Amorphous system freeze concentration

Spectroscopic methods, such as FT-infrared (FTIR) and Raman spectroscopy detect changes in molecular vibrational characteristics in noncrystalline solid and supercooled liquid states. Various nuclear magnetic resonance (NMR) techniques and electron spin resonance (ESR) spectroscopy, however, are more commonly used, detecting transition-related changes in molecular rotation and diffusion (Champion et al. 2000). These methods have been used for studies of the amorphous state of a number of sugars in dehydrated and freeze-concentrated systems (Roudaut et al. 2004). 

Fig. 1 Freeze concentration in an amorphous system. The product temperature is shown as a broken line whereas the percentage of unfrozen water is shown as a solid line. (From Ref.. )...

There are, however, other ways in which conventional drying could, in principle, be performed at temperatures above Tg, without deleterious results. If a solution could be treated so that one or several components can undergo crystallisation, then the crystals formed would serve as a substitute for the subliming ice crystals and could mechanically protect the porous, amorphous matrix against structural collapse. The efficacy of this type of formulation for freeze-drying purposes has been demonstrated for the system water-sucrose-NaCl. " Its practical applications may, however, be limited to products where the bioactive component is not chemically degraded in the freeze-concentrate at temperatures above Tg. 

Figure 12. Schematic state diagram of temperature vs. w% solute for an aqueous solution of a hypothetical small carbohydrate (representing a model frozen food system), illustrating the critical relationship between Tg and freezer temperature (Tf), and the resulting impact on the physical state of the freeze-concentrated amorphous matrix. (Reproduced with permission from reference 18. Copyri t 1988 Cambridge.)...

On cooling a typical copolymer melt, one observes, after the customary supercooling, crystallization of pure A. The melt must thus increase to some degree in concentration B as predicted by the liquidus line of Fig. 4.23. But in copolymer systems, one neither reaches the liquidus concentration, nor observes the eutectic point. The system freezes to a metastable state before the eutectic temperature is reached. Usually only one component crystallizes in random copolymers. All of the component B and a large fraction of A remain in the amorphous portion of the semicrystalline sample. For a more extensive discussion of the irreversible melting of homopolymers and copolymers see Ref. 57, Chapters IX and X. 

Carbohydrates and proteins are typical hydrophilic components of concentrated food systems. These components tend to form amorphous, noncrystalline structures at low water contents (White et al. 1966 Slade et al. 1991 Roos 1995). Well-known food processes resulting in glass formation by amorphous or partially amorphous food components include baking, extrusion, dehydration and freezing (Roos 1995). In these processes, removal of water as part of the manufacturing process results in the formation of a noncrystalline, amorphous state, which is extremely sensitive to water and may show various time-dependent changes causing loss of quality and reduced shelf life. 

Fig. 32. Magnetic phase diagram pertaining to the amorphous alloy system Laoi,(, Gd Au 2o Proposed by Poon and Durand (1978). The log of the freezing temperatures TJ is plotted versus the log of the Gd concentration.

---