Food Chemistry

10.1 Milk 513

Table 10.15.Some physical characteristics of lactose

α-Lactose β-Lactose Equilibrium

mixture

Melting point
(◦C) 223 a 252. 2 a
Spec. rotation
[α]^20 D 89. 435. 0

Equilibrium in aqueous solutionb
0 ◦C1. 00 1. 80
20 ◦C1. 00 1. 68
50 ◦C1. 00 1. 63

Solubility in waterc
0 ◦C5. 045. 111. 9
25 ◦C8. 621. 6
39 ◦C12. 631. 5
100 ◦C7094. 7 157. 6
aAnhydrous.bRelative concentration.
cg Lactose/100 g water.

The great solubility difference between the two
anomers is noteworthy. The sweetness of lactose
is significantly lower than that of fructose, glu-
cose or sucrose (Table 10.16). For people who
suffer under lactose intolerance, dietetic milk
products are produced by treatment withβ-1,4-
galactosidase (cf. 2.7.2.2.7). Glucose and some
other amino sugars and oligosaccharides are
present in small amounts in milk.
Lactulose is found in heated milk products. It
is a little sweeter and clearly more soluble than
lactose. For example, condensed milk contains
up to 1% of lactulose, corresponding to an iso-
merization of ca. 10% of the lactose present. The
formation proceeds via theLobry de Bruyn–van
Ekensteinrearrangement (cf. 4.2.4.3.2) or via
Schiffbase. Traces of epilactose (4-O-β-D-glacto-

Table 10.16.Relative sweetness of saccharose, glucose,
fructose and lactosea

Saccharose Glucose Fructose Lactose

0. 
   
   
   
   50. 
       
       
       
       90. 
           
           
           
           41. 9
           
           
           
           
       
       
       
       
   
   
   
   


1. 
   
   
   
   8. 
      
      
      
      34. 
          
          
          
          215. 7
          
          
          
          
      
      
      
      
   
   
   
   


2. 
   
   
   
   12. 
       
       
       
       78. 
           
           
           
           720. 7
           
           
           
           
       
       
       
       
   
   
   
   


3. 
   
   
   
   21. 
       
       
       
       816. 
            
            
            
            733. 3
                 aResults are expressed as concentration % for isosweet
            
            
            
            
       
       
       
       
   
   
   
   

aqueous sugar solutions.

pyranosyl-D-mannose) are also formed on heat-

ing milk.

10.1.2.3 Lipids

The composition of milk fat is presented in Ta-

ble 10.17. Milk fat contains 95–96% triglycerols.

Its fatty acid composition is given in Table 10.18.

The relatively high content of low molecular

weight fatty acids, primarily of butyric acid, is

characteristic of milk. Although linoleic acid

dominates in the lipids occurring in feed, the

content of this fatty acid is very low in milk

fat (Table 10.17). The reason was found to

be that microorganisms living in the rumen

hydrogenate the linoleic acid to oleic acid and

stearic acid with the formation of conjugated

linoleic acid (CLA, cf. 3.2.1.2) and vaccenic acid

as intermediates, as shown in Fig. 10.13. It is

possible to increase the concentration of linoleic

Table 10.17.Milk lipids

Lipid fraction Percent of the total lipid

Triacylglycerols 95–96

Diacylglycerols 1 .3–1. 6

Monoacylglycerols 0 .02–0. 04

Keto acid glycerides 0 .9–1. 3

Hydroxy acid glycerides 0.6–0. 8

Free fatty acids 0 .1–0. 4

Phospholipids 0 .8–1. 0

Sphingolipids 0. 06

Sterols 0 .2–0. 4

Fig. 10.13.Biohydrogenation of linoleic acid in the ru-

men of ruminants