Chemistry Reference
In-Depth Information
mobility induced by glass transition beginning at a temperature as low as T
0
(the onset of transition T
1
) (Simatos and Blond, 1993). ESR showed that
transition T
2
corresponded to an important increase in mobility, suggesting
that ice melting was beginning at that temperature (LeMeste and Simatos,
1980). Recently, in the case of gluten, the minor endothermic peak observed
just before the major ice melting peak was assigned to the melting of ice
confined in capillaries (Kontogiorgios and Goff, 2006). Although the authors
related the existence of confined water to the porous microstructure of gluten,
one could envisage the same explanation for other food systems. Water
confined between the ice crystals formed during the first freezing step could
freeze as small crystals, the melting of which would give the endothermic feature
T
2
. The latter has been shown to develop as a peak during an annealing treat-
ment in sugar solutions (Simatos and Blond, 1993; Champion, pers. comn.).
11.5.6.2. Can T
g
0
Be Raised? Effect of ''Stabilizers'' in Ice Cream
As stability of frozen products at a given temperature was hypothesized
to increase with a raise in T
g
0
(Levine and Slade, 1988), the possibility of
raising T
g
0
through the addition of high molecular weight compounds was
explored in many studies. The influence of macromolecules on the glass
transition temperature of frozen solutions was reviewed, e.g. by Simatos
et al. (1995a), and can be summarized as follows. When added at a rather
high concentration to solutions of low molecular weight solutes, polymers
induce an increase in the value of T
2
. The values of T
0
and T
1
, however, do not
increase, which indicates that the temperature at which some mobility
appears is not raised on addition of the polymer. The T
1
temperature of
frozen sucrose or trehalose solutions, to which egg albumin, gelatin, corn-
starch or a mixture of the three was added (at the solids content ratio 1/1), was
even reported to decrease (by 2-78C), although the glass transition tempera-
tures of the dry mixtures were strongly raised (by 9-598C) and the tempera-
tures T
m
0
(i.e. T
2
) were raised by 3-118C (Singh and Roos, 2005, 2006).
Macromolecules (various polysaccharides and gelatine) are commonly
added to ice cream mixes as ''stabilizers'' as they are known to exert a
beneficial effect on the texture of the final product. At the very low concen-
trations that are used in the ice cream industry, these stabilizers do not induce
a visible effect on the DSC T
g
0
(T
2
) values (Levine and Slade 1988; Goff et al.,
1993; Blond 1994). The main mechanism of the favourable effect of these
stabilizers on the sensory properties of texture may relate to the perception in
the mouth, either through a change in the viscoelastic properties of the
unfrozen phase or any other sensory effect. Stabilizers, however, have been
shown to reduce the recrystallization rate in ice creams stored at temperatures
above T
g
0
(Caldwell et al., 1992; Hagiwara and Hartel, 1996). In frozen
