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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.

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Summary to Date

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  1. 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.

  2. Almost always heterogeneous nucleation is faster than homogeneous nucleation. • More solid will then deposit on the nuclei (crystal growth) until the supersaturation is relieved.

  3. 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.

  4. Magnify Mechanism of Growth Melt Crystal There is a surface tension between phases “Solid” molecules strongly attract other ice molecules

  5. Magnify Magnify Small Crystals “Solid” molecules in small crystals are less strongly attached than those in large crystals

  6. The Kelvin Equation r = crystal radius s = crystal solubility

  7. <<disappear>> diffusion diffusion diffusion diffusion Ostwald Ripening

  8. Stages in Crystal Growth • Nucleation (homogeneous or heterogeneous) • Growth (no change in crystal number) • “Perfection”

  9. What if it doesn’t crystallize? (at a molecular level, how do things crystallize?)

  10. 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

  11. Molecular mobility Thermodynamic pressure for phase transition Glass transition temperature Melting point Temperature

  12. 15 12 0 20-100oC RUBBER GLASS SOLUTION Log h Tg Tm T

  13. 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

  14. 50 mm Ice Crystals in Ice Cream

  15. Sensory Effects of Ice Crystals Sensory smoothness Crystal detectability 25 50 Ice Crystal Size (mm)

  16. Thermodynamics of Crystallization Solution Temperature Glass Concentration

  17. 100% Water frozen 50% 0% 0 -10 -20 -30 -40 Temp /°C Freezing Point Curve

  18. Simplified Flow Chart -18°C -15°C -5°C Freezer Hardening Distribution

  19. Mix is cooled to about -10oC Vigorous mixing Air is incorporated The Ice-Cream Freezer

  20. Effect of Dasher 1. Ice crystals grow from cold wall 2. Dasher cuts off dendrite 3. Dendrite grows in barrel center

  21. Domestic vs Commercial Freezers Why do commercial freezers make smoother ice cream than domestic freezers? • Boiling ammonia –30°C • Saturated brine –10°C

  22. 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

  23. Distribution • Plant freezers • Refrigerated distribution • Store freezers • Domestic freezers transfer transfer transfer How does the number and size of crystals change during distribution?

  24. 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)

  25. Effect of Unfrozen Matrix Diffusion Tg? Stabilizers?

  26. 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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