Chemistry Reference
In-Depth Information
rapidly and results in lactose crystallization in various forms, depending on
temperature and water content (Haque and Roos, 2005).
Crystallization of amorphous lactose in dairy powders and in ice cream
during storage is one of the principal causes of loss of product quality
(Supplee, 1926; Troy and Sharp, 1930). Supplee (1926) reported that milk
powders sorbed large amounts of water at low storage relative humidities.
This water often induced changes in properties of the powder, and the water
content decreased at higher humidity conditions due to crystallization. Troy
and Sharp (1930) reported that drying of milk and whey by spray drying and
roller drying produced a glass, composed of a non-crystalline mixture of
-
and
-lactose. Water sorption by whey powders caused plasticization and
subsequent hardening of the material owing to lactose crystallization.
Herrington (1934) found that lactose glasses were stable at room tem-
perature if they were protected from water. The existence of lactose in the
glassy state in dairy products and lactose crystallization at high storage
humidities have been confirmed in numerous studies. These studies have
used polarized light microscopy, electron microscopy, differential scanning
calorimetry (DSC), nuclear magnetic resonance (NMR) and X-ray techni-
ques to analyse the physical state of lactose in dairy powders (King, 1965; Lai
and Schmidt, 1990; Roos and Karel, 1990; Jouppila et al., 1997; Haque and
Roos, 2005). As shown in Figure 2.5, water sorption by most dehydrated
dairy products, which contain lactose, shows a characteristic break in the
sorption isotherm, indicating lactose crystallization (Berlin et al., 1968a,b;
Jouppila and Roos, 1994a,b; Haque and Roos, 2006).
The crystallization behaviour of amorphous lactose in milk products is
also temperature dependent. Berlin et al. (1970) observed that the relative
humidity at which the break in sorption isotherms appeared was dependent
on temperature, which was confirmed by Warburton and Pixton (1978). An
increase in storage temperature shifted the break to a lower relative humidity.
The temperature dependence of the water sorption properties of crystallizing
amorphous sugars can be explained by changes in their physical state. DSC
thermograms of milk powders show a glass transition and a crystallization
exotherm for the amorphous lactose fraction (Jouppila and Roos, 1994b).
Water plasticization decreases the T
g
of lactose and a higher water content
causes lactose crystallization at a lower temperature. Water plasticization of
non-crystalline lactose and associated depression of the T
g
to a lower tem-
perature indicates that the break in the lactose sorption isotherm is both
temperature and time dependent.
Amorphous lactose may crystallize in a complex manner in a number of
crystalline forms and the form produced depends on the relative humidity and
temperature. According to Vuataz (1988), lactose crystallizes as the anhy-
drous
-form at relatively low water activities or as
-lactose monohydrate
