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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.

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Summary to date
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 (DG>0) and volume energy gains giving a free energy barrier for the formation of small crystals.



  • Inventors: homogeneous nucleation. 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.


Mechanism of growth

Magnify homogeneous nucleation.

Mechanism of Growth

Melt

Crystal

There is a surface tension between phases

“Solid” molecules strongly attract other ice molecules


Small crystals

Magnify homogeneous nucleation.

Magnify

Small Crystals

“Solid” molecules in small crystals are less strongly attached than those in large crystals


The kelvin equation
The Kelvin Equation homogeneous nucleation.

r = crystal radius

s = crystal solubility


Ostwald ripening

<< homogeneous nucleation.disappear>>

diffusion

diffusion

diffusion

diffusion

Ostwald Ripening


Stages in crystal growth
Stages in Crystal Growth homogeneous nucleation.

  • Nucleation (homogeneous or heterogeneous)

  • Growth (no change in crystal number)

  • “Perfection”


What if it doesn t crystallize

What if it doesn’t crystallize? homogeneous nucleation.

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


Fondant manufacture
Fondant Manufacture homogeneous nucleation.

  • Cook to 114-120°C

  • Cool quickly and gently to 45°C

  • Vigorously mix until all clarity is lost and a creamed malleable mass is formed

  • Ripen and mature for 24 h


Molecular mobility homogeneous nucleation.

Thermodynamic pressure for phase transition

Glass transition temperature

Melting point

Temperature


15 homogeneous nucleation.

12

0

20-100oC

RUBBER

GLASS

SOLUTION

Log h

Tg

Tm

T


Solution homogeneous nucleation.

60

30

0

S.S.

soln.

Temp, oC

TE

Ice+Soln.

Tg

Ice+S.S. Soln.

Glass

Ice+S.S. glass

0

Conc. Solute, %

100


50 homogeneous nucleation.mm

Ice Crystals in Ice Cream


Sensory effects of ice crystals
Sensory Effects of Ice Crystals homogeneous nucleation.

Sensory smoothness

Crystal detectability

25

50

Ice Crystal Size (mm)


Thermodynamics of crystallization
Thermodynamics of Crystallization homogeneous nucleation.

Solution

Temperature

Glass

Concentration


Freezing point curve

100% homogeneous nucleation.

Water frozen

50%

0%

0

-10

-20

-30

-40

Temp /°C

Freezing Point Curve


Simplified flow chart
Simplified Flow Chart homogeneous nucleation.

-18°C

-15°C

-5°C

Freezer

Hardening

Distribution


The ice cream freezer

Mix is cooled to about -10 homogeneous nucleation.oC

Vigorous mixing

Air is incorporated

The Ice-Cream Freezer


Effect of dasher
Effect of Dasher homogeneous nucleation.

1. Ice crystals grow from cold wall

2. Dasher cuts off dendrite

3. Dendrite grows in barrel center


Domestic vs commercial freezers
Domestic vs Commercial Freezers homogeneous nucleation.

Why do commercial freezers make smoother ice cream than domestic freezers?

  • Boiling ammonia –30°C

  • Saturated brine –10°C


Hardening
Hardening homogeneous nucleation.

  • At the freezer exit the product is packed

    and cooled to -18°C in a tunnel freezer

100%

How does the number and size of crystals change in the hardening room?

Water frozen

50%

0%

0

-10

-20

-30

-40

Temp /°C


Distribution
Distribution homogeneous nucleation.

  • Plant freezers

  • Refrigerated distribution

  • Store freezers

  • Domestic freezers

transfer

transfer

transfer

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


Ice coarsening
Ice Coarsening homogeneous nucleation.

  • 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 (>55 mm)


Effect of unfrozen matrix
Effect of Unfrozen Matrix homogeneous nucleation.

Diffusion

Tg?

Stabilizers?


Simplified flow chart1
Simplified Flow Chart homogeneous nucleation.

-18°C

-15°C

-5°C

Freezer

Hardening

Distribution

~35 mm

50% Frozen

~45 mm

80% Frozen

Nucleation

Growth

Growth

Melting

Growth

Ripening


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