Chemistry Reference
In-Depth Information
also reduces the rate of diffusion of lactose to the surface of lactose crystals
(Hunziker, 1946).
Studies on the solubility of lactose have been published by several authors,
in water (e.g., Hudson, 1908 (see Visser, 1982); Herrington, 1934; Visser, 1982),
in ethanol (Machado et al., 2000; Tze-Shuong, 2004) or in solution containing
sucrose also (Hartel, 1993).
I.5.
Crystallization of Lactose in Sweetened Condensed Milk
and Related Products
Lactose in milk powder, spray-dried lactose or inhaler-grade lactose usually
occurs as a combination of
-hydrate crystals, anhydrous
-crystals and
amorphous lactose ''glass''. Amorphous lactose is formed during rapid eva-
poration or drying, along with the
-and
-lactose present initially in the
concentrate. The glass form of lactose is also formed in ice cream during its
rapid cooling and storage below its glass transition temperature (T
g
-308C).
-Lactose is obtained when crystallization occurs from ethanol (at
various concentrations and temperatures; Olano et al., 1983), or from water
above 93.58C. When dried in clusters,
-lactose looks like an uneven-sided
diamond (Holsinger, 1988); when suspended in saturated solution, it looks
like bundled straws. It is sweeter and much more soluble than
-lactose.
In solution,
-lactose exists in equilibrium with
-lactose. At room
temperature,
- and
-lactose co-exist at a ratio of 1.6:1 (or 61%
, 39%
).
Despite the greater proportion of
-lactose, the usual crystalline lactose in
intermediate-moisture foods is
-lactose. This is because its solubility is much
lower than that of
-lactose (below 93.58C). On supersaturation, it crystal-
lizes from solution as
-lactose monohydrate, leaving
-lactose in solution.
On mutarotation, the concentration of
-lactose in solution is replenished, as
-lactose is converted to
-lactose. Mutarotation and crystallization continue
as long as the solution is supersaturated and stop when the saturation point is
reached.
-Lactose monohydrate (C
12
H
22
O
11
.H
2
O) crystals are typically toma-
hawk in shape when grown in supersaturated lactose solution. In SCM or in
supersaturated lactose solution containing 62% sucrose, however, the crystals
are no longer tomahawk in shape, but resemble short, truncated pyramids
with flat rhomboid bases and apices (Hunziker, 1946). The volume of lactose
crystals in SCM is equal to that of a sphere with its diameter (D) calculated as
L/1.33, where L is the longest axis of the crystal (Hunziker, 1946).
The crystals are sparingly soluble in water and are hard and gritty in the
mouth. According to Zhizhin et al. (1971), SCM becomes powdery in mouth-
feel when crystals of lactose exceed 16 mm, but Kruk et al. (1974) reported that
