Presentation on theme: "Summary to Date Solutions are thermodynamically stable within a range of temperatures and compositions. Solutions more concentrated than their limit will."— Presentation transcript:
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 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 = crystal radius s = crystal solubility
Ostwald Ripening diffusion >
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 °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 point Glass transition temperature
Log T TgTm o C GLASS RUBBER SOLUTION
Solution Glass S.S. soln. Ice+Soln. Ice+S.S. glass TETE TgTg Conc. Solute, %0100 Temp, o C Ice+S.S. Soln.
50 m Ice Crystals in Ice Cream
Sensory Effects of Ice Crystals Ice Crystal Size ( m) Crystal detectability Sensory smoothness 25 50
Thermodynamics of Crystallization Glass Solution Concentration Temperature
Freezing Point Curve Temp /°C % 50% 100% Water frozen
The Ice-Cream Freezer Mix is cooled to about -10 o C 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 % 50% 100% Water frozen How does the number and size of crystals change in the hardening room?
Distribution Plant freezers Refrigerated distribution Store freezers Domestic freezers 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?