Chemistry Reference
In-Depth Information
Milk powders with lactose hydrolysed to galactose and glucose show no
break in their sorption isotherms (San Jose et al., 1977; Jouppila and Roos,
1994a). It was suggested that crystallization of individual sugars in the pro-
tein-glucose-galactose mixture was delayed in comparison to lactose crystal-
lization in skim milk and whey powders. Skim milk powders containing
hydrolysed lactose showed a T
g
well below that of amorphous lactose. Powders
produced from skim milk containing galactose and glucose as a result of
enzymatic hydrolysis of lactose had an anhydrous T
g
at 498Candawater
content of 2.0 g/100 g of solids reduced the T
g
to 248C (Jouppila and Roos,
1994b). Haque and Roos (2006) have shown that the T
g
of lactose-containing
anhydrous skim milk powders is close to that of lactose at 1058C. However, a
number of T
g
values for amorphous lactose have been reported, which reflect
the sensitivity of the transition to composition and water. Various criteria are
also used to locate the transition temperature in DSC thermograms and it may
be taken from the onset or mid-point of the transition.
Galactose and glucose show glass transitions at 30 and 318C (Roos, 1993),
respectively. Although Kalichevsky et al. (1993a,b) found that sugars had only a
small effect on the T
g
of casein, the T
g
of milk powders containing hydrolysed
lactose seems to be higher than is suggested by the T
g
values of the component
sugars. The T
g
of milk powders is significantly reduced by lactose hydrolysis,
which presumably is the main cause of stickiness during processing and storage,
as well as of hygroscopic characteristics and higher susceptibility of the powder
to non-enzymatic browning reactions. It should also be noted that although
lactose is a reducing sugar, the hydrolysis of one mole of lactose produces two
moles of reducing sugars, i.e. one mole of galactose and one mole of glucose.
Lactose crystallization in dairy powders results in increasing rates of
non-enzymatic browning and other deteriorative changes (Labuza and
Saltmarch, 1981; Saltmarch et al., 1981; Miao and Roos, 2004). Saltmarch
et al. (1981) found that the rate of browning at 458C increased rapidly above a
w
of 0.33 and showed a maximum between a
w
of 0.44 and 0.53. The maximum
rate of browning occurred at a lower a
w
than was found for other foods. The
maximum rate was coincident with extensive lactose crystallization which was
observed from scanning electron micrographs. The rate of browning was
significantly lower in a whey powder which contained precrystallized lactose.
The loss of lysine was also found to be most rapid at water activities which
allowed lactose crystallization (Saltmarch et al., 1981). Crystallization of
amorphous lactose in closed containers increases water activity very rapidly
and accelerates the browning reaction in comparison with the rate of the
reaction at the same temperature but at a constant water activity (Kim et al.,
1981). Compositional factors and crystallization behaviour of different sugars
may also enhance lipid oxidation (Shimada et al., 1991) and browning reactions
(Miao and Roos, 2004; Nasirpour et al., 2006).
