Fat Crystallization
APRIL 14, 2015
Fat Crystallization
Crystallization process
Polymorphism
Processes Involved in Crystallization and Storage of Fats
Himawan et. al. 2006
Crystallization: Liquid Fat  Solid Fat
Supercooling
Nucleation
Crystal Growth
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
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)
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


Lower activation free energy for nucleation


Equipment surfaces, emulsifiers, polar lipids, etc.
Less supercooling needed than homogeonous nucleation
Impurities can also inhibit further oil molecules from being
incorporated into growing nuclei
Supercooling
Nucleation
Crystal Growth
Postcrystallization
Events
Primary Nucleation
Secondary Nucleation
 Foreign surfaces have a
 Due to the presence of
different chemical
structure than oil
growing crystals in the
melt or solution
 Foreign surfaces are
crystals with same
chemical structure as
liquid oil
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
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
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

Fat Crystallization
Crystallization process
Polymorphism
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
Polymorphic Forms
β
β’
fat
crystal
α
Polymorphism
α
β’
β
Richards 2007
Polymorphism
β’
β
α
Ghorta et. al. 2002
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
Polymorphism
Different polymorphic states of a particular substance often
demonstrate quite different physical properties
Melting Points of
Beta and Betaprime
Polymorphs of
Some Common
TAGs
Ghorta et. al. 2002
References
Ghorta BS, Dyal SD, Narine SS. 2002. Lipid shortenings: a
review. Food Research International. 35: 1015-1048.
McClements DJ, Decker EA. 2008. Lipids. Fennema’s Food
Chemistry. 4th ed. Damodaran S, Parkin KL, Fennema OR (eds.)
CRC Press. Boca Raton, FL. 155- 216 p.
Narnie SS, Marangoni AG. 1999. Relating structure of fat crystal
networks to mechanical properties. Food Research
International. 32: 227-248.
Richards MP. 2007. Lipids: Functional Properties. Food
Chemistry: Principles and Applications. 2nd ed. Hui YH (ed.)
Science Technology System. West Sacramento, CA: 7-1-7-20 p.