Glassy freezing materials

A fully amorphous (glassy) freeze-dried product, consisting of 5% of a calcitonin gene-related protein (CGRP), 95% lactose and 3% residual water, was heated gently in a sealed DSC capsule. Its Tg is shown by the endothermic step in Scan 1 of Figure 4. Heating beyond Tg makes a material susceptible to more or less rapid physical and/or chemical... 

Polymer glasses are formed best when the macromolecular chains are irregular in structure (atactic, branched, crosslinked) so that crystallization is prevented. Regular (isotactic, syndiotactic unbranched) polymer chains form glasses only if they are cooled down so fast that crystallization is prevented such a quenching procedure freezes the material in the glassy state even if the polymer is able to crystallize. 

Figure 5 presents the results of tensile tests for the HPC/OSL blends prepared by solvent-casting and extrusion. All of the fabrication methods result in a tremendous increase in modulus up to a lignin content of ca. 15 wt.%. This can be attributed to the Tg elevation of the amorphous HPC/OSL phase leading to increasingly glassy response. Of particular interest is the tensile strength of these materials. As is shown, there is essentially no improvement in this parameter for the solvent cast blends, but a tremendous increase is observed for the injection molded blend. Qualitatively, this behavior is best modeled by the presence of oriented chains, or mesophase superstructure, dispersed in an amorphous matrix comprised of the compatible HPC/OSL component. The presence of this fibrous structure in the injection molded samples is confirmed by SEM analysis of the freeze-fracture surface (Figure 6). This structure is not present in the solvent cast blends, although evidence of globular domains remain in both of these blends appearing somewhat more coalesced in the pyridine cast material.

Conclusion. Most foods that are aqueous solutions and/or dispersions can be freeze-concentrated without crystallization of solutes and so become mixtures of ice crystals and a glassy material. The glass is of mixed composition, and roughly 20% of its mass is unfrozen water. The frozen food is stable to all physical changes and to many chemical reactions, provided that the temperature is kept below Tg. 

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