1. Introduction to Food Engineering
FOOD FREEZING
Introduction to Food Engineering

2. Food Freezing The reduction of product temperature to levels below 0 C
Significant reduction in growth rates of microorganisms
Reduce rates of enzymatic and oxidation reactions
Formation of ice crystals reduce Aw

3. Food Freezing
Some products require rapid freezing for small ice crystal, minimum damage to product structure
Other products not influence by structural changes – no need for rapid freezing
Some products cannot freeze rapidly

4. Food Freezing Product quality = freezing time + storage condition
Fluctuations in storage temperature

5. Freezing Systems Aim : remove sensible heat, latent heat of fusion
20 % of water remains in liquid state
To reduce temp in short time
Temp of medium much lower than final product temp
Large convective heat-transfer coefficients

6. 1. Indirect contact systems

7. Plate freezer
Batch or continuous

8. Air-blast freezer Product size/shape not accommodate plate freezing
Package film is the barrier
Source of refrigeration = cold air
Batch or continuous

9. Freezers for liquid foods
Mostly before packaging
Scraped-surface system
60-80 % latent heat removed
Frozen slurry eg. ice cream
Batch or continuous

10. 2. Direct contact systems
No barriers to heat transfer between refrigerant and product

11. Air blast Low-temp air at high speeds Individual quick freezing (IQF)
High convective heat-transfer coefficient
Small product shape
Fluidized-bed IQF

12. Immersion
Nitrogen, CO2, freon

13. Immersion
Disadvantages
Cost of refrigerant
Difficult to recover

14. Frozen-food properties
Water in food changes to solid. Density, thermal conductivity, heat content (enthalpy), apparent specific heat change.

15. Density

16. Thermal Conductivity

17. Enthalpy

18. Apparent Specific Heat

19. Apparent Specific Heat
Below 20 C below initial freezing temp = value of unfrozen food

20. Freezing Time
Plank’s Equation

21. tF = freezing time
 = density
HL = latent heat of fusion
a = size (thickness, diameter)
hc, k
Infinite plate P =1/2 , R = 1/8
Infinite cylinder P = 1/4 , R = 1/16
Sphere P = 1/2 , R = 1/24

22. Example
Spherical food product is being frozen in air-blast tunnel. Initial product temp = 10 C, cold air – 15 C. Product diameter 7 cm, density 1,000 kg/m3. Initial freezing temp – 1.25 C, latent heat of fusion 250 kJ/kg. K product = 1.2 W/m.K. Compute freezing time.

23. P = 1/6 , R = 1/24
hc = 50 W/m2K
tF = x 103 s = 2.04 hr.

24. Limitations to Plank’s Equation
Density values for frozen foods are difficult to locate/measure
Latent heats of fusion – for water & water content of product
Initial & final product temp not accounted for
Thermal conductivity, k, for frozen product not readily available

25. Other Freezing-Time Prediction Methods
a) One-dimensional infinite slab
Valid within following ranges
0.02<NBi< 11, 0.11< NSte<0.36,
0.03<NPk<0.61

26. Cu, Cf = vol specific heat of unfrozen, frozen products

27. L = latent heat

28. b) Ellipsoid Shapes t ellipsoid = tslab/E For infinite slab E = 1
Infinite cylinder E = 2
Sphere E = 3

29.  = dimension factor of ellipsoid

32. Example Lean beef 74.5 % moisture content
Length = 1 m, width = 0.6 m, thickness = 0.25
hc = 30 W/m2K, air temp = -30 C, Ti = 5 C, TF = C, density 1050 kg/m3
Etc.