Chemistry Reference
In-Depth Information
relaxation time (Fujara et al., 1992; Blackburn et al., 1996; Hall et al., 1998).
Since the decoupling temperature was in the range where
and
relaxations
merge (Perez and Cavaill´ 1994), it was suggested that the translational
diffusion of the probe could be facilitated by local motions of the matrix
when the temperature approaches T
g
(Champion et al., 1997b). The practical
importance of this decoupling has to be emphasized, with regard to the
stability of amorphous products: In the vicinity of T
g
, the translational
diffusivity may be 2-5 orders of magnitude higher than that predicted from
the viscosity and the Stokes-Einstein equation. The temperature dependence
also is much weaker (apparent activation energy
¼
59 kJ/mol for the D
trans
of
fluorescein in sucrose below 1.2 T
g
).
For food systems composed of polymers, small molecules and water,
DSE expressions may not be valid to predict diffusion, because the ''macro-
scopic viscosity'' (commonly measured) does not reflect the local environ-
ment of the diffusing species and is not the factor that controls diffusion.
The translational and rotational mobilities of small probes dispersed in
concentrated sucrose solutions (57.5%) were not significantly affected, or
were reduced only slightly, upon addition of, respectively, 1 or 10% poly-
saccharides, in spite of the large increase in viscosity (Contreras-Lopez
et al., 2000).
A study on caseinates provides a good illustration of the mobilization
process induced by water (Figure 11.17). The mobility of a small probe
(monosaccharide analogue) dispersed in the protein matrix was monitored
by ESR. Below a critical water content (0.25 g H
2
O/g dw), all the small
solutes exhibited very slow motions (rotational correlation time,
c
>
10
-7
s). Then, between the above critical value and approximately 1 g
H
2
O/g dry weight, a progressive dissolution of the small solutes was
observed. Once dissolved, solute diffusivity was controlled by the hydro-
dynamic radius of the solute, the water content and the changes in plasticity
of the surrounding protein (LeMeste et al., 1991). Although the glass
transition for caseinate occurs at room temperature for a water content
close to 20% (wet basis), it seems that the glass transition of the polymeric
network and the mobilization process of small solutes, as also that of the
protein side chains, are not closely related: ''(1) the mobilization process of
spin-labelled protein side chains (lysyl residues) started at a water content
(0.5gH
2
O/g dry weight) higher than that for small solutes (data not shown);
(2) the critical water content necessary for the first solute molecule to
become more mobile (i.e. to be dissolved) increases with the size of the
solute (Figure 11.17); (3) internal plasticization of caseinates (resulting from
the glycation of lysyl residues) did not modify the water content where, on the
one hand small solutes (Figure 11.17), and on the other hand, the spin-labelled
side chains, became mobile (LeMeste et al., 1990)''.
