Dairy Chemistry And Biochemistry

PHYSICAL PROPERTIES OF MILK

0.2 -

0.1 -

0.0 -

-0.1 -

-0.2 -

443

1

-0.31. I ' I ' I ' I. I. I. I

0 1 2 3 4 5 6 7

Time (h)

Figure 11.3 Decrease in the redox potential of milk caused by the growth of Lactococcus lactis

subsp. lactis at 25°C.

methylene blue) can be used as an index of the bacterial quality of milk by

measuring the 'reduction time', at a suitable temperature, of milk containing

the dye.

Riboflavin absorbs light maximally at about 450nm and in doing so can

be excited to a triplet state. This excited form of riboflavin can interact with

triplet 0, to form a superoxide anion 0; (or H,O, at low pH). Excited

riboflavin can also oxidize ascorbate, a number of amino acids and proteins

and orotic acid. Riboflavin-catalysed photo-oxidation results in the produc-

tion of a number of compounds, most notably methional(11.1) which is the

principal compound responsible for the off-flavour in milk exposed to light.

Methional

Photo-oxidation of milk constituents was discused in detail by Walstra
and Jenness (1984).

11.4 Colligative properties of milk

Colligative properties are those physical properties which are governed by
the number, rather than the kind, of particles present in solution. The
important colligative properties of milk are its freezing and boiling points
(c. -0.522 and 100.15"C, respectively) and its osmotic pressure (approxi-