Solid and Liquid States of Lactose - Advanced Dairy Chemistry - page 16

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Solid and Liquid States of Lactose

Y.H. Roos

Lactose in dairy systems can exist in various crystalline and non-crystalline
forms. These forms affect lactose behaviour, particularly in processing and
storage of low-water dairy foods. Crystalline -lactose monohydrate and
anhydrous -lactose are well-known solid forms of lactose, which are rela-
tively poorly soluble in water. Its occurrence in two anomeric forms, - and
-lactose, makes its solubility a complex function of temperature. -Lactose
has low solubility in water at room temperature, but mutarotation to equili-
brium quantities of the - and -forms increases the overall solubility of
lactose which increases rapidly with increasing temperature, with a more
rapid increase in the solubility of -lactose. Liquid dairy systems contain
dissolved lactose in a complex chemical environment and lactose is likely to
exist in a composition-, temperature- and process-dependent / -ratio. On
rapid removal of solvent water from dairy liquids on dehydration or freezing,
lactose molecules retain their solution structure and, therefore, amorphous,
non-crystalline solid forms of lactose are typical of dairy powders and frozen
dairy desserts (Roos, 1995; Hartel, 2001).
Amorphous lactose in dairy solids may often exist in a glassy, solid
state or in a syrup-like, super-cooled liquid state. The apparent glass-like
solid state results from a very high viscosity exceeding 10 12 Pa s (White and
Cakebread, 1966). The state transition of amorphous solid- and liquid-like
states occurs over a second-order-type state transition known as the glass
transition (White and Cakebread, 1966), as described in Figure 2.1. The
glass transition involves no latent heat but it can be observed from changes
in heat capacity, thermal expansion coefficient, dielectric properties,
various mechanical and flow properties and molecular mobility (White