Agricultural/Bioengineering Food Processing Unit Operation 2 â Questions and Answers
Question 1: In a counterflow heat exchanger used for milk pasteurization, which configuration provides the greatest thermal efficiency?
- Parallel flow with equal flow rates
- Counterflow with product and heating medium flowing in opposite directions (Correct answer)
- Cross-flow with baffles directing fluid perpendicular to tubes
- Shell-and-tube with product on the shell side only
Correct answer: Counterflow with product and heating medium flowing in opposite directions
Counterflow configuration maintains a more uniform temperature driving force along the entire exchanger length, maximizing heat transfer efficiency.
Question 2: The Reynolds number in a food processing pipeline is 1,800. What flow regime is present?
- Turbulent flow
- Transitional flow
- Laminar flow (Correct answer)
- Plug flow
Correct answer: Laminar flow
A Reynolds number below 2,100 indicates laminar (streamline) flow in a pipe, where fluid layers move parallel without mixing.
Question 3: Which drying method preserves heat-sensitive nutrients best while achieving very low final moisture content?
- Drum drying at atmospheric pressure
- Spray drying with high inlet air temperature
- Freeze drying (lyophilization) under vacuum (Correct answer)
- Fluidized bed drying at 120°C
Correct answer: Freeze drying (lyophilization) under vacuum
Freeze drying removes water by sublimation at low temperatures, minimizing thermal degradation of vitamins, flavors, and bioactive compounds.
Question 4: In centrifugal separation of cream from milk, increasing the bowl speed by a factor of 2 increases the centrifugal force by a factor of:
- 2
- 4 (Correct answer)
- 8
- 16
Correct answer: 4
Centrifugal force is proportional to the square of angular velocity (F = mÏÂēr), so doubling speed quadruples the separating force.
Question 5: What is the primary purpose of blanching vegetables before freezing?
- To sterilize the product for ambient storage
- To inactivate enzymes that cause quality deterioration during frozen storage (Correct answer)
- To reduce the freezing point of cellular water
- To increase the sugar content through starch hydrolysis
Correct answer: To inactivate enzymes that cause quality deterioration during frozen storage
Blanching inactivates peroxidase, lipoxygenase, and other enzymes that cause off-flavors, color changes, and texture degradation during frozen storage.
Question 6: In a rotary drum dryer processing grain, the 'critical moisture content' refers to:
- The equilibrium moisture content at ambient conditions
- The moisture content below which product quality degrades
- The point where the drying rate transitions from constant to falling rate (Correct answer)
- The maximum moisture content allowed by food safety regulations
Correct answer: The point where the drying rate transitions from constant to falling rate
Critical moisture content marks the transition from the constant-rate period (surface evaporation) to the falling-rate period (internal diffusion-controlled drying).
Question 7: A plate-and-frame filter press is operating in constant-pressure filtration. As the filter cake builds up, the filtration rate:
- Remains constant because pressure is maintained
- Increases due to improved cake porosity
- Decreases as resistance through the cake grows (Correct answer)
- Fluctuates cyclically with cake compression
Correct answer: Decreases as resistance through the cake grows
At constant pressure, increasing cake thickness adds resistance so that flow rate must decrease according to Darcy's law.
Agricultural/Bioengineering Food Processing Unit Operation 2
In a counterflow heat exchanger used for milk pasteurization, which configuration provides the greatest thermal efficiency?