Crystallization and tg
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Transcript of Crystallization and tg
![Page 1: Crystallization and tg](https://reader036.fdocuments.us/reader036/viewer/2022062300/55574661d8b42aa8378b4cab/html5/thumbnails/1.jpg)
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 (G>0) and volume energy gains giving a free energy barrier for the formation of small crystals.
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• Almost always heterogeneous nucleation is faster than homogeneous nucleation.
• More solid will then deposit on the nuclei (crystal growth) until the supersaturation is relieved.
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• 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
There is a surface tension between phases “Solid” molecules strongly
attract other ice molecules
Crystal Melt
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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
M
.
..2
s
sln RT
0.01%
0.10%
1.00%
10.00%
100.00%
0.001 0.01 0.1 1
Particle size
incr
ease
in s
olub
ility
r = crystal radiuss = crystal solubility
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Ostwald Ripening
diffusion diffusion diffusion diffusion
<<disappear>>
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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 doesn’t crystallize?
(at a molecular level, how do things crystallize?)
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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
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Temperature
Thermodynamic pressure for phase
transition
Molecular mobility
Melting pointGlass transition temperature
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Log
T
15
12
0
Tg Tm
20-100oCG
LA
SS
RUBBER
SO
LU
TIO
N
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Solution
Glass
S.S.soln.
Ice+Soln.
Ice+S.S. glass
TE
Tg
Conc. Solute, %0 100
Tem
p, o C
60
30
0
Ice+S.S. Soln.
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50 m
Ice Crystals in Ice Cream
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Sensory Effects of Ice Crystals
Ice Crystal Size (m)
Cry
stal
det
ecta
bili
ty
Sen
sory
sm
ooth
ness
25 50
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Thermodynamics of Crystallization
Glass
Solution
Concentration
Tem
pera
ture
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Freezing Point Curve
Temp /°C
0 -10 -20 -30 -40
0%
50%
100%
Wat
er f
roze
n
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Simplified Flow Chart
Freezer Hardening Distribution
-5°C -18°C -15°C
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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
3. Dendrite grows in barrel center
2. Dasher cuts off dendrite
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Domestic vs Commercial Freezers
Why do commercial freezers make smoother ice cream than domestic
freezers?
• Boiling ammonia –30°C
• Saturated brine –10°C
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Hardening
• At the freezer exit the product is packed
and cooled to -18°C in a tunnel freezer
Temp /°C
0 -10 -20 -30 -400%
50%
100%W
ater
fro
zenHow does the
number and size of crystals change in the hardening room?
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Distribution
• Plant freezers
• Refrigerated distribution
• Store freezers
• Domestic freezerstransfer
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 (>55 m)
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Effect of Unfrozen Matrix
Diffusion
Tg?Stabilizers?
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Simplified Flow Chart
Freezer Hardening Distribution
-5°C -18°C -15°C
~35 m50% Frozen
~45 m80% Frozen
NucleationGrowth
Growth MeltingGrowthRipening