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