Dairy Chemistry And Biochemistry

114 DAIRY CHEMISTRY AND BIOCHEMISTRY

Milk from high-

pressure pump

n

Spring-loaded valve

Figure 3.23 Diagram of a milk homogenizer.

Reducing the average diameter of the fat globules to 1 pm results in a
four- to sixfold increase in the fat/plasma interface. There is insufficient
natural membrane to completely coat the newly formed surface or insuffi-
cient time for complete coverage to occur and consequently the globules in
homogenized milk are coated by a membrane which consists mostly of
casein (93% of dry mass, with some whey proteins, which are adsorbed less
efficiently than the caseins) (Figure 3.25). The membrane of homogenized
milk contains 2.3 g protein per lOOg fat (10mg proteinm-'), which is very
considerably higher than the level of protein in the natural membrane
(0.5-0.8g per 1OOg fat), and is estimated to be about 15nm thick. The
casein content in the serum phase of homogenized milk is reduced by about
6-8%.
Homogenization causes several major changes in the properties of milk:

1. Homogenized milk does not cream naturally and the fat is recovered only
   poorly by mechanical separation. This is due in part to the smaller
   average size of the fat globules but failure of the globules in homogenized
   milk to form aggregates, due mostly to the agitation-induced denatura-
   tion of some immunoglobulins, is mainly responsible for the failure to
   cream.