Effects of solutes and freezing front velocity on ice morphology in frozen ovine milk

Abstract

Ovine milk is sometimes stored frozen until processing, and deterioration has been observed during frozen storage. Mechanisms for degradation have been proposed involving morphology of the frozen product. Freezing is also integral to freeze-drying, sometimes applied to ovine milk. Knowledge of factors influencing the frozen morphology in ovine milk may help avoid degradation or aid freeze-drying.Fractions were prepared from ovine milk and used to build milks of decreasing complexity from whole milk, skim milk, skim milk less casein (removed centrifugally), ovine milk ultrafiltration permeate and finally water. Cryo-Scanning-Electron Microscopy, and optical microscopy with a custom-built Bridgman furnace format freezing stage, were used to investigate the static morphology of frozen milk and then the morphology of advancing freezing fronts in ovine milk. Milk frozen slowly showed a significantly different morphology in the frozen state to milk that was frozen rapidly. The morphology of the freezing front was observed to change as the velocity increased, and as the composition of the sample being frozen changed. Increased freezing front velocities lead to the formation of columnar and then dendritic ice morphologies, and the freezing front velocities that lead to transitions in morphology increased with decreasing fluid complexity. A phase diagram was generated showing ice morphology on axes representing solution complexity and ice front velocity.Knowledge gained from the observations was used to design rapid freezing processes that preserve fresh milk properties, which have since been patented.