Food Crystallization Flashcards
Crystallization
When a crystal forms, the molecules
orient themselves in a regular pattern, or
lattice structure.
When a crystal forms, the molecules orient themselves in a regular pattern, or lattice structure.
1 Generation of a supersaturated phase
2 Nucleation
3 Crystal growth
4 Recrystallization
Nucleation is the formation
of crystalline lattice structure from solution or melt
Crystal growth involves
subsequent growth of nuclei
until equilibrium is attained
Recrystallization is
a reorganization of the crystalline structure to a
lower energy state, generally without any further
change in the amount of crystalline phase volume
Generation of a supersaturated phase
: the driving force for crystallization & is a prerequisite before a solid
phase will appear in a saturated solution
When it is Supersaturated,
Solute concentration higher than the equilibrium solubility
The liquid phase must exceed the equilibrium condition and become
supersaturated (solution) or supercooled (melt)
For a melt
cooling below the melting point of a compound
For a solution
produce a concentration
in solution greater than solubility
concentration (sugars, salts, etc)
When solute contents > solubility concentration (Cs),
supersaturated solutions
How to achieve supersaturation?
- Heating the solvent prior to dissolving the solute so that high concentration solutions can be made
- Cooling this solution below the saturation temperature results in supersaturated solution (A - B)
- Evaporating solvent from solution - Increase concentration of solute
Labile zone: Supersaturation is very high
- Nucleation occurs spontaneously
- High nucleation rate, so many small crystals are formed
Metastable zone: Solution state is thermodynamically unstable
- But kinetic barrier prevents spontaneous formation
- Crystal may grow (by adding seed crystal – heterogeneous nucleation) but nucleation is negligible
Undersaturated region: Solution state is thermodynamically stable
- Any crystals added will dissolve
- Nucleation will not occur
Nucleation
Molecules in the liquid state re-arrange &
eventually form into a stable cluster that
organizes into a crystalline lattice.
Primary nucleation
- Formation of crystal nuclei from a solution that
contained no pre-existing crystals
Homogenous nucleatio
- Formation of nuclei within a homogeneous fluid ─ rarely occurs in practical food processing conditions
Heterogenous nucleation
- Initiated by contact with foreign particles and
surfaces
Secondary nucleation
- Formation of crystal nuclei due to the
presence of existing crystals
3rd step Crystal growth
- Nuclei that form can grow to larger size based on the available supersaturation
in the solution
The extent of this crystal growth depends on t
the magnitude of supersaturation
remaining in solution after nucleation has occurred
crystal Growth continues until all of the available supersaturation
has been depleted & the system approaches an equilibrium in phase volume, which depends on
temperature & composition of the system
4th step Recrystallization
- Once equilibrium in phase volume has
been attained, changes still may take place
in the crystalline structure during long term
storage, especially where temperature &
RH are likely to vary over time
Ice-cream/frozen foods
Molecules within ice crystals re-arrange
* Small ice crystals disappear
* Larger crystals grow
Result: Product feels coarse during consumption
Effects of temperature fluctuations
(T°C increase - small ice crystals melt – unfrozen water increase)
as temperature decrease, water will refreeze but
does not renucleate.
(T°C decrease - water refreeze – deposited on large crystal surface – no. of crystals decrease – crystal size increase)
Formation of fat bloom in chocolate during long-term storage
Cocoa butter in fresh chocolate is not in polymorph with lowest energy state
* Gradually re-crystallize into the most stable form
* Formation of visible crystals on chocolate surface (loss of gloss)
Result: Formation of a whitish haze (fat bloom) on the surface
Mechanisms of recrystallization
Ostwald ripening/Accretion/Polymorphic transformation /Melt/refreeze
Chocolate tempering
- Tempering process involves holding chocolate at the appropriate temperature for the proper
time to form the desired crystalline structure. - In commercial operations, the chocolate mass is pumped through a heat exchanger where
the temperature is controlled in different sections to ensure proper crystallization. - Molten chocolate at about 50°C is cooled rapidly in the first section to ~26ºC to initiate crystallization of the β ’ polymorph. The rate of nuclei formation at these conditions is rapid so
that many crystals are formed, which results in an increase in viscosity of chocolate mass. - Since the β -polymorph is desired, the temperature of the chocolate mass is increase to cause melting of the unstable
form & transformation on the more stable β -polymorph
