Food Crystallization

Question 1

Crystallization

Answer 1

When a crystal forms, the molecules
orient themselves in a regular pattern, or
lattice structure.

Question 2

When a crystal forms, the molecules orient themselves in a regular pattern, or lattice structure.

Answer 2

1 Generation of a supersaturated phase
2 Nucleation
3 Crystal growth
4 Recrystallization

Question 3

Nucleation is the formation

Answer 3

of crystalline lattice structure from solution or melt

Question 4

Crystal growth involves

Answer 4

subsequent growth of nuclei
until equilibrium is attained

Question 5

Recrystallization is

Answer 5

a reorganization of the crystalline structure to a
lower energy state, generally without any further
change in the amount of crystalline phase volume

Question 6

Generation of a supersaturated phase

Answer 6

: the driving force for crystallization & is a prerequisite before a solid
phase will appear in a saturated solution

Question 7

When it is Supersaturated,

Answer 7

Solute concentration higher than the equilibrium solubility

Question 8

The liquid phase must exceed the equilibrium condition and become

Answer 8

supersaturated (solution) or supercooled (melt)

Question 9

For a melt

Answer 9

cooling below the melting point of a compound

Question 10

For a solution

Answer 10

produce a concentration
in solution greater than solubility
concentration (sugars, salts, etc)

Question 11

When solute contents > solubility concentration (Cs),

Answer 11

supersaturated solutions

Question 12

How to achieve supersaturation?

Answer 12

• 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

Question 13

Labile zone: Supersaturation is very high

Answer 13

• Nucleation occurs spontaneously
• High nucleation rate, so many small crystals are formed

Question 14

Metastable zone: Solution state is thermodynamically unstable

Answer 14

• But kinetic barrier prevents spontaneous formation
• Crystal may grow (by adding seed crystal – heterogeneous nucleation) but nucleation is negligible

Question 15

Undersaturated region: Solution state is thermodynamically stable

Answer 15

• Any crystals added will dissolve
• Nucleation will not occur

Question 16

Nucleation

Answer 16

Molecules in the liquid state re-arrange &
eventually form into a stable cluster that
organizes into a crystalline lattice.

Question 17

Primary nucleation

Answer 17

• Formation of crystal nuclei from a solution that
  contained no pre-existing crystals

Question 18

Homogenous nucleatio

Answer 18

• Formation of nuclei within a homogeneous fluid ─ rarely occurs in practical food processing conditions

Question 19

Heterogenous nucleation

Answer 19

• Initiated by contact with foreign particles and
  surfaces

Question 20

Secondary nucleation

Answer 20

• Formation of crystal nuclei due to the
  presence of existing crystals

Question 21

3rd step Crystal growth

Answer 21

• Nuclei that form can grow to larger size based on the available supersaturation
  in the solution

Question 22

The extent of this crystal growth depends on t

Answer 22

the magnitude of supersaturation
remaining in solution after nucleation has occurred

Question 23

crystal Growth continues until all of the available supersaturation

Answer 23

has been depleted & the system approaches an equilibrium in phase volume, which depends on
temperature & composition of the system

Question 24

4th step Recrystallization

Answer 24

• 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

Question 25

Ice-cream/frozen foods

Answer 25

Molecules within ice crystals re-arrange * Small ice crystals disappear * Larger crystals grow Result: Product feels coarse during consumption

Question 26

Effects of temperature fluctuations

Answer 26

(T°C increase - small ice crystals melt – unfrozen water increase)

Question 27

as temperature decrease, water will refreeze but does not renucleate.

Answer 27

(T°C decrease - water refreeze – deposited on large crystal surface – no. of crystals decrease – crystal size increase)

Question 28

Formation of fat bloom in chocolate during long-term storage

Answer 28

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

Question 29

Mechanisms of recrystallization

Answer 29

Ostwald ripening/Accretion/Polymorphic transformation /Melt/refreeze

Question 30

Chocolate tempering

Answer 30

* 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