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)

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

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

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)

• 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

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

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)

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.

• 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

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

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

Concept of water activity

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.

Role of water activity

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.

Measuring water activity

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.

Sorption isotherm

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. ).