Water in Dairy Products - Advanced Dairy Chemistry - page 484

E a
RT g ln 10

m ¼ (20)

E a being the apparent activation energy at T g . Since ''fragility'' is a
measure of the sensitivity of amorphous materials to temperature and water
content, it seems to be a relevant parameter to evaluate the efficiency of
ingredients in imparting stability during storage or processability in operations
such as extrusion, flaking or drying. A discussion of the various methods used
to estimate m for food materials can be found in Simatos et al. (1995b).
As is well known, liquid water possesses unusual physical properties
compared to most other liquids. These ''anomalies'' become even more pro-
nounced for supercooled water. Moreover, unlike most other substances,
glassy water can apparently exist in at least two distinct forms (see Angell,
2002, and Debenedetti, 2003, for reviews). Some arguments had been used to
suggest that water undergoes a transition from extreme fragility, in the range
240-273 K, to strong character close to its glass transition (Angell, 1993);
although this view is still a matter of controversy (Debenedetti, 2003), newer
studies have been presented to support the occurrence of this transition in the
range 220-240 K (Angell, 2002; Starr et al., 2003). All low molecular weight
sugars for which data have been published (glucose, fructose, sucrose, mal-
tose, trehalose) can be classified as rather fragile materials; they seem to be
located in a narrow domain of the fragility diagram, with low or no influence
of water content (LeMeste et al., 2002) (Figure 11.14). One may assume that
this is true also for lactose. For proteins, the scarce experimental data seem to
indicate a strong behaviour: m 40.5 for poly- L -asparagine (15-25% water)
(Angell et al., 1994) and similar values for elastin and gluten (Simatos et al.,
1995b). Pullulan-starch blends were found to show strong behaviour
(m 42-51) increasing with water content (Biliaderis et al., 1999). Fragility
was reported to increase in the order: pullulan < dextran and phytoglycogen
< amylopectin and to decrease for amylopectin with increasing water content
(Borde et al., 2002).

11.5.2. Transport Properties

The drastic changes in mechanical properties that occur at the glass
transition led to the assumption of a parallel evolution of the translational
diffusion of water and solutes, resulting in important applications for food
processing and storage. These expectations have been fulfilled only partially.
In liquid systems well above T g , the translational diffusion (D trans ) and
rotational diffusion (D rot ) of molecules can be predicted from the so-called
Debye-Stokes-Einstein relations (DSE):