Water in Dairy Products - Advanced Dairy Chemistry - page 472

understood as a glass transition, because it is observed in extremely short
timescale studies (of the order of ps or ns), very different from the timescale of
molecular diffusion or reorientation ( 100 s) that is considered to be
characteristic of glass transition (Green et al., 1994; Angell, 1995). It was
proposed, instead, that the low temperature transition is actually a strong
water-sensitized relaxation. When the temperature is raised, the progressive
evolution of heat capacity, mechanical relaxation studies and dielectric mea-
surements suggest a continuing build-up of relaxation processes, eventually
merging with the process at some higher temperature, consistent with the
glass transition temperature observed with low-moisture proteins and poly-
peptides (Green et al., 1994). Dielectric measurements in wide frequency and
temperature ranges on haemoglobin (0.8 g water/g protein) showed several
relaxation processes with different temperature dependencies. One of them, a
'' -like'' process (Arrhenius temperature dependence) corresponding to a
100 s at -1038C, was attributed to local motions of polar side groups of the
protein (possibly together with a small amount of bound water molecules).
This process changed at ca. -638C to a more cooperative '' -like'' relaxation
(VTF temperature dependence) (Jansson et al., 2005b, 2006). Similar results
were obtained with myoglobin, although with somewhat lower temperature
levels (Swenson et al., 2006, 2007). There seems to exist a consensus, however,
on the idea that compact proteins will not undergo a true glass transition, the
term ''protein dynamic transition'' being preferred (Doster and Settles, 2005).
This view has much in common with the model proposed by Gregory (1995,
1998). According to the knot-matrix model, knots are regions of the native
structure with efficient packing of residues, higher density and low conforma-
tional mobility, embedded in more flexible matrices. Only the surface and
matrix regions should undergo the low temperature transition, while the
knots remain glassy and rigid, cooperative disruption of the knots occurring
only at denaturation. Protein motions appear to be coupled to the and
relaxation processes in water (Doster and Settles, 2005; Jansson et al., 2005b,
2006; Swenson et al., 2006, 2007).
The state diagram in Figure 11.5 is based on data for the lactose-water
system, because it has been recognized widely that the glass transition beha-
viour of milk powders is very close to that of pure lactose (e.g. Jouppila and
Roos, 1994; Vuattaz, 1999, 2002; Fernandez et al., 2003). This is true what-
ever the fat content of milk is, provided the solid non-fat is considered
(Figure 11.10). A unique line is also obtained when T g is represented against
a w for amorphous lactose, milk powder with varying fat content and whey
powders with various degrees of lactose precrystallized in the form. This
curve is correctly described by a linear expression for 0.12 < a w < 0.65: T g (8C)
¼ -143.6 a w +77.8 (Vuattaz, 2002).