Handbook of Food Science and Technology 3

Figure 1.2
. Diagram of the structure of a native
milk fat globule membrane (Source: [MIC 01]). For a color
version of this figure, see www.iste.co.uk/jeantet/foodscience.zip
Due to the composition of the MFGM, the interfacial tension between the 
fat phase and skimmed milk is low at around 2 mN m
-1
, which makes it very 
sensitive to local perturbations. The total surface of the MFGM is
around 80 m
2
L
-1
of fresh milk. This can, however, be considerably
increased during processing (agitation, homogenization, etc.). The surface 
electrostatic potential of the fat globule, which is close to –13 mV at the 
natural pH of milk, contributes to stability by limiting the risk of flocculation 
and coalescence.
1.1.2.
 Carbohydrates 
Milk contains free carbohydrates, the main one being lactose, and 
carbohydrates bound to proteins. The lactose concentration in mammalian 
milk is inversely proportional to the mineral content, both of which contribute 
to the balance of osmotic pressure. The lactose content of cow’s

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From Milk to Dairy Products 9 
milk varies from 4.8 to 5% (w/w) and represents 97% of total carbohydrates. 
Lactose is a disaccharide composed of a galactose and a glucose unit
(Figure 1.3). It is made from blood glucose in the presence of 
galactosyltransferase and 
α
-lactalbumin. For absorption, lactose should be 
hydrolyzed by 
β
-galactosidase (lactase) secreted by enterocytes in the small 
intestine. The low hydrolysis rate of lactose provides young mammals with 
prolonged energy and a constant blood glucose level between feedings. 
Lactase-deficient individuals cannot digest lactose as it provokes intestinal 
problems (diarrhoea, bloating) when ingested. Galactose and its amino 
derivative galactosamine contribute to the formation of several glycoproteins 
and/or glycolipids.
Figure 1.3
. Chemical structure of lactose 
Lactose has a low solubility (around 18 g/100 g of water at 20°C) 
compared to other carbohydrates: it can crystallize when concentrated in the 
aqueous phase of milk or derivatives (evaporation, freezing, storage in powder 
form). Lactose has a high melting point for a disaccharide (over 200°C). It has 
a low sweetness level (0.3 with reference to sucrose, which has a sweetness 
level of 1). Lactose has one reducing function per molecule, carried by the 
glucose unit. It is thus prone to non-enzymatic browning, which changes the 
flavor and color of foods (Maillard reaction). Enzymatic hydrolysis by
β
-galactosidase combats lactose intolerance, improves the sweetness of
milk and doubles its reducing power, which promotes non-enzymatic 
browning. Lactose is the main substrate for lactic acid bacteria. The 
transformation of lactose to lactic acid lowers the pH of milk and destabilizes 
the dispersed elements, which is the basis of the production of fermented dairy 
products.
O
OH
H
H
H
OH
H
OH
H
OH
O
H
H
O
H
H
OH
H
OH
OH
H
O

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10 Handbook of Food Science and Technology 3