Agricultural/Bioengineering Agricultural/Bioengineering: Food Engineering 3 — Questions and Answers
Question 1: High-pressure processing (HPP) of food at 600 MPa is most effective at inactivating:
- Bacterial spores
- Prions
- Vegetative bacteria and most viruses (Correct answer)
- Bacterial endotoxins
Correct answer: Vegetative bacteria and most viruses
HPP at commercial pressures (400–600 MPa) effectively inactivates vegetative bacteria, yeasts, and molds but is largely ineffective against bacterial spores and prions.
Question 2: Water activity (aw) of 0.85 represents the approximate lower limit for growth of:
- Most molds
- Most xerophilic fungi
- Most spoilage bacteria (Correct answer)
- Osmophilic yeasts
Correct answer: Most spoilage bacteria
Most spoilage bacteria require aw above 0.91–0.95, while 0.85 is near the lower limit for the most salt-tolerant bacteria.
Question 3: In freeze-drying, the primary drying stage involves:
- Evaporation of liquid water at reduced pressure
- Sublimation of ice directly to vapor under vacuum (Correct answer)
- Condensation of water vapor on cold trays
- Adsorption of water onto the product surface
Correct answer: Sublimation of ice directly to vapor under vacuum
Primary drying in freeze-drying removes ice by sublimation — the direct solid-to-vapor phase transition — under high vacuum and controlled low temperature.
Question 4: The emulsifying capacity of soy protein isolate is mainly attributed to its:
- High carbohydrate content
- Amphiphilic structure with both hydrophilic and hydrophobic regions (Correct answer)
- High ash content
- Low molecular weight peptides only
Correct answer: Amphiphilic structure with both hydrophilic and hydrophobic regions
Soy proteins stabilize emulsions by adsorbing at the oil-water interface due to their amphiphilic structure, which has both water-loving and fat-loving regions.
Question 5: Which packaging atmosphere modification is used to slow ripening and extend shelf life of fresh-cut produce?
- Elevated O2 and reduced CO2
- Reduced O2 and elevated CO2 (Correct answer)
- Pure nitrogen atmosphere with no CO2
- Elevated O2 and elevated CO2
Correct answer: Reduced O2 and elevated CO2
Modified atmosphere packaging for fresh-cut produce uses low O2 (2–5%) and elevated CO2 (5–10%) to reduce respiration rate and microbial growth.
Question 6: During spray drying, the 'outlet temperature' of the drying chamber primarily controls:
- Feed pump flow rate
- Final product moisture content (Correct answer)
- Droplet size distribution
- Nozzle atomization pressure
Correct answer: Final product moisture content
The outlet temperature determines the extent of drying and thus the residual moisture content of the spray-dried powder.
Question 7: The phenomenon of 'tempering' chocolate involves:
- Adding sugar to reduce bitterness
- Controlled crystallization of cocoa butter into stable polymorphic Form V (Correct answer)
- Rapid cooling to prevent bloom
- Emulsification with soy lecithin only
Correct answer: Controlled crystallization of cocoa butter into stable polymorphic Form V
Tempering guides cocoa butter into the stable beta (Form V) polymorph, giving chocolate its characteristic snap, gloss, and melt-in-mouth properties.
Agricultural/Bioengineering Agricultural/Bioengineering: Food Engineering 3
High-pressure processing (HPP) of food at 600 MPa is most effective at inactivating: