Crystallization

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Crystallization
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Crystallization

• Texture
• number of crystals
• size distribution of crystals
• shape factor
• Refining operations
• Food processing

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• Desired
• ice crsytallization
• ice cream
• frozen foods
• freeze drying
• sugar crystallization
• confectionery products
• powders
• lipid crystallization
• chocolate
• spreads

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• Undesired
• sugars
• confectionery
• dairy products
• baked goods
• ice
• recrystallization in frozen foods

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• Controlling crystallization
• solubility
• melting/freezing point
• processing parameters
• storage conditions

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• Crystalline structure
• lattice structure
• regular, repeating pattern of molecules
• limited mobility
• amorphous structure
• random orientation of molecules

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• Crystal systems
• seven types
• unit cells combined to form lattice
• depends on the nature of ions, atoms, molecules
• bonding in the lattice
• ionic NaCl
• covalent diamond
• weak molecular forces or hydrogen bonds sugar
• metallic

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• Enantiomorphism
• D and L-form, saccharides and acids
• ? and ? form, reducing sugars
• hydration, ?-lactose (monohydrate)
• ?-lactose (anhydrous)

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• Polymorphism
• different crystal forms
• depending on the structure and arrangement of the
  molecules
• triglycerides
• ? -gt ? -gt ?
• transition to the lowest energy form

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• Crystal shape
• tabular (thin disks)
• prismatic
• acicular (needles)
• speed of crystal growth
• temperature
• presence of impurities
• dendritic growth, tree-like, fast

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• Crystal size distribution
• powder properties
• filtration
• texture
• number of crystals
• size range (threshold)
• shape

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• Crystal size measurements
• sieve analysis
• sedimentation
• microscopy
• Coulter counter
• light scattering

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Solution structure

• driving force
• temperature and concentration
• molecular interactions
• viscosity (molecular mobility)

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• Solution
• Cs saturation concentration, equilibrium
• CltCs undersaturated solution
• crystals dissolve
• CgtCs supersaturated solution
• crystals nucleate and grow

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• Solution
• Cooling
• Evaporating
• Salting out (add a second solvent)
• Melt
• Cooling below freezing point
• Impurities
• decrease solubility
• freezing point depression

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Supersaturation
Sugar concentration
Undersaturated
Saturation Undersaturation
Temperature
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• Melt systems
• driving force Tf - T

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Supersaturation

• S C / Cs
• S ln (C/Cs)
• Sucrose
• S 1.3 (min)
• S 1.2 (20-30 h)
• S 1.1 (150-200 h, metastable)

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• Nucleation
• Primary from solution
• Secondary from crystal
• breeding
• agitation
• contact nucleation

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Homogeneous nucleation

• Critical size for nucleation
• for solution
• for melts

? surface tension v molecular volume k
Boltzman constant ?Hf Latent heat of
formation S Saturation
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Heterogeneous nucleation

• nucleating sites
• dust particles
• vessel wall
• foreign particles
• molecular forces
• hydrogen bonding
• electrostatic forces
• van der Waals forces

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• Optimum temperature for nucleation
• low T high viscosity
• high viscosity low molecular mobility
• Induction time

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• Secondary nucleation
• high supersaturation
• contact nucleation
• crystal lattice broken and form nuclei
• crystal-crystal
• crystal-wall
• crystal-impeller
• fluid shear
• disruption of crystal surface

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Factors affecting nucleation

• Supersaturation
• Temperature
• Viscosity
• Agitation rate
• Impurities

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Crystal growth

• Molecular deposition
• diffusion
• incorporation to lattice
• Ostwald ripening
• small crystals dissolve larger ones grow
• Agglomeration
• polycrystalline formation

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Factors affecting growth

• supersaturation
• temperature
• viscosity
• agitation rate
• additives/impurities
• electric/magnetic/sonic fields

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Storage

• recrystallization
• temperature
• moisture

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• Prevent crystallization
• sugar crystallization (powders, candies)
• ice cream (lactose)
• Prevent recrystallization
• chocolate (fat bloom)
• ice cream (coarseness)
• temperature fluctuations
• increase in mean crystal size

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Recrystallization

• Ostwald ripening
• Rounding
• Fusion
• Polymorphic transformation
• Melt/Refreeze

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