Chemistry Reference
In-Depth Information
suggestions that powders may be labelled as ''non-caking'' at
>
75% crystal-
linity; however, there is no guarantee that lactose and milk powders that are
free from amorphous lactose will not cake.
IV.2.2.
Lactose Crystallization in Milk Powders
The behaviour of lactose in milk powders has important consequences
for other physical and functional properties. First, powder composition and
storage conditions influence crystallization changes. Secondly, it is now
widely accepted that the surface composition and morphology of powder
particles, more than their internal microstructure, are major factors which
dictate inter-particle interaction and behaviour. Thirdly, manipulation of
surface properties during microencapsulation and spray drying is desirable
when attempting to protect sensitive ingredients during subsequent storage
and delivery.
Because of their hydration behaviour, the presence of milk proteins in
powders tends to delay lactose crystallization due to competition for available
water. Crystallization in pure lactose and milk powders starts at 40% and 50%
RH, respectively (Thomas et al., 2004). In the case of whole milk powder,
lactose crystallization does not occur until
66.2% RH, due to the role of
milk fat which is believed to act as a hydrophobic barrier and limits the
diffusion of hydrophilic molecules and the growth of lactose crystals.
The migration of internal fat to the surface of particles during the
storage of milk powders is believed to be facilitated when lactose crystal-
lization creates an internal network of capillary interstices. Morphological
changes, such as surface deformation, also occur due to the build-up of
lactose crystals (Thomas et al., 2004). The fat content of powder has a
positive influence on the surface fat coverage of powder particles, with the
most dramatic effect being evident for powders in the 0-5% fat range (skim
milk powder category), giving rise to a surface fat of 0-35 % (Nijdam and
Langrish, 2006). Nijdam and Langrish (2006) also considered that protein
along with fat preferentially migrate to the surface of particles during drying
on a laboratory scale dryer at an air inlet temperature of 1208C. The
migration of lactose increased when the experimental drying was conducted
at an industrially realistic air inlet temperature of 2008C. However, the
release of water during crystallization is likely to increase viscous flow on
particle surfaces, induce lactose migration and further crystallization to the
point that particle bridging and agglomeration occurs. Deformation of the
surface of particles is related to uneven shrinkage of atomized droplets
during the early stages of spray drying. The surface properties of high-
protein powders prepared from skim milk are known to be dependent on
