148 Chapter 6
(140 ° F to 149 ° F), the powder temperature
must be a minimum of 50 ° C (122 ° F), and the
powder must be fl uidized for at least 5
minutes at 45 ° C (113 ° F) (Pisecky, 1997 ).
Recently an alternative in - situ process for
lecithination of skim milk powders has been
reported. In this process lecithin is added to
the feed introduced into the spray dryer. This
approach was based on the fi nding that the
surface of spray - dried milk powder is domi-
nated by surface - active species and that the
most rapidly adsorbing surface - active agent
dominates the composition of the surface
of a powder particle (Millqvist - Fureby and
Smith, 2007 ).Properties of Milk Powders
Powders in the market must meet general
standard specifi cations for trade (Table 6.1 ).
This is the minimum requirement. The com-
position and microbiological quality of the
milk powder, though essential attributes, do
not always refl ect their performance in their
intended application. A number of tests have
been developed for further characterization
of milk powders. These may be used as
quality control measures or as a guide for
assessing the physical functionality of the
powders.Physical Characteristics of Powders
The physical characteristics of milk powders
must be understood if they are to be fi t for
the intended application.Bulk Density
Bulk density is a measure of the weight of
powder that can be contained in a set or
known volume. It is also referred to as
packing density and can be expressed in g/
cm^3 , kg/m^3 , or g/100 ml (Pisecky, 1997 ). The
bulk density is a consideration for packaging,
particularly when transport is being costed on
a volume basis. Bulk density also has anmanufacture of milk protein concentrate
powders (75% protein in powder), impaired
hydration properties were obtained when the
outlet temperature of the spray dyer was
increased from 65 ° C (149 ° F) to 95 ° C (203 ° F)
(at a constant inlet temperature of 250 ° C;
482 ° F). Detrimental effects on hydration
properties of MPC powders were also
obtained with an increase in the inlet tem-
perature (Castro de and Harper, 2001 ; Castro -
Morel de and Harper, 2003 ).
A number of comprehensive descriptions
of the engineering aspects of spray drying are
available (Masters, 1991 and 2002 ; Mujumdar,
1995 ; Pisecky, 1997 ). Further developments
in process control techniques for optimiza-
tion of milk powder production require an
improved understanding of the thermody-
namic properties of the milk and concentrate
and the interactive effects of time, tempera-
ture, and shear during the course of milk
powder manufacture (O ’ Callaghan and
Cunningham, 2005 ).
Instantization
Instantization is carried out to improve wet-
tability, dispersibility, and the free - fl ow prop-
erties of milk powders. Instantizing may be
achieved by returning the fi ne powder parti-
cles into the dryer close under the rotary
atomizer or spray nozzles so that the particles
aggregate to form agglomerated powder.
Lecithin is often used to improve the prop-
erties of instantized milk powders (Sanderson,
1978 ). A traditional method of application
involves dissolving the lecithin in butter oil
and spraying over the agglomerated milk
powder, either internally or in a fl uidized bed
external to the dryer. The process requires
strict adherence to controlled temperatures
during powder manufacture to allow both the
hydrophilic and lipophilic components of the
lecithin to interact with the free fat in a
molten state. Hence, when using a wetting
agent such as 30% to 50% lecithin in an oil
solvent, the mixture should be 60 ° C to 65 ° C