Water in Dairy Products - Advanced Dairy Chemistry - page 487

PALS, together with density measurements, also provide information
concerning the changes in nanostructure when water is sorbed on carbohy-
drates (Kilburn et al., 2004, 2005). The mean hole volume was found to
increase strongly, and linearly, with water content (0.1 < a w < 0.75 at
258C). The partial molar volume of water in the carbohydrate glassy matrix
was estimated to be close to the van der Waals' volume of water (7.1 cm 3 /
mol), i.e. much lower than its value in the liquid state (18 cm 3 /mol). Water
influences the average hole size, on the one hand, by filling the smallest voids
in the glassy matrix and, on the other hand, by increasing the mobility of the
polymer chains, which allows the coalescence of the smallest voids under the
surface tension driving force (Kilburn et al., 2004). Whereas water acts as a
plasticizer mainly by interacting with the polymer H-bonding, conversely, the
plastifying action of low molecular weight sugars is suggested to be due to a
reduction of the number of molecular entanglements because of their small
size (Kilburn et al., 2005). The differences in the glass nanostructure observed
as a function of the molecular weight have important technological implica-
tions, as the barrier properties of carbohydrates in the glassy state increase
with decreasing molecular weight, although their glass transition temperature
decreases (Kilburn et al., 2005).
Several sub-T g relaxations can be observed in biopolymers and low
molecular weight sugars ( and relaxations). Their origin is still being
discussed. As observed for polysaccharides, they could correspond to the
rotation of lateral groups ( relaxation at low temperature) or to local
conformational changes of the main chain ( relaxation closer to T ) (Mont`s
et al., 1998; Einfeldt et al., 2004; Bidault et al., 2005). The E a value ranges
between 40 and 70 kJ/mol for relaxation in sucrose (Figure 11.14), maltose
and glucose. Sub-T g relaxations are also observed in complex food products,
such as bread (Roudaut et al., 1999a,b) (Figure 11.9). The influence of water
on these processes is less well known than its plastifying action on T g . For
polysaccharides, T was found to decrease for increasing water content
(Borde, 1999; Lievonen and Roos, 2003; Poirier-Brulez et al., 2006).
Water at low concentration may induce an increase in rigidity, although
T g is decreased. This hardening effect, observed with starch and cereal pro-
ducts under high deformation conditions (Attenburrow et al., 1992; Nicholls
et al., 1995; Fontanet et al., 1997; Roudaut et al., 1998; Li et al., 1998; Chang
et al., 2000; Konopacka et al., 2002), is known as antiplastification for
polymers (Vrentas et al., 1988). This antiplastifying effect of water in starch
systems was attributed to a short-range reorganization (Fontanet et al.,
1997), a density increase by filling the defects in the glass structure (Benczedi,
1999; Chang et al., 2000).
When stored below T g , a glass is subject to a slow evolution of many of
its properties. This ''physical ageing'' is the result of a microstructure