1. Fat Crystallization
April 14, 2015

2. Crystallization process
Fat Crystallization
Crystallization process
Polymorphism

3. Processes Involved in Crystallization and Storage of Fats
Himawan et. al. 2006

4. Crystallization: Liquid Fat  Solid Fat
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events

5. Postcrystallization Events
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events
Supercooling – temperature below melting point
Liquid oils must be cooled below their thermodynamic melting points before crystal formation is observed
Fats can remain liquid at temperatures below melting point
Continuous supercooling:
At low degree – crystal growth predominates
At high degree – nucleation predominates
Thermodynamic driving force – required to form the smallest stable solid entity from a liquid phase
Free energy barrier for the formation of the solid phase
Encourages liquid lamellar structure formation
Grows to a critical size before forming a solid nucleus

6. Postcrystallization Events
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events
Nucleation
Activation free energy for nucleation is lowered by increases in supercooling
Lipid molecules collide and become associated
 ordered domains
Beyond a certain size, further addition of molecules to such ordered domains result in a decrease in the Gibbs free energy of the system and when such ordered domains grow beyond a critical size, r*, a nucleus is formed (Ghorta et. al. 2002)

7. Postcrystallization Events
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events
Homogenous Nucleation
No impurities in the oil (rare)
Heterogenous nucleation – Primary or Secondary
Impurities act as catalytic nucleation sites for crystal growth
Equipment surfaces, emulsifiers, polar lipids, etc.
Lower activation free energy for nucleation
Less supercooling needed than homogeonous nucleation
Impurities can also inhibit further oil molecules from being incorporated into growing nuclei

8. Postcrystallization Events
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events
Primary Nucleation
Secondary Nucleation
Foreign surfaces have a different chemical structure than oil
Due to the presence of growing crystals in the melt or solution
Foreign surfaces are crystals with same chemical structure as liquid oil

9. Postcrystallization Events
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events
Crystal Growth
Stable nuclei grow into crystals by incorporating molecules from the liquid oil at the solid-liquid interface
Crystal growth rate dependent on:
Mass transfer of molecules from liquid to solid-liquid interface
Incorporation of molecules within crystal lattice
Removal of heat generated by crystallization process
Growth increases with increasing degree of supercooling until reaches maximum rate, then decreases
As solid fraction increases, individual crystals begin to touch each other which slows crystal growth

10. Nucleation and crystal growth rates have different temperature-dependencies, which account for differences in the number and size of fat crystals produced under different cooling regimes
McClements and Decker 2008

11. Postcrystallization Events
Supercooling
Nucleation
Crystal Growth
Postcrystallization Events
Postcrytallization Events
Sintering
Increase in hardness
Formation of solid bridges between fat crystals
Network formation due to mutual forces of adhesion
Polymorphic transformation
Rearrangement of TAG within crystals
Less stable  more stable rearrangement
Diffusion of TAG between glycerol molecules  crystal composition change in mixed crystals
Ostwald ripening
Net growth in average size of crystals

12. Crystallization process
Fat Crystallization
Crystallization process
Polymorphism

13. Lipid Crystal Structure//Polymorphism
Crystals can exist in different forms (polymorphs)
Subcell structure + layered structure  polymorphs
Subcell – packing mode of hydrocarbon chains of TAG
Layered – repetitive sequence of the acyl chains which form a unit lamella along hydrocarbon axis
Type of crystalline form depends on molecular structure and composition of the lipids (TAG), as well as the environmental conditions during crystallization

14. Polymorphic Forms
fat crystal β β’ α

15. Polymorphism α β’ β
Richards 2007

16. Polymorphism β’ β α
Ghorta et. al. 2002

17. Polymorphic Forms & Thermodynamic Stability
Thermodynamic stability, and thus melting point of the three forms decreases in the order: β β’ α
Lowest activation energy
Least thermodynamic stability
Lowest melting point
Formed first
Highest free energy
Polymorphic forms go through successive modifications until the most stable stage is reached

18. Melting Points of Beta and Beta-prime Polymorphs of Some Common TAGs
Polymorphism
Different polymorphic states of a particular substance often demonstrate quite different physical properties
Melting Points of Beta and Beta-prime Polymorphs of Some Common TAGs
Ghorta et. al. 2002

19. References
Ghorta BS, Dyal SD, Narine SS Lipid shortenings: a review. Food Research International. 35: McClements DJ, Decker EA Lipids. Fennema’s Food Chemistry. 4th ed. Damodaran S, Parkin KL, Fennema OR (eds.) CRC Press. Boca Raton, FL p. Narnie SS, Marangoni AG Relating structure of fat crystal networks to mechanical properties. Food Research International. 32: Richards MP Lipids: Functional Properties. Food Chemistry: Principles and Applications. 2nd ed. Hui YH (ed.) Science Technology System. West Sacramento, CA: p.