CONCLUSIONS
The results from mapping the frozen state suggest that cryoconcentration is preserved in the frozen state and the observed
macro-cryoconcentration values during freezing were in general agreement with frozen state values. The extent of macro-cryoconcentration
(measured by cores) varied between the top and bottom halves of the frozen block. The significantly higher macro-cryoconcentration
in the lower half of the cryowedge can be explained by the presence of disaccharide in the formulation and formation of temperature-induced
density gradients. When freezing a protein solution containing an additive that shows significant changes in density with
temperature, a cooling-induced density gradient is created that leads to convective flow of solution. This causes the lower
regions of the solution to cryoconcentrate to a greater extent than the upper regions. However, looking fundamentally at the
changes occurring during the freezing process tells us that the true extent of (micro-)cryoconcentration experienced by the
protein in the inter-dendritic spaces can be estimated from a phase or state diagram for the major solute component. This
is significantly higher than what is measured as macro-cryoconcentration. Active freezing systems can affect the overall distribution
of ice and solute in the matrix, i.e., the macro-cryoconcentration, but not the micro-cryoconcentration experienced by the
solute and protein embedded in the matrix.
Parag Kolhe is a principal scientist, Alanta Lary is a senior scientist, Steven Chico is an associate scientist, Elisabeth Holding was an associate scientist, and Satish K. Singh is a research fellow, all at Pfizer, Inc., Chesterfield, MO, 636.247.9979, satish.singh@pfizer.com
.
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