# Food Processing Unit Operations Questions and Answers – Drying – Equilibrium Moisture Content

This set of Food Processing Unit Operations Interview Questions and Answers for Experienced people focuses on “Drying – Equilibrium Moisture Content”.

1. The capillary condensation theory plays a major role in ________ equation.
a) Kelvin
b) Harkins-Jura
c) Chung-Pfost
d) Henderson

Explanation: The Kelvin equation depends on the relationship between vapor pressure of water present in capillary and saturated vapor pressure at same temperature.

2. Henderson equation is based on ______
b) enthalpy
c) latent heat
d) entropy

Explanation: Gibbs adsorption relates to the change in concentration of a component in contact with surface with change in surface tension, which results in corresponding change in surface entry which is explained in Henderson equation.

3. The _______is the average material moisture content at which the drying rate begins to decline.
a) critical moisture content
b) equilibrium moisture content
c) free moisture content
d) bound moisture content

Explanation: The critical moisture content is the average material moisture content at which the drying rate begins to decline. It depends upon the ease of moisture movement through the solid, and hence, upon the pore structure of the solid, sample thickness and drying rate.

4. What is hysteresis effect?
a) Difference between desorption and adsorption curve
b) Difference between desorption and absorption curve
c) Difference between relative humidity and adsorption curve
d) Difference between relative humidity and absorption curve

Explanation: Hysteresis effect is defined as the difference between desorption and adsorption curve. This effect is detected due to the shrinkage effect during desorption that changes the water binding properties of the food product. Therefore, same path of EMC is not observed during adsorption.

5. __________ was used to describe relationship between the water activity and equilibrium moisture content.
a) Kelvin model
b) Harkins-Jura model
c) Chung-Pfost model
d) GAB model

Explanation: GAB modelwas used to describe relationship between the water activity and equilibrium moisture content. In Guggenheim, Anderson, de Boer (GAB) model, the sample is kept in the desiccators which contains saturated Potassium nitrate solution to maintain the relative humidity at 90% at 40°C and are kept in a temperature controlled chamber. Moisture content and water activity of each sample is determined using hot air oven and water activity meter.
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6. Which of the following statements about moisture sorption isotherms are correct?
Statement 1: It describes the relationship between moisture content and water activity in food.
Statement 2: Water activity always increases as moisture content increases.
a) True, False
b) True, True
c) False, True
d) False, False

Explanation: The moisture sorption isotherm is a graph where it indicates water activity where the water is adsorbed into and desorbed from a product held at constant temperature. This relationship is complex and unique for each product. Water activity almost always increases as moisture content increases, but the relationship is not linear.

7. __________ equation is based on potential field theory.
a) Kelvin
b) Harkins-Jura
c) Chung-Pfost
d) Henderson

Explanation: Chung-Pfost equation is based on potential field theory. This equation provides fairly accurate EMC values of grains between 20 to 90% relative humidity values.

8. Harkins-Jura equation does not predict satisfactory EMC values when relative humidity is __________
a) more than 30%
b) more than 40%
c) more than 50%
d) more than 60%

Explanation: Harkins-Jura equation does not predict satisfactory EMC values when relative humidity is more than 30%. In this equation, it deals with the work required to absorb or desorb a water molecules, the sum of work required to overcome vapor molecule to come on the surface and work necessary for condensation.

9. The __________ helps to decide the stability of food at particular moisture content in the given environment.
a) equilibrium moisture content
b) psychometric charts
c) equilibrium moisture content and psychometric charts
d) chemical properties and psychometric charts

Explanation: The equilibrium moisture content helps to decide the stability of food at particular moisture content in the given environment. It depends on the partial pressure of the water vapor in the surrounding air and relatively weak on the air temperature.

10. What is the relationship between equilibrium moisture content (EMC) and free moisture content (FMC)?
a) FMC=Total water content -EMC
b) FMC=Total water content + EMC
c) FMC=Total water content × EMC
d) Total water content = EMC × FMC

Explanation: EMC (equilibrium moisture content) of a hygroscopic substance is defined as the substance that is surrounded at least partially by air where it neither gains nor loses the moisture. FMC(free moisture content) is the amount of water that is free to evaporate from solid.

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