Dairy Ingredients for Food Processing

Processing Principles of Dairy Ingredients 121

can be applied in several ways. The feed can

be sprayed on drums, placed on the bottom

by an applicator roll or placed on the top of

the drum by several applicators (Figure 4.18 ).

Spray Drying

Spray drying is the transformation of liquid

dairy products into powders by spraying

liquid into a controlled fl ow of hot air within

a drying chamber. It is a single - step continu-

ous process. Spray - dried power consists of

single particles or agglomerates and the fi nal

properties of the product depend upon the

physical and chemical properties of the feed

material as well as the dryer design and oper-

ation. The feed is pumped from a balance

tank to an atomizing device in the drying

chamber. The drying air is drawn from the

atmosphere and fi ltered and heated. The

proposition and if milk or skim milk were
to be dried directly, the resulting powder
would be very fi ne and the cost of drying
prohibitive.
Concentration (see above) precedes the
drying step, in which a four - fold concentra-
tion of solids is achieved by economical
means. The concentrate is the feed material
that introduced into the dryer. Moisture
removal involves either roller or spray drying.

Drum or Roller Drying

In roller drying, milk or milk products can be
dried in a thin fi lm on an internally steam -
heated drum and the dried product removed
from the exterior of the drum by a doctor
blade. Roller drying requires less space, can
be installed in existing facilities, is made of
equipment that is relatively simply con-
structed, is easy to commission, is easy to
clean, and can handle sticky products (e.g.,
mixtures of cereals and milk). The steam
consumption is 1.1 to 1.2 kg/kg of water
evaporated. The major disadvantages of the
roller dryers are: the solubility of the powders
is less than that obtained from spray drying,
they have greater amounts of scorched par-
ticles, and the solids concentrations of the
feed are lower. Roller drying is not suitable
if high - drying capacities or high - powder
solubility are desired. However, there are
some niche uses of this technology. Roller
dryers often manufacture milk powder for
use in meat applications in which high water
binding capacity is desirable. High water
binding is the result of the denaturation of
milk proteins in roller drying. Roller - dried
powders also are desired by chocolate manu-
facturers because of the high free fat content
of these powders.
Roller dryers are classifi ed according to
the number of drums, method of placing the
product on the surface of the drum, and the
pressure surrounding the drum. In either type
of drum dryer (single or double; atmospheric
or vacuum) the feed (material being dried)

Figure 4.18. Schematic of a trough - fed roller dryer.

Reproduced with permission from Tetra Pak.

Milk

Heating medium

Air for pneumatic

transportation and cooling