ICAR AICE-JRF/SRF Chilling, Freezing & Frozen Food Technology: practice questions

1 · Chilling and chilled storage of foods · hard

Non-proteolytic Clostridium botulinum is a particular hazard in long-life, reduced-oxygen chilled foods because it

1. grows only above 30 °C but forms toxin very quickly once the pack warms up
2. is usually destroyed completely by chilling to 4 °C for about one day
3. produces gas and odour that always warn the consumer before toxin forms
4. can grow and form toxin down to about 3 °C

2 · Freezing-point depression and unfrozen water · hard

For the same food with initial freezing point −1.5 °C, the fraction of freezable water frozen at −3 °C is about

1. 92 %
2. 25 %
3. 0 %
4. 50 %

3 · Freezing-point depression and unfrozen water · medium

Frozen foods conduct heat better than unfrozen foods mainly because

1. frozen foods typically contain more air, which is an excellent conductor of heat
2. ice conducts heat about four times better than liquid water
3. freezing removes all fat, which is the main insulator in foods
4. ice has a lower density than water, so heat moves faster through it

4 · Ice nucleation, crystal growth and recrystallisation · hard

Large frozen items such as whole tuna can crack when frozen very rapidly in liquid nitrogen because

1. liquid nitrogen often reacts chemically with fish proteins and weakens the muscle structure
2. rapid freezing removes all the water from the surface as vapour
3. water expands on freezing and the rigid outer shell cannot accommodate the core
4. the core contracts on freezing and pulls the surface inwards

5 · Freezing-time prediction: Plank's equation and its refinements · hard

Plank's equation, infinite slab (P = 1/2, R = 1/8): fish 5 cm thick, ρ = 1,000 kg/m³, latent heat 250 kJ/kg, Tf = −1.5 °C, air at −30 °C, h = 20 W/m² K, frozen k = 1.5 W/m K. The freezing time is closest to

1. 1.6 h
2. 3.6 h
3. 1.2 h
4. 4.6 h

6 · Freezing-time prediction: Plank's equation and its refinements · hard

The same 5 cm fish slab (ρ = 1,000 kg/m³, λ = 250 kJ/kg, Tf = −1.5 °C, k = 1.5 W/m K) is frozen in a plate freezer with medium at −30 °C and h = 200 W/m² K. By Plank's equation the freezing time is closest to

1. 487 min
2. 18 min
3. 49 min
4. 0.8 min

7 · Freezing-time prediction: Plank's equation and its refinements · medium

For a thin product frozen in still air (very low h), doubling its thickness changes the freezing time by a factor of about

1. 4
2. 2
3. 1
4. 0.5

8 · Time-temperature tolerance and shelf life of frozen foods · hard

A frozen vegetable has practical storage lives of 360 days at −24 °C, 270 days at −18 °C and 90 days at −12 °C. It spends 100 days at −24 °C, 40 days at −18 °C and 20 days at −12 °C. Its remaining life at −18 °C is about

1. 110 days
2. 95 days
3. 155 days
4. 195 days

9 · Time-temperature tolerance and shelf life of frozen foods · medium

Codex texts and many national laws define quick-frozen foods as foods held at the thermal centre at

1. −18 °C or colder
2. −5 °C or colder
3. 0 °C or colder
4. −40 °C or colder

10 · Supercooling, isochoric and other emerging freezing methods · medium

Dehydrofreezing of fruits and vegetables involves

1. freezing and then complete drying
2. partial drying before freezing
3. freezing in a dehumidified room
4. thawing and refreezing twice