ICAR AICE-JRF/SRF Evaporation, Dehydration & Concentration of Foods: practice questions

1 · 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

2 · 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

3 · Drying theory: constant-rate and falling-rate periods · hard

In the constant-rate period, the heat transfer coefficient is 30 W/m² K, the air is at 60 °C with wet-bulb temperature 30 °C, and the latent heat at 30 °C is 2,430 kJ/kg. The drying rate is closest to

1. 0.370 kg/m² h
2. 1.33 kg/m² h
3. 2.67 kg/m² h
4. 1333 kg/m² h

4 · Drum drying · medium

A producer wants a light-coloured, aroma-rich instant fruit powder but has only a drum dryer available. Which modification best limits heat damage?

1. Use a vacuum drum dryer so the film dries at a lower temperature
2. Raise the steam pressure in the drum to finish drying faster
3. Apply a thicker film so that the product spends longer on the drum
4. Recycle the dried flakes back onto the drum for a second pass

5 · 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

6 · Spray drying: atomisation, powder properties and agglomeration · hard

Lactose in whey concentrate is often pre-crystallised before spray drying to

1. remove the lactose completely so that only protein remains
2. avoid hygroscopic amorphous lactose that makes powder sticky
3. increase the amount of amorphous lactose for better solubility
4. convert lactose into glucose and galactose for sweetness

7 · Freeze concentration and membrane concentration · medium

In freeze concentration, a ripening vessel after the scraped-surface crystalliser is used to

1. melt all the ice so that it can be returned to the juice
2. freeze the concentrate solid for storage
3. add sugar to raise the freezing point
4. let small ice crystals dissolve and large ones grow so they separate cleanly

8 · 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

9 · 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

10 · Quality changes in dried foods and rehydration · medium

Sulphur dioxide or sulphites are traditionally used on dried apricots mainly to

1. increase water activity for a softer texture
2. add sweetness to the dried fruit
3. sterilise the fruit so that no drying is needed
4. prevent browning and help retain colour and vitamin C