
0004 Lactose may appear in different crystal poly-
morphs, depending on the crystallization conditions.
Each polymorph has its specific properties. As an
example, a-lactose (which crystallizes as a hydrate
containing equimolar amounts of lactose and water)
has very hard crystals and is not hygroscopic, as with
b-lactose. However, a hydrous b-lactose (which crys-
tallizes at temperatures above 93.5
C) forms small
crystals with a solubility up to 10 times higher than
that of a-lactose. Because of this higher solubility,
b-lactose is experienced sweeter than a-lactose.
0005 The solubility of a-lactose is more temperature-
dependent than that of b-lactose, and the solubility
curves intersect at 93.5
C. A solution at 60
Ccon-
tains approximately 59 g of lactose per 100 g of water.
Therefore, if a 50% solution of lactose (approx. 30 g
of b- and 20 g of a-) at 60
C is cooled to 15
C, then
at this temperature, the solution can contain only 7 g
of a-lactose or a total of 18.2 g per 100 g of water at
equilibrium. Therefore, a-lactose will crystallize very
slowly out of solution as irregularly sized crystals,
which give rise to a sandy, gritty texture.
0006 a-andb-lactose differ considerably in solubility
and in the temperature dependence of solubility. If
a-lactose is brought in water, much less dissolves at
the outset than later. This is because of mutarotation:
a-lactose is converted to b, hence the a-concentration
diminishes, and more a can dissolve. If b-lactose is
brought in water, more dissolves at the outset than
later (at least below 70
C): on mutarotation, more
a-lactose forms than can stay dissolved, and a-lactose
starts to crystallize.
0007 The solution of lactose is temperature-dependent,
and solutions are capable of being highly supersatur-
ated before spontaneous crystallization occurs, and
even then, crystallization may be slow. In general,
supersolubility at any temperature equals the satur-
ation (solubility) value at a temperature of 30
C
higher. The low solubility of lactose, coupled with
its capacity to form supersaturated solutions, is of
considerable practical importance in the manufacture
of concentrated milk products. This is of prime im-
portance in sweetened condensed milk, and crystal
size must be controlled if a product with a desirable
texture is to be produced. (See Carbohydrates: Clas-
sification and Properties.)
0008 Usually, a-lactose crystallizes as a hydrate contain-
ing one molecule water of crystallization. The crystals
are very hard, slightly hygroscopic, often fairly large,
and dissolve slowly. The water of crystallization
is very strongly bound. Above 93.5
C, anhydrous
b-lactose crystallizes from an aqueous solution.
b-lactose is not very hydroscopic, and it dissolves
quickly; its solubility is good. Therefore, dehydrating
a-hydrate is difficult. It may cause problems when
determining the dry matter of milk and milk prod-
ucts; this determination implies evaporation of water
at elevated temperatures. Maintaining the tempera-
ture > 93.5
C during the assay is paramount to pre-
vent the formation of a-lactose hydrate crystals.
0009Amorphous lactose is formed during rapid drying,
as in a spray drier. It is present in the glassy state,
which means that many properties, including hard-
ness, density, and specific heat, are similar to those of
crystalline sugar but that the packing of the molecules
does not show perfect order. Amorphous lactose con-
tains at least a few percent of water and can quickly
dissolve on addition of water. But then, a-lactose
hydrate may start to crystallize. If the water content
of the amorphous lactose is low, say 5%, crystalliza-
tion is postponed. However, the product attracts
water from moist air, and when moisture content
rises about 8%, a-lactose hydrate starts to crystallize
(at room temperature). The postponed crystallization
is an important factor in relation to spray-dried
powders made from skim milk or whey because it
leads to hard lumps in the powder, and eventually,
the whole mass of powder turns into one solid cake.
0010This caking is caused by the crystallization of lac-
tose, as it causes the powder particles, largely consist-
ing of lactose, to grow together (to sinter). Since
water is needed for crystallization of a-lactose, caking
does not occur at a low a
w
, below 0.4. At a higher
temperature, crystallization can occur far more read-
ily, a
w
being higher; moreover, the viscosity of the
highly concentrated lactose solution (essentially the
continuous phase of the powder particles) is lower,
leading to a more rapid nucleation, hence crystalliza-
tion. The susceptibility of caking, especially higher in
whey powder, is considerably reduced if most of the
lactose is crystallized before the drying (in the concen-
trate). Such precrystallized powder is usually called
‘nonhygroscopic,’ which may be a misnomer because
the powder concerned does not attract less water (this
is determined by its a
w
in relation to that of the air),
but the effects differ.
Sensory Properties and Digestibility
0011Lactose is a disaccharide that yields d-glucose and d-
galactose on hydrolysis. Lactose occurs in both a- and
b-forms, which differ in the steric configuration on
the H and OH around the C-1 glucose. Lactose is not
as sweet as sucrose or fructose. For example, 1.08,
2.2, and 4.91% sucrose are of equivalent in sweetness
to 3.75, 7.5, and 15% equilibrium lactose solutions.
b-lactose is 1.05–1.22 times as sweet as a-lactose, but
this small difference in sweetness is of no practical
value because equilibration of the anomers occurs
rapidly in solution.
LACTOSE 3473