CJW-+Chapter+3


 * Crystallization**
 * Crystalline phase in foods à impact on quality, texture and shelf life.

Example: Crystalline microstructure of cocoa butter in chocolate. -provide the desired snap when a mould bar is broken - notable difference in snap btw. a good dark chocolate & a milk chocolate due to diff in crystal microstructure.
 * Main components that form crystalline phase are
 * 1) Water,
 * 2) sugar,
 * 3) lipids and
 * 4) starches
 * other components:salts, organic acid, proteins,and emulsifiers

it is a physical transformation (phase transition) of a liquid, solution, or gas to a crystal.
 * crystallization- describes several different phenomena related to the formation of a crystalline lattice structure

molecules in amorphous material are more randomly oriented & move about freely.
 * when a crystal forms, the molecules orient themselves in a regular pattern/ lattice structure.

-nature of the force and -interaction between individual molecules
 * crystal lattices are composed of an ordered layering of molecules, which dependent on


 * crystallization can be induced from a solution (sugar solution) or from a melt (liquid fat)

**Principles of Crystallization**

1. Generation of supersaturated state 2. Nucleation - formation of crystalline lattice structure from solution or melt 3. Growth- subsequent growth of nuclei until equilibrium is attained 4. Recrystallization- a reorganization of the crystalline structure to a lower energy state, generally without any further change in the amount of crystalline phase volume.
 * Four steps in crystallization:

2&3 require thermodynamic driving force (TDF)

For a melt, TDF= undercooling- by lowering the temperature below the melting point.
 * In the case of crystallization from a solution, TDF= supersaturation- concentration of a solute exceed its equilibrium solubility

Although any supersaturation state in principle cause crystallization from a solution, crystallization usually does not occur unless the supersaturation exceeds a certain threshold/ metastable limit.

The width of metastable zone depends on the nature of solute and condition of crystalllzation
 * Figure1** Solubility- supersolubility curve showing equilibrium solubility, supersaturation and metastable limit.

**Nucleation**

1. homogeneous- rare in food processing 2. heterogeneous- initiated by contact with foreign particles and surfaces
 * Primary- formation of crystal nuclei from a solution that contained no preexisting crystals

- formation of nuclei due to the presence of existing crystals - under certain conditions (temperature & concentration) within metastable zone, new nuclei might formed due to presence of existing nuclei. important in refining of sugars, freeze concentration of fluid foods and fractionation of fats.
 * Secondary
 * known as **contact secondary nucleation**



- not begin until reasonably high driving force or a critical supersaturation - however nucleation increases dramatically with small increases in driving force - occurs initially at lower supersaturation driving forces - increases at a slower rate than for homogeneous nucleation
 * Homogeneous
 * Heterogeneous

Factors influencing nucleation:

1. Supersaturation 2. Temperature 3. Viscosity 4. Cooling rate 5. Agitation rate 6. Additives and impurities

**Crystal Growth** 1. supersaturated condition remains in the system 2. molecule have sufficient mobility to move to the crystal interface and orient into the lattice
 * once nuclei have been formed in supersaturated system, they grow at a rate dependent on conditions in surrounding.
 * crystal continue to grow as long as

1. all available supersaturation is consumed 2. system reaches the phase equilibrium
 * Growth stops when-

Assuming no new nuclei are formed, crystal growth continues until the system reaches the equilibrium point defined by phase diagram.
 * in solution systems, the extent of crystal growth is based on the supersaturation.
 * in melt systems, crystal growth until the system attains a thermal equilibrium or all of the material has crystallized.


 * Kinetics of Crystal Growth**




 * Types of Crystal Lattice Structure**

Such cases may be due to
 * The complexity of the molecular structure may lead to differences in the crystal lattice structure.
 * crystallization of different types of isomers of molecules present e.g. enantiomers and anomers
 * crystallization of hydrates, where water are incorporated into the crystal lattice along with the crystallizing species
 * polymorphism, where complex molecules crystallize in more than on molecular arrangement depending on conditions
 * conversion between enantiometric & hydrates forms may occur.


 * Rapid cooling generally results in formation of less stable polymorphous


 * Controlling crystallization**

1. Control to produce desired crystalline structure either numerous small crystal or larger crystal

2. Control to prevent crystallization e.g hard candy, milk and whey powder, honey and ice cream - way to prevent crystallization- low mobility or inhibit by addition of other molecules

3. Control of change in crystalline structure (recrystallization) - evidenced y increase in the mean size and spread in the width of a crystal size distribution.


 * Mechanism of Recrystallization**

1. Ostwald ripening=> small crystal dissolve/ melt and large crystal grow

2. Accretion- crystals that are very close to each other tend to grow together to form one large crystal e.g. ice crystal in ice cream

3. Polymorphic transformation- less stable crystal forms change to a more stable forms - the subsequent release of latent heat during transformation may cause substantial problem in product.

4. Melt/ refreeze- change in the phase volume of crystalline materials occurs as temperatures fluctuate during storage, accompanied by a change in crystalline structure.


 * Factors affecting Control of Recrystallization**

1. Heat & Mass Transfer Rate

2. Product Formulation

3. Post- Processing Effect