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Crystallization and tg


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Crystallization and tg

  1. 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 (∆G>0) and volume energy gains giving a free energy barrier for the formation of small crystals.
  2. 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. 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. 4. Mechanism of GrowthCrystal Melt There is a surface tension between phases “Solid” molecules strongly attract other ice molecules
  5. 5. Small Crystals“Solid” molecules in smallcrystals are less stronglyattached than those in largecrystals
  6. 6. The Kelvin Equation s 2.γ .M increase in solubility 100.00%RT ln = s∞ ρ .r 10.00% 1.00% 0.10%r = crystal radius 0.01%s = crystal solubility 0.001 0.01 0.1 1 Particle size
  7. 7. Ostwald Ripening <<disappear>>diffusion diffusion diffusion diffusion
  8. 8. Stages in Crystal Growth• Nucleation (homogeneous or heterogeneous)• Growth (no change in crystal number)• “Perfection”
  9. 9. What if it doesn’t crystallize? (at a molecular level, how do things crystallize?)
  10. 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. 11. Thtra erm ns od iti yn on a mi y cp res ob ilit m su r u la r ef lec or ph Mo as e Glass transition Melting point temperature Temperature
  12. 12. 15 20-100oC 12 SOLUTION GLASSLog η RUBBER 0 Tg T Tm
  13. 13. 60 Solution S.S. soln. 30Temp, oC 0 TE Ice+Soln. Tg Ice+S.S. Soln. Glass Ice+S.S. glass 0 Conc. Solute, % 100
  14. 14. Ice Crystals in Ice Cream 50 µm
  15. 15. Sensory Effects of Ice Crystals Sensory smoothness Crystal detectability 25 50 Ice Crystal Size (µm)
  16. 16. Thermodynamics of Crystallization SolutionTemperature Glass Concentration
  17. 17. Freezing Point Curve 100%Water frozen 50% 0% 0 -10 -20 -30 -40 Temp /°C
  18. 18. Simplified Flow Chart -5°C -18°C -15°CFreezer Hardening Distribution
  19. 19. The Ice-Cream Freezer• Mix is cooled to about -10oC• Vigorous mixing• Air is incorporated
  20. 20. Effect of Dasher1. Ice crystals 2. Dasher cuts 3. Dendrite growsgrow from cold off dendrite in barrel centerwall
  21. 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. 22. Hardening • At the freezer exit the product is packed and cooled to -18°C in a tunnel freezer 100%How does the Water frozennumber and size 50%of crystals changein the hardeningroom? 0% 0 -10 -20 -30 -40 Temp /°C
  23. 23. Distribution• Plant freezers transfer• Refrigerated distribution transfer• Store freezers transfer• Domestic freezers How does the number and size of crystals change during distribution?
  24. 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 µm)
  25. 25. Effect of Unfrozen Matrix Diffusion Tg? Stabilizers?
  26. 26. Simplified Flow Chart -5°C -18°C -15°C Freezer Hardening Distribution ~35 µm ~45 µm 50% Frozen 80% FrozenNucleation Growth MeltingGrowth Growth Ripening