Chemistry Reference
In-Depth Information
products of lactose hydrolysis. Water plasticization occurs only in the solids-
non-fat fraction, and state diagrams describe the solids-non-fat properties of
dairy systems.
The lactose-water system is a binary solute-solvent mixture. Water, as
a small molecular mass solvent, acts as a strong plasticizer and a significant
depression of the glass transition temperature, T
g
, occurs at low water con-
tents (Slade and Levine, 1991). The plasticization behaviour of amorphous
polymer-solvent systems is often modelled using the Gordon-Taylor rela-
tionship (Gordon and Taylor, 1952), which allows modelling of the glass
transition temperature depression with increasing water content. The Gor-
don-Taylor relationship is shown in equation (1), where w
1
and w
2
are weight
fractions of solids and water, respectively, T
g1
and T
g2
are the glass transition
temperatures of respective components and k is a constant:
T
g
¼
w
1
T
g1
þ
kw
2
T
g2
w
1
þ
kw
2
(1)
The constant, k, in equation (1) can be derived from experimental data
for T
g
at various water contents (Roos, 1995). Water plasticization of lactose
has been shown to follow this equation which allows its use for establishing
the glass transition curve in the state diagram of lactose (Roos and Karel,
1991a). The Gordon-Taylor equation has also been applied to predict water
plasticization of dairy powders (Jouppila and Roos, 1994b; Haque and Roos,
2006), casein (Kalichevsky et al., 1993a,b) and a number of other food
systems (Roos, 1995). Although numerous values have been reported for
the glass transition temperature of non-crystalline water, the glass transition
temperature for amorphous water is often taken as -1358C (Sugisaki et al.,
1968). Several equations other than the Gordon-Taylor relationship are
available for predicting the effects of water plasticization and composition
on the T
g
of dairy solids (Roos, 1995).
Most state diagrams show equilibrium melting temperatures of ice at
various water contents and kinetic limitations for ice formation. Ice formation
ceases at temperatures where the equilibrium ice melting temperature
approaches the glass transition of the freeze-concentrated solutes in an unfro-
zen solute matrix. Kinetically limited ice formation may be described as non-
equilibrium ice formation, which is a typical phenomenon in rapidly cooled
carbohydrate solutions and probably the most common form of ice formation
in frozen dairy systems, including ice cream and frozen yoghurt. One of the first
studies reporting non-equilibrium freezing was that of Troy and Sharp (1930),
who found that rapid freezing of ice cream resulted in freeze-concentration and
