Handbook of Food Science and Technology 3

Figure 1.13.
 Processing diagram for the production of low-heat skimmed milk powder
The concentrate obtained by vacuum evaporation can be dried using 
various drying techniques, which are differentiated according to energy cost 
and powder quality; the most common method is spray drying, which involves 
spraying the product (liquid or suspension) into a hot gas stream so as to 
obtain a powder almost instantaneously. It is thus a form of entrainment drying 
where air acts both as a heater and a carrier for the water removed from the 
concentrate: the air is dry and hot when entering the drying tower and moist 
and cooler when it leaves. In Europe, industrial drying systems usually have 
an evaporation capacity of 0.5 – 4.5 tons h
-1
, requiring air flows of 1×10
4
to 
12×10
4
m
3
h
-1
, and production cycles varying from 4 – 24 h on average. 
Operating a skimmed milk drying facility involves the control of many 
thermodynamic, physical and technological parameters, such as incoming and 
outgoing air temperatures (180 – 280°C and 80 – 90°C respectively), velocity 
and relative humidity of the air (5 – 10% for outgoing air), temperature and 
viscosity of the concentrate, spray nozzle type (high pressure, two-fluid or 
turbine), type of tower (1–3 stages) and so on. Under these conditions, it is 
possible to obtain a skimmed milk powder with a
w 
of 0.2 (25°C) and 4% 
residual moisture. Figures 1.13 and 1.14 are diagrams showing the production 

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34 Handbook of Food Science and Technology 3 
of low-heat skimmed milk powder and the production of reduced whey protein 
content skimmed milk powder prepared by combining microfiltration and 
ultrafiltration.
Figure 1.14.
 Processing diagram for the production of skimmed milk
powder by microfiltration and ultrafiltration (Primin
®
 powder) 
1.3.3.2.
 Physical properties of milk powders 
The quality of milk powder depends on many factors, including milk 
quality before drying and the implementation of the drying process itself.
[PIS 81] and [MAS 91] classified the main properties of milk powders into 
two categories (Figure 1.15): 
– properties inherent to the product (biochemical, microbiological, etc.); 
– properties inherent to the process (functional properties and any defects). 

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From Milk to Dairy Products 35 
Figure 1.15. 
Main properties of milk powders [PIS 81, MAS 91]
Particle size