Chemistry Reference
In-Depth Information
packed, because temperature gradients cause moisture movement
within the bags during storage (Bronlund and Paterson, 2008; Pater-
son and Bronlund, 2009). This is the major cause of caking of bulk
lactose. It has been demonstrated that for caking to occur, the relative
humidity in the air spaces within the product must rise above 80% so
that significant amounts of capillary condensation can occur.
A temperature gradient within the product will cause moisture to move
from the hot region to the cold region (Paterson and Bronlund, 2009). It is
this moisture movement, caused by day-night temperature fluctuations as the
product is transported about the world, that can cause free-flowing product
to arrive at its destination as solid 900 kg blocks (Bronlund and Paterson,
2008; Paterson and Bronlund, 2009). In order to prevent caking during
transport, it is vital that the moisture level be reduced to below a critical
moisture content. This water content can be determined most easily by
measuring the water activity and relating this to the moisture content via
the isotherm for the lactose crystals which is shown in Figure 4.3 (Bronlund
and Paterson, 2004). It is obvious from this figure that the water activity is a
much more sensitive index than the moisture content and it is also quicker
and easier to measure. Hence, it is recommended that the water activity
should be the preferred quality control method of determining whether a
dried product is suitable for shipping or long-term storage.
1.0%
12°C
20°C
30°C
37°C
tss model
0.9%
0.8%
0.7%
0.6%
0.5%
0.4%
0.3%
0.2%
0.1%
0.0%
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Water activity
Figure 4.3.
Effect of temperature on the adsorption isotherm of
-lactose monohydrate, with
the third stage sorption (tss) model fitted to the data. (Reproduced from Bronlund and Paterson,
2004, with permission.).
