polymorphism. polymorphism the occurrence of several different crystal forms for a same compound
TRANSCRIPT
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POLYMORPHISM
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Polymorphism
The occurrence of several different crystal forms for a same compound.
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Packing Types of Fatty Acids and Position
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Fatty Acid Crystal Structure
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Cross-Sectional Structures of Triglycerides
Alpha
Beta Prime Beta
Freedom of molecular motion
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Hexagonal
Triclinic
Orthorhombic
a
b
c
b c
c
b a
a 120º
Crystal forms
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Space Arrangements of Triglyceride Crystals
11
22
33
Tuning fork form Chair form
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Double Chain Length Structures of Triglycerides
Alpha Beta-Prime Beta (Vertical tuning fork) (Tilted tuning fork) (Stacked chair)
D
DD
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Transformation of Crystal Structures
Simple
Collapse
Cataclysmic
Chain shift
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Types of Fatty Acids and Positions for Double Chain Length
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Molecular Arrangement in Trilaurin Lattice
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Triple Chain Length Structures of Triglycerides
D
D D
Alpha Beta-Prime Beta (Vertical tuning fork) (Tilted tuning fork) (Stacked chair)
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Types of Fatty Acids and Positions for Triple Chain Length
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sin
= 4
3.76
Å
63º38’
c =
48.
84 Å
63º38’
b = 7.38 Å
a = 5.54 Å
Unit Cell of Stearic Acid
= COOH
= CH3
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Crystal Structure of Oleic Acid
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Analytical Methods for Polymorphism Study
1. X-ray diffraction analyses
2. Microscopic analyses
3. IR spectroscopic analyses
4. Thermal analyses
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Characteristics of Triglygeride Polymorphs Alpha Beta-Prime Beta
________________________________________________________________________
X-ray Diffraction Analyses
Hexagonal Orthorhombic Triclinic
Tuning fork Tuning fork Chair form
Acyl groups oriented Acyl groups are tilted Acyl groups are tiltedat 90 to the plane of 68-70 from plane of about 59 from thethe glyceryl group the glyceryl group plane of the glyceryl
groups
Vertical chain Tilted chain Tilted chainorientation orientation orientation
Longest long Intermediate Shortest longspacing long spacing spacing
Randomly ordered In-between Highly ordered
Most loosely packed More closely packed Most closely packed________________________________________________________________________
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Characteristics of Triglyceride Polymorphs
Alpha Beta-Prime Beta
Microscopic Analyses
Platelet Fine needle Long needle
5 1 25-50
Infrared Spectroscopic Analyses
A singlet at 720 cm -1 A doublet at A singlet at 717 cm -1
719 and 727 cm-1
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Characteristics of Triglyceride Polymorphs
Alpha Beta-Prime Beta
Thermal Analyses
Thermodynamically Thermodynamically Most stable formmost unstable unstable
Lowest melting point Intermediate melting Highest meltingpoint point
Color Analyses
Translucent In-between Opaque___________________________________________________________________
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Dilatometric Curve of Tristearin Polymorphic Form
Spec
ific
vol
ume
Temperature C
Alpha
Beta
30 50 700.94
0.96
0.98
1.02
1.04
1.06
1.08
1.10
1.00
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Formation of Triglyceride Polymorphs
Alpha Beta-Prime Beta
________________________________________________________________________
Rapid cooling Slow cooling Very slow coolingof liquid fat of liquid fat of liquid fat
_ _ _ _ _ _ _ _ Polymorphic Polymorphictransformation of transformation ofthe alpha form beta-prime
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Activation Energy of Crystal Nucleation
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Factors for Different Crystals Formation
Tristearin
Melt
Drop into dry ice pieces
Solid
Room temperature for 2 weeks
Heat
Heat
Heat
Heat
Heat
Melt at 73 C
Melts at 55 C and Resolidification
Melts at 64 C
Melt at 73 C
and Resolidification
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APPLICATION OF POLYMORPHISM IN FOODS
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Crystal Form Preference of Oils
Beta-type Beta-Prime Type
Coconut oil Cottonseed oil
Corn oil Herring oil
Olive oil Menhaden oil
Lard Milk fat
Palm kernel oil Palm oil
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Application of Polymorphism in Foods
The incorporation of air, plasticity and consistency, and solid-liquid ratio are important characteristics of shortenings.
These physical characteristics in turn depend upon the polymorphic forms of the fats used and the methods of preparation.
Plasticity and consistency, as well as solid-liquid ratio, depends on the melting range, proper tempering is required to form the mixed crystals required for a broad melting range.
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Shortening
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1. Incorporated Air
Beta-prime crystals large amount of small air bubblesBeta-crystals small amount of large air bubbles
Beta-Prime Crystal ShorteningSmall crystalsWhiter, creamier, more tender, smoother textureUniform and glossy texture
Beta-Crystal ShorteningLarge clustered crystalsA waxy or grainy texture
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Beta-prime crystal shortening helps the incorporation of an abundant quantity of small air bubbles in batters for good volume,
texture and tenderness of baked goods.
Natural lard (palmitic acid at 2 position is 64%) -crystal (OPS)
Rearranged lard (palmitic acid at 2 position is 24%) ’crystal (POS)
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Lard conversion methods from beta to beta-prime
Interesterification Interesterification and hydrogenation Winterization (destearinization) Addition of cottonseed oil and/or tallow flakes
(beta- prime)
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2. Plasticity and Consistency
Plasticity is the changes in consistency as a function of temperature.
Consistency is the apparent hardness at a temperature
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Margarine
Spreadability
Plasticity
Water-in-Oil Emulsion:
Scraped surface heat exchanger Partial solidification Crystallizer Crystallization to have desired plastic
properties
Blend of soybean oil () and hydrogenated cottonseed oil (prime) for margarine
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Confectionary Fat (Enrobing Fat)
Cocoa Butter – Short melting range at mouth temperature
Melting range of confectionary fat:Fatty acid compositionProper crystallization
Cocoa Butter: 80% of cocoa butter is disaturated triglyceride.
SOS 20%POS 55%POP 5%
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Beta-prime beta form Chocolate “bloom” – white spots and dull surface appearance
Small crystal structure a good glossy textureBeta Large clustered crystal structure
POS determines characteristic cocoa butter texture
POS: Alpha form 17.0CBeta-prime form 27.0C
Beta form 35.5C
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Tempering
A process which permits transformation to the
proper polymorphic form.
During crystallization, the heat of transformation
must be removed to avoid melting and a later
conversion into large beta crystal.
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Melting Points of Triglycerides vs. Chain Length
Fatty Acid Chain Length