Crystallization and tg
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Transcript of Crystallization and tg
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.
• Almost always heterogeneous nucleation is faster than homogeneous nucleation.
• More solid will then deposit on the nuclei (crystal growth) until the supersaturation is relieved.
• 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
There is a surface tension between phases “Solid” molecules strongly
attract other ice molecules
Crystal Melt
Small Crystals
“Solid” molecules in small crystals are less strongly attached than those in large crystals
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
Ostwald Ripening
diffusion diffusion diffusion diffusion
<<disappear>>
Stages in Crystal Growth
• Nucleation (homogeneous or heterogeneous)
• Growth (no change in crystal number)
• “Perfection”
What if it doesn’t crystallize?
(at a molecular level, how do things crystallize?)
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
Temperature
Thermodynamic pressure for phase
transition
Molecular mobility
Melting pointGlass transition temperature
Log
T
15
12
0
Tg Tm
20-100oCG
LA
SS
RUBBER
SO
LU
TIO
N
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.
50 m
Ice Crystals in Ice Cream
Sensory Effects of Ice Crystals
Ice Crystal Size (m)
Cry
stal
det
ecta
bili
ty
Sen
sory
sm
ooth
ness
25 50
Thermodynamics of Crystallization
Glass
Solution
Concentration
Tem
pera
ture
Freezing Point Curve
Temp /°C
0 -10 -20 -30 -40
0%
50%
100%
Wat
er f
roze
n
Simplified Flow Chart
Freezer Hardening Distribution
-5°C -18°C -15°C
The Ice-Cream Freezer
• Mix is cooled to about -10oC
• Vigorous mixing• Air is incorporated
Effect of Dasher
1. Ice crystals grow from cold wall
3. Dendrite grows in barrel center
2. Dasher cuts off dendrite
Domestic vs Commercial Freezers
Why do commercial freezers make smoother ice cream than domestic
freezers?
• Boiling ammonia –30°C
• Saturated brine –10°C
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?
Distribution
• Plant freezers
• Refrigerated distribution
• Store freezers
• Domestic freezerstransfer
transfer
transfer
How does the number and size of crystals change during distribution?
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)
Effect of Unfrozen Matrix
Diffusion
Tg?Stabilizers?
Simplified Flow Chart
Freezer Hardening Distribution
-5°C -18°C -15°C
~35 m50% Frozen
~45 m80% Frozen
NucleationGrowth
Growth MeltingGrowthRipening