Fat Crystallisation

Question 1

Fat Crystallisation Impact on stability

Answer 1

Fat crystal formation in O/W emulsions leads to a decrease in emulsion stability

e.g. in salad dressings if oil phase starts to crystallise the emulsion breaks at once

However, in an ice cream system some crystallisation is essential to create foam structure
W/O fatty spreads are stabilised by fat crystals

Question 2

Ice cream food complex involves what molecules & the food structure has what characteristics

Answer 2

water & sugar
milk protein 
fat globules
ice crystals 
air bubbles

–> partial coalescence of fat globules

Question 3

Margarines and water-in-oil fatty spreads are stabilised by ? and how? gives what effects?

Answer 3

by fat crystals

the fat crystals also interact and aggregate to form a three-dimensional network

• -> Long-term stability to dispersed water droplets
• -> Solid-like (“spreadable”) texture to the food product

Question 4

Mechanism of emulsion breakdown

Answer 4

1) emulsion
2) crystallisation at interface
3) penetration of any surfactant “film” into continuous phase
4) formation of a lipid crystal bridge

Question 5

diagram of Crystallisation at surface in oil-in-water emulsions

Answer 5

Continuous phase (water)
Emulsion droplet (Oil)
Fat Crystals
“ membrane” of droplet

slide 25

Question 6

Fat crystallisation in practice (4)

[hint: no of crystals; boiling/melting point ; crystal growth; what forms around droplets]

Answer 6

• Droplets rarely contain single discrete crystals
  Usually mixed both in terms of composition and polymorphic form
• In emulsion droplets the fat will often supercool and remain liquid below its normal melting point
  ~ already below point at which would expect it to crystallise
• When nucleation does take place, crystal growth is rapid and is unlikely to produce discrete crystals
• Crystal plates in practice form concentric layers around the droplets
  ~ not a single crystal that penetrates any surface layer, but imperfections in the surface crystal layer (dislocations)

Question 7

Factors governing crystal penetration (7)

Answer 7

Relative free energies of the surface
–> crystal wetted preferentially by the oil phase leads to low/no penetration
crystal wetted preferentially by the water phase high penetration/emulsion breakdown

• Crystal characteristics such as size, shape and orientation
• Deformability of the crystals
• -> film penetration much less likely if crystal can conform to the droplet
• Alpha (α) crystals more liquid than beta (β)
• -> See notes on polymorphism (alpha are least stable polymorph while beta are most stable polymorph)
• Mixed crystal systems give rise to softer structures (deformable) –> do not pack as well
• Polydisperse lipids more stable (better packing) for example mixtures of triglycerides in ice cream mixes
• thickness of the surfactant layer

Question 8

With protein stabilised emulsions (e.g. milk), what can reduce the effective hydrodynamic thickness? by ____?

Answer 8

the effect of small molecule surfactants could be to reduce the effective hydrodynamic thickness

by displacing protein and reducing stability

Question 9

what factors can effect protein hydration ? (5)

Those factors can also influence the _____ of the surface layer and hence ________

Answer 9

(size, orientation, temp, pH and ionic environment)

influence the thickness of the surface layer and hence the emulsion stability

Question 10

Does fat crystallisation increase or decrease the stability of a typical salad dressing?

Answer 10

decrease

Question 11

Why does the crystallisation of fats into mixed crystals (both alpha and beta) influence the effect on emulsion stability?

Answer 11

–?

Question 12

Which is the least stable fat crystal polymorph?

Answer 12

Alpha (α) crystals