LACTOSE 31Table 2.3 Some physical properties of the two common forms of lactose (modified from Jenness
and Patton, 1959)
Property a-H ydrate 8- Anhydride
Melting point" ("C) 202 252
Solubility in water (g 100 rn1-l) at 20°C I 50
Specific gravity (20°C) 1.54 1.59
Specific heat 0.299 0.285
Heat of combustion (kJ mol-')^5687 5946
"Decomposes; values vary with rate of heating, ti-hydrate loses water at 120°C.
bValues on anhydrous basis, both forms mutarotate to f55.4".Specific rotaltionb [a]:' + 89.4" +35"Some properties of c(- and !-lactose are summarized in Table 2.3. Mixed
a/! crystals, e.g. asp3, can be formed under certain conditions. The relation-
ship between the different crystalline forms of lactose is shown in Figure 2.8.Lactose glass. When a lactose solution is dried rapidly, viscosity increases
so quickly that crystallization is impossible. A noncrystalline form is
produced containing a- and !-forms in the ratio at which they exist in
solution. Lactose in spray-dried milk exists as a concentrated syrup or
amorphous glass which is stable if protected from air, but is very hygro-
scopic and absorbs water rapidly from the atmosphere, becoming sticky.2.2.7 Problems related to lactose crystallization
The tendency of lactose to form supersaturated solutions that do not
crystallize readily causes problems in many dairy products unless adequate
controls are exercised. The problems are due primarily to the formation of
large crystals, which cause sandiness, or to the formation of a lactose glass,
which leads to hygroscopicity and caking (Figure 2.9).
Dried milk and whey. Lactose is the major component of dried milk
products: whole-milk powder, skim-milk powder and whey powder contain
c. 30, 50 and 70% lactose, respectively. Protein, fat and air are dispersed in
a continuous phase of amorphous solid lactose. Consequently, the behav-
iour of lactose has a major impact on the properties of dried milk products.
In freshly made powder, lactose is in an amorphous state with an a/!
ratio of 1 : 1.6. This amorphous lactose glass is a highly concentrated syrup
since there is not sufficient time during drying for crystallization to proceed
normally. The glass has a low vapour pressure and is hygroscopic, taking
up moisture very rapidly when exposed to the atmosphere. On the uptake
of moisture, dilution of the lactose occurs and the molecules acquire
sufficient mobility and space to arrange themselves into crystals of a-lactose