Chemistry Reference
In-Depth Information
(see below) and more prone to shrinkage or loss of air, all which limit its shelf
life (Marshall et al., 2003). These are all a result of enhanced mobility of
constituents within the ice cream structure. This is one of the major limita-
tions of the use of a high level of lactose in ice cream/frozen dairy dessert
formulations.
III.4.
Potential for Lactose Crystallization
The crystallization of lactose in general has been well studied and is reviewed
in Chapters 1, 2 and 3.I. The crystallization of lactose in ice cream has also
been well studied over many decades because, in this specific application,
crystallization leads to the serious texture defect known as sandiness (Zoller
and Williams, 1921; Nickerson, 1954, 1956, 1962; Livney et al., 1995). The
solubility of
-lactose is 7 g/100 g water at 208C, while the solubility of
-lactose is 50 g/100 g water. The mutarotation equilibrium is 1.6
:1.0
,
so the final total solubility of lactose is 18.2 g/100 g water at 208C at this ratio
(Fox and McSweeney, 1998). Solubility and mutarotation are both a function
of temperature, so at 08C the solubility of
-lactose is approximately 2-3 g/
100 g water to provide a total lactose solubility closer to 11 g/100 g water
(Nickerson, 1956) and solubility continues to decline into the sub-zero region.
The initial concentration of lactose in ice cream may be expected to be
approximately 9-10 g/100 g water, depending on formulation (Marshall et al.,
2003). However, the process of freeze-concentration due to the formation
of ice is critical to an understanding of lactose crystallization in ice cream, as it
contributes much more strongly to supersaturation than does decreasing
temperature in the absence of freezing. Solutes become freeze-concentrated
in an ever-decreasing volume of solvent as the temperature is lowered and
more ice is formed. The water in this unfrozen phase forms an equilibrium
ratio with ice at any given temperature. The removal of solvent (water) by
freezing results in a doubling of the lactose concentration at the temperature
of extrusion from the ice cream freezer (-58C) and concentrations of 3-5
at -10 to -208C as freeze-concentration continues.
It should be obvious from the above discussion that lactose has greatly
exceeded its solubility (saturation) level in frozen ice cream and, from a
thermodynamic point of view, could easily crystallize. Increasing supersa-
turation favours crystallization (Hartel and Shastry, 1991; Hartel, 2001).
However, the first step of crystallization is nucleation of the lactose and this
process is constrained kinetically by both high viscosity and low temperature
in the unfrozen phase, thus maintaining lactose in the supersaturated, non-
crystalline state. This increased viscosity and decreased temperature, which
decreases the driving force for crystallization, overwhelms the effect of
