Handbook of Food Science and Technology 3

Figure 1.10.
 Processing diagram for the production of microfiltered
whole milk (VRF = Volume reduction factor) 
1.3.1.3.
 Microfiltered milk 
Microfiltration (1.4 µm) is used to obtain liquid milk with the original 
flavor intact and a shelf life of around 21 days. Dispersed elements and 
microorganisms are concentrated at temperatures of around 50°C in the 
retentate (often called “bacterial retentate”), while all other constituents are 
transferred to the permeate (microfiltrate; Figure 1.10). In order to increase 
Microfiltrated full fat milk
99,55 L
Full fat raw milk
100 L
Cream
10 L
Skimmed 
milk
90 L
MF 1,4 µm
VRF = 20
Microfiltrate
85,5 L
Retentate
4,5 L
90°C – 20s
MF 1,4 µm
VRF = 10
Microfiltrate
4,05 L
Retentate
0,45 L

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From Milk to Dairy Products 29 
yield and the number of decimal reductions obtained, double filtration is 
generally performed to achieve a volume reduction factor (VRF) of 200. Milk 
is pre-skimmed and the cream is reincorporated to the microfiltrate after a 
specific heat treatment.
Combined with a moderate heat treatment, the shelf life of microfiltered 
milk can range from 35 days (treatment of 20 s at 72°C) to six months 
(treatment of 6 s at 96°C). 
1.3.2.
 Fermented milk products 
The bacterial conversion of lactose forms the basis of a wide variety of 
fermented products (yoghurt, kefir, kumis, etc.) and is one of the oldest 
methods used for stabilizing milk. Fermentation causes the formation of an 
acidic (or alcoholic) gel consisting of a network of proteins and fat globules 
trapped in the aqueous phase. Yoghurt is the most popular fermented milk 
product and is obtained exclusively by the growth of lactic acid bacteria 
Streptococcus salivarius 
subsp
 thermophilus 
and 
Lactobacillus delbrueckii 
subsp.
 bulgaricus
,
 
which should be inoculated simultaneously. Any products 
containing bacteria other than these cannot be called yoghurts but are 
fermented milk. In yoghurt, lactic acid bacteria should be viable, active and 
present in abundant quantities (
∼
10
7
bacteria g
-1
); the lactic acid content must 
not be less than 0.7 % (w/w) in products sold to consumers [MAH 00]. Many 
molecules generated during fermentation, other than lactic acid, contribute to 
the sensory (diacetyl, acetaldehyde, etc.) and health (bioactive peptides,
β
-galactosidase, etc.) qualities of fermented products.
There are two types of yoghurts: set and stirred yoghurt. In the case of set 
yoghurt, fermentation occurs directly in the container; these are usually natural 
or flavored yoghurts. In the case of stirred yoghurt, fermentation occurs in 
tanks prior to stirring, smoothing (up to total liquefaction of the gel in the case 
of drinking yoghurts) and packaging; these are generally smooth natural or 
fruit yoghurts.
1.3.2.1.
 Standardization of milk 
The standardization of milk in the production of yoghurt helps to achieve 
the qualitative requirements of the finished product. It mainly concerns total 
solids as well as the protein and fat content. Total solids are generally higher 
for set yoghurts than for stirred yoghurts. Enriching milk with proteins

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30 Handbook of Food Science and Technology 3 
(to around 5 g per kg
-1
) contributes to the firmness of the gel and prevents the 
risk of phase separation. This is achieved either by the addition of powder 
(skimmed milk powder, whey protein concentrate powder), evaporation or 
membrane technology (ultrafiltration, reverse osmosis).
In addition, carbohydrates such as sucrose or glucose are often added to 
sweetened or fruit yoghurts. Polysaccharides (pectin, xanthan, etc.) can also be 
used as stabilizers in fruit yoghurts.
1.3.2.2.
 Homogenization 
The homogenization of milk used for fermentation has a number of 
objectives: it improves the firmness of the gels obtained after fermentation, 
increases their water retention capacity and reduces syneresis. It also prevents 
creaming during yoghurt production, in particular during the static incubation 
period in containers or fermentation tanks. Homogenization is usually carried 
out in the rising phase of pasteurization at a pressure of around 20 MPa and a 
temperature between 60 and 90°C. During homogenization, the lipid interface 
is covered with proteins (casein micelles, whey proteins). The protein coating 
of homogenized fat globules is involved in the formation of the protein 
network during acidification [LUC 98].
1.3.2.3.
 Heat treatment
By modifying the physicochemical properties of proteins, the heat 
treatment of milk (around 90°C/10 minutes) has a significant impact on the 
rheological properties of lactic gels. Through heat denaturation, whey proteins 
(more than 90%) form soluble covalent aggregates or aggregates bound to 
κ
-
casein on the surface of casein micelles. By changing the micelle surface, heat 
treatment causes an increase in the pH of acid gelation of milk, an increase in 
gel firmness and a reduction in its syneresis (Figure 1.11). Furthermore, heat 
treatment creates a favorable environment for the growth of lactic acid bacteria 
by destroying undesirable microorganisms and potential competitors to lactic 
acid bacteria, lowering the redox potential, contributing to the production of 
formic acid, and so forth.
1.3.2.4.
 Fermentation
After heat treatment, milk is cooled to between 40 and 45°C and inoculated 
with starter culture, resulting in acidification in either a tank or individual 
containers. In the case of yoghurt, starter cultures include 
Streptococcus 
thermophilus
and 
Lactobacillus bulgaricus
. They grow synergistically

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From Milk to Dairy Products 31 
(Figure 1.12) and can be differentiated by their optimum growth temperature, 
but also by their acidifying capacity and flavor production. Thus, the 
proportion of strains added during inoculation and the incubation temperature 
determine the sensory properties of the products. In addition, some strains 
release exopolysaccharides into the medium, which affect the rheological 
properties of the gel.