Handbook of Food Science and Technology 3

Figure 1.11.
 Diagram of the influence of heat treatment on the protein
constituents of milk and rheological properties (G’) of the gels obtained 
Figure 1.12.
 Synergistic action of starter cultures in yoghurt 
0
100
200
300
400
500
600
700
4
4,5
5
5,5
6
pH
G' (Pa)
Without heat treatment
With heat treatment
Heat treatment
90°C/10min
Casein 
micelle
Micellar 
aggregate
Soluble 
aggregate
Whey 
proteins
Acidification

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32 Handbook of Food Science and Technology 3 
During acidification, casein micelles covered with whey protein aggregates 
are destabilized and begin to associate when the pH of the medium drops 
below 5.5. This results in molecular rearrangement, leading to the formation of 
a gelled protein network that includes homogenized fat globules. The firmness 
of the network increases with the degree of acidification [TAM 99]. When the 
pH reaches 4.6, yoghurt is cooled to around 5°C in order to control the 
metabolic activity of the starter cultures. While set yoghurts (container 
fermentation) are cooled to 5°C in a single stage, stirred yoghurts (tank 
fermentation) are cooled in two stages. In the first stage, carried out in a plate 
heat exchanger, the yoghurt is cooled to 15–20°C. After stirring and 
smoothing, the yoghurt is then poured into containers and cooled to 5°C.
1.3.3.
 Milk powder 
1.3.3.1.
 Drying of milk
After bactofugation to eliminate dispersed elements (butyric acid bacteria 
spores, casein fines), whole, skimmed or standardized milk is heat-treated 
before drying; it can also undergo different concentration operations 
(microfiltration, ultrafiltration, nanofiltration) that modify the ratio between 
milk components. Some components (polysaccharide, minerals, vitamins, etc.) 
can be dispersed in milk. After standardization, the liquid is homogenized, 
concentrated by vacuum evaporation and finally spray-dried or drum-dried 
(see Volume 2).
The concentration of milk and its derivatives by vacuum evaporation (the 
process of removing water by boiling) is based on lowering the boiling point 
of the liquid (and therefore the processing temperature) by reducing pressure. 
Vacuum is used for two main reasons: on the one hand, the temperature 
difference between the dairy product to be concentrated and the heating 
surface of the falling film evaporator is greater for a given heating steam 
pressure, which can reduce steam consumption by increasing the evaporation 
capacity and/or using more effects; on the other hand, it can evaporate heat-
sensitive solutions. The most common apparatus in the dairy industry is the 
multiple effect evaporator, which incorporates a falling film evaporator 
equipped with thermal vapor recompression and mechanical vapor 
recompression systems. The energy cost of removing 1 ton of water is 
between 360 and 1080 kWh. The maximum boiling at the beginning of the 
cycle (first effect) is normally less than 70°C, corresponding to an absolute 
pressure of 30,664 Pa. The evaporation capacity of industrial evaporators 

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From Milk to Dairy Products 33 
varies from 10 to 30 tons h
-1
. A concentration cycle lasts between 10 and 20 h. 
Theoretically, the average residence time of the product in an industrial 
evaporator is between 10 and 20 minutes: fouling of the evaporation tubes due 
to the precipitation of calcium phosphate causes a gradual increase in 
temperature throughout the entire evaporation unit of 10 – 15°C.