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 GrowthCrystal Melt There is a surface tension between phases “Solid” molecules strongly attract other ice molecules
Small Crystals“Solid” molecules in smallcrystals are less stronglyattached than those in largecrystals
Simplified Flow Chart -5°C -18°C -15°CFreezer Hardening Distribution
The Ice-Cream Freezer• Mix is cooled to about -10oC• Vigorous mixing• Air is incorporated
Effect of Dasher1. Ice crystals 2. Dasher cuts 3. Dendrite growsgrow from cold off dendrite in barrel centerwall
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 100%How does the Water frozennumber and size 50%of crystals changein the hardeningroom? 0% 0 -10 -20 -30 -40 Temp /°C
Distribution• Plant freezers transfer• Refrigerated distribution transfer• Store freezers transfer• Domestic freezers 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 µm)
Effect of Unfrozen Matrix Diffusion Tg? Stabilizers?