Sometimes I freeze milk. If I defrost it within two or three weeks of freezing, it is fine. But if I defrost it after two or three months, it separates into a watery part and a thicker, white part. Why? How does it change during the time that it is frozen and apparently inert?
• Milk is a solution containing a suspension of particles – a structure that concentrates surprising quantities of insoluble materials into a liquid that normally remains stable until it reaches the stomach, where enzymes curdle it so that it stays put long enough for proper digestion.
Freezing creates a mesh of ice crystals, trapping particles of butterfat, membranes from cells, micelles (protein particles that hold astonishing concentrations of nutrients in suspension, especially calcium salts) and regions of imperfectly crystalline ice containing dissolved materials. This “apparently inert” mass is anything but. Constant recrystallisation ruptures cells and micelles, driving dissolved materials out of the solution, and stranding proteins between the growing crystals, where they become distorted and denatured.
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The materials trapped between the ice crystals become concentrated into less and less solvent (water). The process is slow, but fats, proteins and insoluble calcium phosphates from the ruptured micelles agglomerate, react chemically and entangle physically, congealing and resisting resolution or redispersal.
It is still nutritious and might be good in cheese or ice cream, but too chewy to be pleasant in tea.
Jon Richfield, Somerset West, South Africa