Agricultural/Bioengineering Food Processing Unit Operation 5 — Questions and Answers
Question 1: In fluidized bed drying of particulate food, the minimum fluidization velocity is reached when:
- All particles are entrained and carried out of the bed by the airstream
- The drag force on particles exactly equals their weight in the bed (Correct answer)
- Particle temperature exceeds the wet-bulb temperature of the drying air
- Air velocity equals twice the terminal settling velocity of the particles
Correct answer: The drag force on particles exactly equals their weight in the bed
At minimum fluidization velocity, upward drag and buoyancy forces exactly balance particle weight, causing the bed to expand and become fluidized.
Question 2: Which type of pump is most appropriate for moving a shear-sensitive food liquid such as whole berry yogurt without damaging the fruit pieces?
- Centrifugal pump operating at 3,600 RPM
- Positive displacement rotary lobe pump (Correct answer)
- Gear pump with tight clearances
- Turbine pump with high impeller speed
Correct answer: Positive displacement rotary lobe pump
Rotary lobe pumps move product gently through large cavities with minimal shear, preserving the integrity of delicate particulates.
Question 3: The D-value in food microbiology is defined as:
- The temperature required to kill all organisms in one minute
- The time in minutes at a given temperature to reduce the microbial population by 90% (one log cycle) (Correct answer)
- The decimal reduction temperature for a standard organism
- The minimum lethal dose of thermal energy for a pathogen
Correct answer: The time in minutes at a given temperature to reduce the microbial population by 90% (one log cycle)
D-value (decimal reduction time) is the time needed at a constant temperature to destroy 90% of a specific microbial population.
Question 4: In a pneumatic conveying system for flour, 'dense phase' conveying differs from 'dilute phase' primarily in that it:
- Uses higher air velocities to keep all particles suspended
- Operates at lower velocities with higher product-to-air ratios, reducing particle degradation (Correct answer)
- Requires open-ended pipes rather than a closed loop
- Is only suitable for wet or sticky materials
Correct answer: Operates at lower velocities with higher product-to-air ratios, reducing particle degradation
Dense phase conveying uses high pressure at low velocity with high solid loading, minimizing particle breakage and attrition compared to high-velocity dilute phase.
Question 5: Water activity (aₓ) of a food product is defined as:
- The percentage of total water content by weight
- The ratio of the partial pressure of water in the food to the vapor pressure of pure water at the same temperature (Correct answer)
- The temperature at which a food releases bound water
- The concentration of dissolved solids per unit of free water
Correct answer: The ratio of the partial pressure of water in the food to the vapor pressure of pure water at the same temperature
Water activity equals p/p₀, the equilibrium relative humidity divided by 100, and determines microbial growth potential and chemical reaction rates.
Question 6: In aseptic processing and packaging of juice, the product is sterilized:
- Inside the sealed package using retort heating
- Continuously in a heat exchanger before being filled into pre-sterilized containers (Correct answer)
- By irradiation after the package is sealed
- By adding chemical preservatives prior to packaging
Correct answer: Continuously in a heat exchanger before being filled into pre-sterilized containers
Aseptic processing sterilizes product and packaging separately in continuous flow, then combines them in a sterile environment, enabling UHT treatment with minimal quality loss.
Question 7: The overall heat transfer coefficient (U) in a food heat exchanger is most significantly reduced when:
- Product flow velocity is increased
- A fouling layer deposits on the heat transfer surface (Correct answer)
- Heat exchanger wall thickness is decreased
- Product viscosity is reduced by heating
Correct answer: A fouling layer deposits on the heat transfer surface
Fouling deposits act as an insulating layer with very low thermal conductivity, adding fouling resistance that dramatically reduces the overall U value.
Agricultural/Bioengineering Food Processing Unit Operation 5
In fluidized bed drying of particulate food, the minimum fluidization velocity is reached when: