Dry Milk Ingredients 147Figure 6.2. Spray dryer.
The concentrate is atomized, using a rotary
atomizer or a nozzle, to obtain small droplets
of concentrate. Water is rapidly evaporated
from the droplet surface when it is initially
mixed with the hot air in the drying chamber.
The resultant dried powder particles are sepa-
rated from the drying air by cyclones, col-
lected, and packaged. Spray - dried milk
powders have a globular shape, with a con-
voluted surface and a porous structure. The
method of atomization used affects the
microstructure of the powder, with nozzle
atomization resulting in lower occluded air
and higher powder bulk density (Caric and
Kalab, 1987 ; Tong, 2001 ).
Although the inlet air temperature of the
dryer is high (above 170 ° C; 306 ° F), there is
minimal heat damage to proteins during
spray drying because of evaporative cooling
as water is removed rapidly and the particle
is dried. The temperature of the particle is
low (below 70 ° C; 158 ° F) until the milk
powder droplet is dried. The outlet air tem-
perature of the dryer has the greatest effect
on heat damage because this is the tempera-
ture the dried powder particle approaches
(Singh and Newstead, 1992 ). Lactosylation
of milk proteins, which is the conjugation of
lactose to protein and is related to heat
damage, can occur during skim milk powder
manufacture. It is promoted with the use of
high outlet air temperatures during drying
(Guyomarc ’ h et al., 2000 ).
The operating conditions of spray dryers
can affect the solubility characteristics of
milk powders. The detrimental effects of
high - temperature drying conditions are more
pronounced during manufacture of high -
protein milk powders compared to standard
skim and full - cream milk powders. In the