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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

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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.


Summary to date

  • Almost always heterogeneous nucleation is faster than homogeneous nucleation.

  • More solid will then deposit on the nuclei (crystal growth) until the supersaturation is relieved.


Summary to date

  • 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.


Mechanism of growth

Magnify

Mechanism of Growth

Melt

Crystal

There is a surface tension between phases

“Solid” molecules strongly attract other ice molecules


Small crystals

Magnify

Magnify

Small Crystals

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


The kelvin equation

The Kelvin Equation

r = crystal radius

s = crystal solubility


Ostwald ripening

<<disappear>>

diffusion

diffusion

diffusion

diffusion

Ostwald Ripening


Stages in crystal growth

Stages in Crystal Growth

  • Nucleation (homogeneous or heterogeneous)

  • Growth (no change in crystal number)

  • “Perfection”


What if it doesn t crystallize

What if it doesn’t crystallize?

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


Fondant manufacture

Fondant Manufacture

  • 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


Summary to date

Molecular mobility

Thermodynamic pressure for phase transition

Glass transition temperature

Melting point

Temperature


Summary to date

15

12

0

20-100oC

RUBBER

GLASS

SOLUTION

Log h

Tg

Tm

T


Summary to date

Solution

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


Summary to date

50 mm

Ice Crystals in Ice Cream


Sensory effects of ice crystals

Sensory Effects of Ice Crystals

Sensory smoothness

Crystal detectability

25

50

Ice Crystal Size (mm)


Thermodynamics of crystallization

Thermodynamics of Crystallization

Solution

Temperature

Glass

Concentration


Freezing point curve

100%

Water frozen

50%

0%

0

-10

-20

-30

-40

Temp /°C

Freezing Point Curve


Simplified flow chart

Simplified Flow Chart

-18°C

-15°C

-5°C

Freezer

Hardening

Distribution


The ice cream freezer

Mix is cooled to about -10oC

Vigorous mixing

Air is incorporated

The Ice-Cream Freezer


Effect of dasher

Effect of Dasher

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

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

  • Boiling ammonia –30°C

  • Saturated brine –10°C


Hardening

Hardening

  • 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

  • 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

  • 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

Diffusion

Tg?

Stabilizers?


Simplified flow chart1

Simplified Flow Chart

-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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