Agricultural/Bioengineering Food Processing Unit Operation 3 — Questions and Answers
Question 1: Which unit operation best describes the removal of alcohol from fermented beer using a vacuum and gentle heat?
- Distillation under pressure
- Vacuum stripping (thin-film evaporation) (Correct answer)
- Reverse osmosis membrane separation
- Ion exchange demineralization
Correct answer: Vacuum stripping (thin-film evaporation)
Vacuum stripping or thin-film evaporation lowers the boiling point of ethanol, allowing gentle removal with minimal flavor loss.
Question 2: The HTST (High Temperature Short Time) pasteurization condition for fluid milk in the US is:
- 63°C for 30 minutes
- 72°C for 15 seconds (Correct answer)
- 85°C for 5 seconds
- 121°C for 2.4 minutes
Correct answer: 72°C for 15 seconds
US FDA mandates HTST pasteurization at a minimum of 72°C (161°F) for at least 15 seconds to eliminate pathogens including Coxiella burnetii.
Question 3: In extrusion cooking of breakfast cereals, the die pressure primarily controls:
- Feed moisture content entering the barrel
- Product expansion ratio and texture after exiting the die (Correct answer)
- Screw rotational speed and throughput
- Residence time in the preconditioner
Correct answer: Product expansion ratio and texture after exiting the die
Die pressure determines the degree of flash evaporation and starch expansion upon exiting, directly controlling puff texture and bulk density.
Question 4: Membrane filtration processes are ranked by pore size from largest to smallest as:
- Microfiltration → Ultrafiltration → Nanofiltration → Reverse Osmosis (Correct answer)
- Reverse Osmosis → Nanofiltration → Ultrafiltration → Microfiltration
- Ultrafiltration → Microfiltration → Reverse Osmosis → Nanofiltration
- Nanofiltration → Reverse Osmosis → Microfiltration → Ultrafiltration
Correct answer: Microfiltration → Ultrafiltration → Nanofiltration → Reverse Osmosis
MF has the largest pores (~0.1–10 µm), followed by UF (~0.001–0.1 µm), NF (~0.001 µm), and RO which rejects nearly all solutes.
Question 5: In a spray drier, atomization of the feed liquid is performed to:
- Increase droplet size to extend residence time
- Maximize surface area-to-volume ratio for rapid moisture evaporation (Correct answer)
- Reduce inlet air temperature requirements
- Prevent powder agglomeration in the drying chamber
Correct answer: Maximize surface area-to-volume ratio for rapid moisture evaporation
Atomization creates millions of tiny droplets with enormous surface area, allowing water to evaporate within seconds even at moderate temperatures.
Question 6: Which psychrometric property of air is held constant during adiabatic (isenthalpic) humidification in a spray-drying system?
- Dry-bulb temperature
- Relative humidity
- Wet-bulb temperature (Correct answer)
- Dew point temperature
Correct answer: Wet-bulb temperature
During adiabatic saturation, enthalpy is constant and the process follows a constant wet-bulb temperature line on the psychrometric chart.
Question 7: What is the function of a homogenizer valve in milk processing?
- To separate cream from skim milk by centrifugal force
- To force milk through a narrow gap, disrupting fat globules to prevent creaming (Correct answer)
- To pasteurize milk by generating frictional heat
- To remove air bubbles from the product before packaging
Correct answer: To force milk through a narrow gap, disrupting fat globules to prevent creaming
High-pressure homogenization forces milk through a tiny valve gap, reducing fat globule size from ~4 µm to <1 µm to create a stable emulsion.
Agricultural/Bioengineering Food Processing Unit Operation 3
Which unit operation best describes the removal of alcohol from fermented beer using a vacuum and gentle heat?