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Lactose Solubility and Lactose Crystal Formation

II. Lactose Crystal Formation

Blue Valley Research Laboratories, Chicago, Illinois

ABSTRACT

1. The crystalline form of lactose was studied. The crystals were precipitated from diverse solutions and mixtures, such as lactose-in-water, lactose-in-sucrose solutions of high and low concentrations, lactose-in-milk, plain condensed milk and sweetened condensed milk.
   
2. Lactose crystals belong to Class C₂. They are monoclinic sphenoidal and have only one axis of symmetry. They have trapezoidal side faces and rhombic tops and bottoms. The fully developed lactose crystal has in addition beveled faces at the base and apex which may terminate in a sharp edge giving the crystal a distinct tomahawk appearance. It may have ten faces and its length is approximately 1.8 times its width at its widest portion.
   
3. Fully developed, typical lactose crystals, as described above, are characteristic of the crystals obtained from supersaturated lactose solutions in water, in skimmilk, in whole milk, in milk serum, and in plain condensed milk (unsweetened). It is plainly evident, therefore, that the presence in the milk solution of the milk colloids and milk fat does in no way modify the appearance of the lactose crystal obtained.
   
4. The presence of sucrose in the supersaturated lactose solution causes an apparent modification in the appearance of the lactose crystals. The crystals so produced are short and stubby and lack the full development at base and apex. In the case of dilute sucrose solutions, such as solutions containing 14 per cent sucrose, this change is only slight, the lactose crystals possessing the full development, but they are shorter and thicker than the crystals from solutions containing no sucrose. This is the type of crystal that occurs in sandy ice cream. In the case of concentrated sucrose solutions, such as solutions containing 62 per cent sucrose, the change in the appearance of the lactose crystal is very marked. The crystals resemble short, truncated pyramids with flat rhomboid base and apex, the beveled faces at base and apex being entirely absent. While crystallographically these crystals are the same, they are apparently not fully developed and give a different appearance, the habit of the crystals being probably the only element affected. This is the type of sugar crystal found in sweetened condensed milk.
   
5. In contradistinction to the lactose crystals, the sucrose crystals, while also monoclinic sphenoidal, present a vastly different appearance. They do not have pyramidal form, all their faces being rhomboid and all opposite sides generally being parallel. Furthermore, the sucrose crystals grow to relatively very large size and have a great tendency to twine together, which appears not to be the case with lactose crystals. These differences in appearance are very noticeable where lactose and sucrose are allowed to crystallize out from solutions supersaturated with both sugars. Such solutions yield typical crystals of each sugar independent of each other.