Agricultural/Bioengineering Food Processing Unit Operation 4 — Questions and Answers
Question 1: In a continuous screw conveyor used to move grain, doubling the screw pitch while keeping rotational speed constant will:
- Halve the conveying capacity
- Double the conveying capacity (Correct answer)
- Have no effect on capacity
- Quadruple the conveying capacity
Correct answer: Double the conveying capacity
Conveyor capacity is directly proportional to pitch distance; doubling pitch moves twice the material per revolution at the same RPM.
Question 2: The F₀ value in thermal process calculation represents:
- The time in minutes at 121.1°C (250°F) that delivers an equivalent lethal effect to the actual process (Correct answer)
- The temperature in °C at which microbial destruction rate doubles
- The number of decimal reductions needed for commercial sterility
- The ratio of actual to theoretical heat penetration rate
Correct answer: The time in minutes at 121.1°C (250°F) that delivers an equivalent lethal effect to the actual process
F₀ is the equivalent processing time at the reference temperature of 121.1°C, integrating the lethal effect of the actual time-temperature profile.
Question 3: Which mechanism dominates heat transfer in a can of solid-pack tuna during retort processing?
- Natural convection throughout the entire can
- Forced convection assisted by can rotation
- Conduction from the can wall through the product (Correct answer)
- Radiation from hot steam to the can surface
Correct answer: Conduction from the can wall through the product
Solid-pack tuna has no free liquid, so heat must conduct slowly through the dense fish tissue from the container wall to the thermal center.
Question 4: A food plant evaporator operates as a triple-effect system. Compared to a single-effect evaporator with the same evaporation duty, steam economy is approximately:
- Equal because total energy required is the same
- Three times better (uses ~1/3 the steam) (Correct answer)
- Nine times better due to exponential savings
- Dependent solely on product viscosity
Correct answer: Three times better (uses ~1/3 the steam)
In a triple-effect evaporator, steam from one effect heats the next, so roughly 1 kg of steam can evaporate ~3 kg of water, tripling steam economy.
Question 5: In size reduction of grains, Rittinger's law states that energy required is proportional to:
- The cube of the particle diameter
- The new surface area created during grinding (Correct answer)
- The ratio of feed to product particle sizes
- The volume reduction of the material
Correct answer: The new surface area created during grinding
Rittinger's law applies to fine grinding and holds that energy input is proportional to the new surface area generated during comminution.
Question 6: During evaporative concentration of fruit juice, boiling point elevation (BPE) causes the juice to:
- Boil at a lower temperature than pure water at the same pressure
- Boil at a higher temperature than pure water at the same pressure (Correct answer)
- Freeze at a higher temperature than the original juice
- Have the same boiling point as water when sufficiently dilute
Correct answer: Boil at a higher temperature than pure water at the same pressure
Dissolved solutes (sugars, acids, minerals) elevate the boiling point above that of pure water at the same pressure, requiring more energy to evaporate.
Question 7: What is the primary advantage of using a scraped-surface heat exchanger (SSHE) for processing high-viscosity foods like tomato paste?
- It requires less cleaning time than plate heat exchangers
- It continuously removes the product film from the heat transfer surface, preventing burn-on and maintaining high heat transfer rates (Correct answer)
- It operates at lower pressures than tubular heat exchangers
- It eliminates the need for temperature control in the product
Correct answer: It continuously removes the product film from the heat transfer surface, preventing burn-on and maintaining high heat transfer rates
Rotating blades scrape the viscous product off the heated wall, preventing fouling and maintaining the thin boundary layer needed for efficient heat transfer.
Agricultural/Bioengineering Food Processing Unit Operation 4
In a continuous screw conveyor used to move grain, doubling the screw pitch while keeping rotational speed constant will: