Subtopic: Concentration and drying of liquid foods – evaporators, tray, drum and spray dryers
Question Statement:
Which of the following statements regarding the operation of industrial spray dryers for food concentrates are CORRECT?
Options:
(A) In a co-current flow spray dryer, the hottest inlet air contacts the wettest atomized droplets, keeping product temperature low at the local wet-bulb temperature due to evaporative cooling.
(B) Wall deposition of sticky powder occurs when droplet surface temperature exceeds its glass transition temperature ($T_g$).
(C) Rotary disk atomizers are generally more tolerant of viscous feeds and produce a more uniform droplet size distribution than pressure swirl nozzles.
(D) Counter-current spray drying is preferred for highly heat-sensitive enzymes and probiotics.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Analysis of spray dryer characteristics:
- Option A is CORRECT: In co-current spray dryers, inlet air ($160\text{--}200^\circ\text{C}$) enters alongside atomized spray. Rapid evaporation maintains the droplet temperature near the wet-bulb temperature ($40\text{--}50^\circ\text{C}$), protecting thermolabile components.
- Option B is CORRECT: Amorphous sugars undergo glass-to-rubbery transition when $T_{\text{surface}} > T_g + 20^\circ\text{C}$, becoming highly sticky and adhering to chamber walls.
- Option C is CORRECT: Rotary atomizers handle high-viscosity slurries with minimal clogging compared to narrow orifice nozzles.
- Option D is INCORRECT: Counter-current flow exposes the driest powder to the hottest incoming dry air, causing severe thermal degradation.
```
[Feed Liquid]
|
v
(Rotary Atomizer)
/ | \
[Hot Air] -> v <- [Hot Air]
| (Co-Current Zone) |
| Rapid Evaporative |
| Cooling |
\ | /
\ v /
\ (Dried Powder) /
\ | /
v
[Powder Exit]
```