Chemistry Reference
In-Depth Information
the lactose content of milk was depleted, e.g., use of ultrafiltration to produce
MPC, the surface of the resulting powder particles after spray drying was
smoother than skim milk powder particles (Mistry, 2002). Hence, lactose
would seem to be a major determinant of the surface morphology of spray-
dried particles in milk powders, and as a result has consequences for shelf-life
properties at or above ambient temperature. However, a very low air inlet
temperature (120-1258C) to the spray dryer was used during that particular
study.
IV.4.
Phase Transitions During Milk Drying
After years of technological innovation, food material science is now being
applied increasingly to improve our understanding of what happens during
spray drying. The material in this instance is dominated by the behaviour of
the amorphous glass structure of rapidly solidified lactose (
:
ratio of
1.2-1.4) during the conversion of milk concentrate to powder. This amor-
phous lactose coats the protein and fat globules in spray-dried milk powder
particles. Thus, by combining material phase transitions with physico-
chemical changes, a more effective approach to optimizing concentration
and spray drying is envisaged. The phase diagram of milk may now be
augmented by tracking changes in glass transition (T
g
). A graphical repre-
sentation by Roos (2002) of hypothetical powder particle temperature during
spray drying on a plot of T
g
vs. water content provides guidance on the
identification of appropriate conditions for optimal operation, e.g., use of
integrated static beds within the primary drying chamber and promotion of
agglomeration. Vuatez (2002) studied sorption isotherms in conjunction with
glass transition as a function of concentration for both whole milk and skim
milk, and proposed a universal relationship between glass transition tempera-
ture and water activity. The same author established differences in phase
transitions during lactose crystallization in whole milk and skim milk. With
whole milk, phase transitions were evident during nucleation of (i)
-hydrate
lactose crystals in a supersaturated milk concentrate and (ii)
-anhydrous
crystals in a rubbery glassy powder, while in the case of skim milk nucleation
of a mixture of anhydrous
- and
-lactose occurred.
The physical state of skim powders is effectively determined by lactose;
Jouppila and Roos (1991) showed that the T
g
of such powders was almost
equal to that of pure lactose (see Chapter 2). Stickiness is also recognized as a
surface phenomenon that is governed by the T
g
behaviour of lactose. Hence,
current research aims to produce ''sticky curves'' in conjunction with T-T
g
plots in order to identify sticky and non-sticky conditions.
