Chemistry Reference
In-Depth Information
I.8.3.
Effect of Lactose Crystallization on the Water Activity
of Sweetened Condensed Milk
The typical composition of SCM is sucrose (44-46%), lactose (10-12%),
milk salts (0.25-0.35%), fat (8%), proteins (8-9%) and moisture (26-28%).
The corresponding a
w
is less than 0.840 upon equilibration at 258C after
seeding, or less than 0.800 before crystallization of lactose. The increase in
a
w
from 0.800 to 0.840 is due mainly to the crystallization of lactose (rather
than possible changes to the milk salts equilibrium) because the a
w
remains
low (e.g., at 0.800) even after 24 h, if SCM is left undisturbed and not seeded.
Similar behavior is observed in whole milk powder (Thomsen et al., 2005); its
a
w
increased from 0.23 to 0.46 (after 147 days of storage), upon gradual
transition of lactose glass to
-monohydrate.
The sucrose in SCM is undoubtedly the primary solute responsible for
lowering its a
w
. The desired S/(S+W) ratio (S
¼
% sugar, W
¼
% water) is
0.625:0.645. At a ratio of 0.625, the corresponding a
w
is less than 0.85. Below
a ratio of 0.625, a
w
will be higher than 0.850 (microbiologically unstable) and
above 0.645, sucrose will crystallize out of solution, especially when the
product is stored below room temperature.
According to the British Columbia Centre of Disease Control (1997),
the a
w
of a sucrose solution can be estimated using an empirical equation:
a
w
¼
1.00/(1+ 0.27n), where n is the number of moles of sucrose in 100 g water
(1 mole sucrose is 342 g). Based on this, the contribution of sucrose alone, to
the a
w
of SCM, is 0.884 (45% sucrose in 27% water or 167 g sucrose per 100 g
water). It follows that the further reduction to 0.800 (prior to lactose crystal-
lization) or to 0.840 (after crystallization) is contributed by lactose and other
solutes (milk salts and proteins).
I.8.4.
Effect of Lactose Crystallization on the Measurement
of Moisture/Total Solids in Sweetened Condensed Milk
As explained above, lactose crystallizes as
-monohydrate on concen-
tration or drying. As a result, the total solids (TS) in SCM are increased by the
water of hydration in the monohydrate crystals. Depending on the conditions
of drying, the measured value can become very high, when compared to
values obtained by the Karl Fisher (wet) titration method (non-drying).
Conceivably, rapid drying encourages lactose to dry in a glassy form,
so there would be little or no lactose monohydrate to increase the TS.
However, due to the high concentration of sucrose in SCM, and depending
onthesamplesizerelativetothedrying surface area, sucrose glass, when
formed on the surface of the drying sample, can significantly inhibit com-
plete drying.
