GATE Evaporation, Dehydration & Concentration of Foods: practice questions

1 · Psychrometry, humidification and dehumidification · medium

As air passes over wet food in an adiabatic dryer, its state follows approximately a line of constant

1. wet-bulb temperature
2. dry-bulb temperature
3. humidity ratio
4. relative humidity

2 · Psychrometry, humidification and dehumidification · medium

Air at 30 °C has a water vapour partial pressure of 2.0 kPa; the saturation vapour pressure at 30 °C is 4.246 kPa. Its relative humidity is closest to

1. 212 %
2. 47 %
3. 1.3 %
4. 2.0 %

3 · Evaporation: boiling-point elevation and heat transfer · easy

10,000 kg/h of juice at 12 % solids is concentrated to 48 % solids. The water evaporated is

1. 2,500 kg/h
2. 7,500 kg/h
3. 3,600 kg/h
4. 9,750 kg/h

4 · Single- and multiple-effect evaporators; steam economy · medium

Backward feed in a multiple-effect evaporator is preferred when

1. the product is heat-sensitive and must never meet the hottest effect
2. no pumps are allowed between effects
3. the concentrate is viscous and the feed is cold
4. the feed is already above the first effect's boiling point

5 · Falling-film, forced-circulation and plate evaporators · easy

The evaporator type most widely used for concentrating milk and fruit juices is the

1. open steam-jacketed pan
2. falling-film evaporator
3. short-tube natural-circulation evaporator
4. agitated thin-film evaporator

6 · Falling-film, forced-circulation and plate evaporators · medium

A rising-film (climbing-film) evaporator differs from a falling-film evaporator in that

1. expanding vapour drives the film up the tubes, so a larger ΔT is needed
2. the liquid flows down the tubes under gravity, so it works with very small differences
3. the liquid is pumped at high velocity with boiling suppressed inside the tubes
4. the liquid is spread by rotating blades on a heated cylinder wall

7 · Spray drying: atomisation, powder properties and agglomeration · medium

Agglomeration (instantising) of a spray-dried powder improves

1. oxidative stability, by removing all fat from the particle surfaces
2. bulk density, by producing the finest possible single particles
3. wettability and dispersibility, so the powder reconstitutes without lumps
4. microbial safety, by sterilising the powder during rewetting

8 · Osmotic dehydration and intermediate-moisture foods · easy

Intermediate-moisture foods typically have a water activity of about

1. 0.1 to 0.3
2. 0.95 to 1.0
3. 0.6 to 0.9
4. 0.3 to 0.5

9 · Freeze concentration and membrane concentration · medium

The heat that must be removed to freeze 1 kg of water (latent heat of fusion) is about

1. 2,260 kJ
2. 2,840 kJ
3. 334 kJ
4. 4.18 kJ

10 · Quality changes in dried foods and rehydration · hard

The selective diffusion theory explains aroma retention in spray-dried products by stating that

1. a dried surface passes water far more readily than aroma molecules
2. aroma molecules evaporate faster than water from the droplet throughout drying
3. aromas are destroyed by heat and replaced by Maillard flavours
4. aromas bind to the chamber walls and return to the powder later