Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

444 Part 4: Milk

ββ

α

β(1→4)

Galactose Glucose

Lactose

O-β-D-Galactopyranosyl-(1→4)-α-D-Glucopyranose: α-Lactose

O-β-D-Galactopyranosyl-(1→4)-β-D-Glucopyranose: β-Lactose

β

Anomeric carbon

1

2

(^41) α β
6
5
3
(^41)
CH 2 OH
H
HO O
H
OH
H
H
OH
H
CH 2 OH
H
H O
H
OH
O
H
OH
O
OH
H
(1→4) 6
5
O
O
O
OH
O
O
OOH
(^23)
OH
H
H
OH
2
Figure 24.1.Structures of lactose.
The functional aldehyde group at the C-1 position of the glu-
cose moiety exists mainly in the hemiacetal form, forming a
cyclic structure and, consequently, C-1 is a chiral, asymmet-
ric, carbon. Therefore, like all reducing sugars, lactose can ex-
ist as two anomers,αandβ, which have markedly different
properties. From a functional viewpoint, the most important of
these properties are differences in solubility and crystallisation
characteristics between the isomers;α-lactose crystallises as a
monohydrate, while crystals ofβ-lactose are anhydrous. Since
crystallineα-lactose contains 5% H 2 O, the yield of this anomer
is higher than that ofβ-lactose and this must be considered when
expressing the concentration of lactose. The solubility ofα-and
β-lactose in water at 20◦C is approximately 7 g/100 mL and
50 g/100 mL, respectively. However, the solubility ofα-lactose
is much more temperature-dependent than that ofβ-lactose and
the solubility curves intersect at approximately 93.5◦C (see Fox
and McSweeney 1998).
At equilibrium in aqueous solution, lactose exists as a mixture
ofαandβanomers in the approximate ratio of 37:63. When an
excess ofα-lactose is added to water, approximately 7 g/100 mL
dissolve immediately, some of which mutarotates to give anα:β
ratio of 37:63, leaving the solution unsaturated with respect to
bothα-andβ-lactose. Furtherα-lactose then dissolves, some of
which mutarotates toβ-lactose. Solubilisation and mutarotation
continue until two conditions exist, that is approximately 7 g
of dissolvedα-lactose/100 mL of water and anα:βratio of
37:63, giving a final solubility of approximately 18.2 g/100 mL.
Whenβ-lactose is added to water, approximately 50 g/100 mL
dissolve initially but approximately 18.5 g of this mutarotates
toα-lactose, which then exceeds its solubility and some lactose
crystallises. This upsets theα:βratio and moreβ-lactose mutaro-
tates toα-lactose, which crystallises. Mutarotation ofβ-lactose
and crystallisation ofα-lactose continue until approximately 7 g
and 11.2 g ofα-andβ-lactose, respectively, are in solution.
Although lactose has low solubility in comparison with
other sugars, once dissolved, it crystallises with difficulty and
forms supersaturated solutions. Highly supersaturated solutions
(greater than twofold saturated) crystallise spontaneously but
if the solution is only slightly supersaturated (one to twofold),
lactose crystallises slowly and forms large, sharp, tomahawk-
shaped crystals ofα-lactose. If the dimensions of the crystals
exceed approximately 15μm, they are detectable on the tongue
and palate as a sandy texture. Crystals ofβ-lactose are smaller
and monoclinical in shape. In the metastable zone, crystallisation
of lactose is induced by seeding with finely powdered lactose
(Fig. 24.2). Since the solubility ofα-lactose is lower than that of
theβanomer below 93.5◦C,α-lactose is the normal commercial
form.
When concentrated milk is spray-dried, the lactose does not
have sufficient time to crystallise during drying and an amor-
phous glass is formed. If the moisture content of the powder
is kept low (<4%), the lactose glass is stable, but if the mois-
ture content increases to about 6%, for example on exposure
of the powder to a high-humidity atmosphere, the lactose will
crystallise asα-lactose monohydrate. If extensive crystallisation
occurs, an inter-locking mass of crystals is formed, resulting in
‘caking’ of the powder, which is a particularly serious problem
in whey powders owing to the high content of lactose∼70%).
The problem is avoided by pre-crystallising, as much as possi-
ble, of the lactose before drying, which is achieved by seeding
the concentrated solution with finely powdered lactose.
Controlled agglomeration (caking) is used to improve the wet-
tability of spray-dried milk powder This product has poor wet-
tability because the small particles swell on contact with water,
thereby blocking the channels between the particles (see Kelly
et al. 2003 for review). The wettability (often incorrectly re-
ferred to as ‘solubility’) of spray-dried milk powder may be
improved by controlling the drying process to produce milk