Summary to Date

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Summary to Date

• Solutions are thermodynamically stable within a
  range of temperatures and compositions.
  Solutions more concentrated than their limit will
  tend to crystallize until the residual solution
  is back at the saturation limit.
• Crystal formation requires the formation of a
  nucleus which can be a slow process due to the
  trade off between surface energy costs (DGgt0) and
  volume energy gains giving a free energy barrier
  for the formation of small crystals.

2

• Almost always heterogeneous nucleation is faster
  than homogeneous nucleation.
• More solid will then deposit on the nuclei
  (crystal growth) until the supersaturation is
  relieved.

3

• Inventors Alan J. Forage William J. Byrne
• Assignee Arthur Guinness Son Co., Ltd.
• The gas pod in the can is blow molded with
  nitrogen (N).
• A laser zaps a hole in the pod. (they
  experimented with holes between 0.2mm and 2.5mm
  finding that 0.61 mm as ideal)
• Pod is inserted in the bottom of can.
• Can is filled with CO2/N supersaturated stout. N
  is present at 1.5 v/v min up to 3.5 v/v. (FYI,
  vol/vol is the number of volumes of gas which are
  dissolved in a unit volume of beverage at 760mm
  of Hg 15.6 oC) CO2 is present at between 0.8
  and 1.5 v/v.
• During filling, foam rises to top of can. This
  clears the air.
• A charge of liquid N is added to the stout.
• Can is sealed.
• As liquid N boils off in can during
  pasteurisation (60 oC for 15-20 min), top of can
  pressurizes and forces the stout into the pod,
  thus compressing the ambient pressure N in the
  pod.
• Equilibrium is reached at about 25 psi.

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Mechanism of Growth
Melt
Crystal
There is a surface tension between phases
Solid molecules strongly attract other ice
molecules
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Small Crystals
Solid molecules in small crystals are less
strongly attached than those in large crystals
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The Kelvin Equation
r crystal radius s crystal solubility
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Ostwald Ripening
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Stages in Crystal Growth

• Nucleation (homogeneous or heterogeneous)
• Growth (no change in crystal number)
• Perfection

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What if it doesnt crystallize?

• (at a molecular level, how do things crystallize?)

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Fondant Manufacture

• Cook to 114-120C
• Cool quickly and gently to 45C
• Vigorously mix until all clarity is lost and a
  creamed malleable mass is formed
• Ripen and mature for 24 h

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Molecular mobility
Thermodynamic pressure for phase transition
Glass transition temperature
Melting point
Temperature
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Ice Crystals in Ice Cream
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Sensory Effects of Ice Crystals
Sensory smoothness
Crystal detectability
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Ice Crystal Size (mm)
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Thermodynamics of Crystallization
Solution
Temperature
Glass
Concentration
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Freezing Point Curve
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Simplified Flow Chart
-18C
-15C
-5C
Freezer
Hardening
Distribution
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The Ice-Cream Freezer

• Mix is cooled to about -10oC
• Vigorous mixing
• Air is incorporated

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Effect of Dasher
1. Ice crystals grow from cold wall
2. Dasher cuts off dendrite
3. Dendrite grows in barrel center
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Domestic vs Commercial Freezers

• Why do commercial freezers make smoother ice
  cream than domestic freezers?
• Boiling ammonia 30C
• Saturated brine 10C

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Hardening

• At the freezer exit the product is packed
• and cooled to -18C in a tunnel freezer

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How does the number and size of crystals change
in the hardening room?
Water frozen
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0
0
-10
-20
-30
-40
Temp /C
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Distribution

• Plant freezers
• Refrigerated distribution
• Store freezers
• Domestic freezers

transfer
transfer
transfer
How does the number and size of crystals change
during distribution?
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Ice Coarsening

• Ice cream may coarsen during storage,
  particularly if
• stored too warm
• temperature fluctuation during storage
• Coarsened product is associated with a cold, icy,
  and gritty mouthfeel
• Caused by many, large crystals (gt55 mm)

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Effect of Unfrozen Matrix
Diffusion
Tg? Stabilizers?
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Simplified Flow Chart
-18C
-15C
-5C
Freezer
Hardening
Distribution
35 mm 50 Frozen
45 mm 80 Frozen
Nucleation Growth
Growth
Melting Growth Ripening