Food Processing Unit Operations Multiple Choice Questions on “Drying Rate”.
1. Rate of drying depends on which of the following statement?
Statement 1: Vapor pressure of water at drying temperature.
Statement 2: Vapor pressure in the external environment.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: b
Clarification: Rate of drying depends on 1) equilibrium vapor pressure of water in the food 2) moisture of the food 3) vapor pressure of water at drying temperature and 4) vapor pressure in the external environment.
2. Which of the following statements are true about drying?
Statement 1: Latent heat of vaporization is not necessary.
Statement 2: Moisture must be transported out from the food.
a) True, False
b) True, True
c) False, True
d) False, False
Answer: c
Clarification: Latent heat of vaporization is necessary as it converts its liquid into vapor and the moisture must be transported out from the food to obtain dehydrated product.
3. What happens to the drying rate when it reaches critical moisture content?
a) Declines
b) Increases gradually
c) Stops
d) Becomes constant
Answer: a
Clarification: The critical moisture content is defined as the average material moisture content at which the drying rate begins to decrease. The point that shifts to the Declining drying period from Constant drying period is known as critical moisture content
4. What is the independent factor of the constant rate period?
a) Moisture content
b) Temperature
c) Pressure
d) Humidity
Answer: a
Clarification: The constant rate period is independent of moisture content in the rate of drying curve. In this period, diffusion of moisture takes place where the droplet maintains saturated surface conditions and the evaporation takes place at constant rate.
5. At what stage the case hardening of a material takes place?
a) Constant rate period
b) Falling rate period
c) Initial rate period
d) Equilibrium
Answer: a
Clarification: The case hardening is defined as the formation of a hard skin on the surface of foods which slows down the drying rate and may leads to the mold growth. It is due to quick drying during the constant rate period and this can be stopped by using cooler drying air.
6. Name the phenomenon which transfers moisture during second falling drying rate period?
a) Liquid diffusion
b) Capillary movement
c) Vapor diffusion
d) Liquid diffusion, capillary movement, and vapor diffusion.
Answer: d
Clarification: During second falling drying rate period, the moisture transfer is due to the liquid diffusion, capillary movement, and vapor diffusion because the drying rate becomes independent of conditions outside the solid.
7. What type of drying takes place when the food has high fat contents?
a) Slow
b) Fast
c) Constant
d) Quick
Answer: a
Clarification: Higher fat contents results in the slower drying rate as water is trapped within the food. The size of the food affects the drying rate in both the constant and falling rate periods.
8. How does humidity affect drying rate?
a) No effect
b) Directly proportional
c) Indirect proportional
d) Linear increase
Answer: b
Clarification: The rate of evaporation depends mainly on the relative humidity than on the temperature. But both the relative humidity and temperature are correlated. When the air temperature increases, it absorbs more liquid and the relative humidity decreases. Low relative humidity helps in faster drying.
9. What is drying constant?
a) Mechanisms of heat and mass transport phenomena
b) Mechanism of heat phenomena
c) Mechanism of mass transport phenomena
d) Mechanism of drying phenomena
Answer: a
Clarification: Drying constant describes the mechanisms of heat and mass transport phenomena and investigates the influence that certain process variables exert on moisture removal processes. Drying constant is measured through experimental studies of material moisture content removal versus time at various drying conditions.
10. What happens to the drying rate during constant rate of drying period?
a) It reduces
b) It increases
c) Constant
d) Becomes negligible
Answer: b
Clarification: During the constant rate period, free moisture persists on the surfaces and the rate of evaporation modifies very little as moisture content decreases. During this period, drying rates are high and have high inlet air temperatures than in subsequent drying stages that can be used without detrimental effect to the product. There is a gradual and relatively small increase in the temperature of the product during this period.