Chemistry Reference
In-Depth Information
lactose content - the higher the lactose content, the more wrinkled the
surface (Mistry et al., 1992). Rosenberg and Young (1993) found that the
structure of whey protein isolate (WPI)-based spray-dried microcapsules
differed from that of other milk-derived powders. Deformation was evident
in WPC-based particles, but not when WPI was used, which suggests that
lactose (present only in the former ingredient) was also a contributory
factor. The physical effects appear to be due to the solidification of particle
wall solids before completion of expansion during droplet/particle dehydra-
tion. Higher drying temperatures eliminated the tendency towards the for-
mation of shallow deformations. Rosenberg and Young (1993) believe that
there is a critical viscosity determining the tendency for dents to occur,
below which surface tension-driven effects are sufficient to smooth out
morphological irregularities. Hence, high concentrations of whey proteins
and fat in product formulations play roles in limiting surface folds.
Lactose glass is recognized as the main encapsulant of milk fat in whole
milk powder (WMP) and spray-dried dairy-like emulsions made with WPC
and WPI (Buma 1971; Young et al., 1993a,b). Lactose is proving to be useful
in microencapsulation studies aimed at optimization of ingredient formula-
tion and process technology (homogenization; spray drying) to yield spray-
dried powder particles with defined characteristics in terms of active
ingredient (e.g. sensitive oil/fat) protection (Kelly, 2007). The selection of
''wall'' materials and the manner in which they solidify during powder particle
dehydration is key to determining the amount of free fat formed, especially in
the case of high-fat powders. The porosity of such ''wall'' materials may be
critical when protecting sensitive materials such as fish oils (Keogh et al.,
2001). While protein is the principal emulsifying agent at the concentrate
preparation stage, the presence of carbohydrate is additionally important
during the subsequent spray drying step of whey protein-based emulsions.
Lactose in its amorphous state acts as a hydrophilic filler or sealant that
significantly limits diffusion of solvent through the wall towards the micro-
encapsulated anhydrous milk fat droplets (Young et al., 1993a). Crystalline
lactose, on the other hand, reduces microencapsulation efficiency by facil-
itating better diffusion of solvent.
IV.3.
Microstructure of Milk Powders
The surface of skim and whole milk powder particles is typically character-
ized by an irregular shape, shrunken appearance and shrivelled-looking sur-
face. Puffed-looking powder particles may also occur if the powder has been
exposed to unfavourable temperatures during drying. Such occurrences cause
entrapped air to expand when heated and lead to rupture of the surface. When
