Handbook of Food Science and Technology 3

Figure 1.9.
 Time–temperature diagram of pasteurization 
1
10
100
1000
10000
100000
50
55
60
65
70
75
80
85
90
Ti
m
e (
s)
Temperature (°C)
Destruction zone
of pathogens
Destruction of tubercle 
bacillus
Destruction of alkaline 
phosphatase 
5% destruction of whey 
proteins
Destruction of 
peroxydase 

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From Milk to Dairy Products 27 
Two types of heat treatment are generally used for milk: 
– 
High temperature short time (HTST) pasteurization
(71–72°C/15–40 s): 
This is used for high-quality raw milk. From a sensory and nutritional 
perspective, high pasteurization has little impact: alkaline phosphatase is 
inhibited but peroxidase remains active. The use-by date of HTST pasteurized 
milk is 7 days after packaging (glass bottle, carton, polyethylene or aluminium 
container). 
– 
Flash pasteurization
(85 – 90°C/1 – 2 s): This is used for poor quality 
raw milk. Phosphatase and peroxidase are inhibited.
Long-life milk
Long-life milk has undergone sterilization, the purpose of which is to 
destroy all microorganisms; in return, the sensory and nutritional quality is 
altered compared to pasteurized milk. Sterilization levels are defined based on 
a 12 decimal reduction of 
Clostridium botulinum
. In long-life milk, shelf-life 
is limited by slow time-dependent physicochemical changes in the product 
(precipitation, gelation, etc.). 
Sterilized milk 
Milk is pre-sterilized (135–150°C/3–10 s) after homogenization (in the 
case of milk containing fat). It is then cooled to 70–80°C and bottled (high-
density polyethylene) before undergoing a second sterilization (115°C/15–20 
min) followed by rapid cooling. This has a negative impact on color and flavor 
due to the Maillard reaction. The shelf life is around 150 days. In order to 
prevent lipid oxidation, such milk is stored away from light, generally in 
opaque containers. From a nutritional perspective, such heat treatments lead to 
a loss of thiamine and vitamins B
12
and B
6
. 
Ultra high temperature (UHT) milk 
Milk is heated to 135–150°C for 1–6 s. This process helps to preserve the 
original nutritional and sensory qualities of the milk because the z value of the 
Maillard reaction is greater than that of microbial inactivation. Its shelf life is 
around 120 days. This limit is imposed to ensure physicochemical stability 
against precipitation, flocculation and gelation due to the partial proteolysis of 
casein by residual plasmin or heat-resistant bacterial proteases.

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28 Handbook of Food Science and Technology 3 
UHT treatment is either direct or indirect, depending on the materials used: 
– in the case of direct UHT treatment, food-grade steam is injected into 
milk preheated to 80°C, where it condenses releasing the latent heat of 
evaporation. The resulting dilution is corrected during cooling by expansion of 
the mixture in a partial vacuum chamber; 
– in the case of indirect treatment, there is no contact between the milk and 
the steam. The treatment is carried out with plate or tubular heat exchangers. 
The limiting factor of the process is the gradual fouling caused by the 
precipitation of protein/mineral complexes on the walls of the exchanger: 
- homogenization is carried out in either the rising or the falling phase; in 
the latter case, it is necessary to ensure sterilization of the homogenizer, 
- the intensity of heat treatments applied is related to the quantity of 
lactulose in UHT milk.