of wheat, corn, soybean, rice and other grains as sorghum, sunflower
seeds, barley, oats, etc., are dried in grain dryers. In the main agricultural countries, drying comprises
the reduction of moisture from about 17-30% to values between 8 and 15% moisture content,
depending on the grain.
• Osmotic Dehydration of Fruits and Vegetables:
It is done by immersing the sample in concentrated solutions of salt or sugar. The base of osmotic
treatment is osmosis.
• Sun-drying:
To dry in the sun, hot, dry, breezy days are best. A minimum temperature of 86°F is needed with
higher temperatures being better. It takes several days to dry foods out-of-doors. Because the weather
is uncontrollable, sun drying can be risky. Fruits dried in the sun are placed on trays made of screen
or wooden dowels. Screens need to be safe for contact with food. The best screens are stainless steel,
teflon coated fiberglass or plastic.

9. • Hot air drying and microwave Vacuum:
Mature fresh vegetables collected
Washed, sorted, peeled and sliced
Uniformly distributed over the tray
Hot air dried at 60⁰C
Mature fresh vegetables collected
Washed, sorted, peeled and sliced
Turmeric slices placed inside a glass round
flask(vacuum chamber connected to the
container via a cold water condenser)
Container placed inside the microwave and dried

10. Difference between Concentration and Dehydration
Concentrated Foods Dehydrated Foods
1. Moisture content = (65-70)%
2. Preferably stored refrigerators
3. Can be stored for about 6-8
months
1. Moisture content = 5%
2. Stored in dry place
3. Can be stored for more than a year if
stored in an air tight container

11. Water activity is defined as the ratio of the vapor pressure of water in a material (p) to the vapor
pressure of pure water (po) at the same temperature
aw = p/po

12. Influencing microbial spoilage.
Textural properties of foods.
Controlling water migration of multi-component products.
Stability of powders and dehydrated products during storage.
Determining shelf life of products.

13. 1. Dewpoint :-
Accurate, fast and simple to use.
Measurement range is 0.030 to 1.000 with a resolution of ±0.001 and
accuracy of ±0.003.
Measurement time less than 5mins.
2. Capacitanc hygrometers :-
Do not require cleaning.
Less accurate than dew point hygrometers.
Should have regular calibration checks.

14.  Water activity is related to water content in a non-linear relationship is known as a moisture sorption isotherm curve.
Determined experimentally by measuring water content at several water activity values.
 Moisture sorption isotherms are sigmoidal in shape for most foods, and a moisture sorption isotherm prepared by adsorption
(starting from the dry state) will not necessarily be the same as an isotherm prepared by desorption (starting from the wet state).
This phenomenon of different aw vs moisture values by the two methods is called moisture sorption hysteresis and is exhibited by
many foods. Hysteresis represents the difference in aw between the absorption and desorption isotherms (Figure).
 A hydrated food can be dehydrated to remove moisture until the desired aw is reached (desorption) or completely dehydrated and
then re-hydrated to the desired aw (absorption). A food is more stable against microbial spoilage when its aw is adjusted by
absorption rather than by desorption.

15. Absorption and desorption isotherms for a food system at a given temperature are plotted using aw along
the horizontal axis and the % water along the vertical. When plotted, the phenomenon of hysteresis can
be observed (Fig. ).