Dairy Chemistry And Biochemistry

62 DAIRY CHEMISTRY AND BIOCHEMISTRY

lack of Gal-1-P : uridyl transferase leads to the accumulation of Gal and

Gal-1-P. The latter interferes with the synthesis of glycoproteins and

glycolipids (important for membranes, e.g. in the brain) and results in

irreversible mental retardation within 2-3 months if the consumption of

galactose-containing foods is continued. The incidence of this disease, often

called 'classical galactosaemia', is about 1 in 60 000.

The ability to metabolize galactose decreases on ageing (after 70 years),

leading to cataract; perhaps this, together with the fact that mammals

normally encounter lactose only while suckling, explains why many people

lose the ability to utilize lactose at the end of childhood.

2.7 Determination of lactose concentration

Lactose may be quantified by methods based on one of five principles:

1. polarimetry;


2. oxidation-reduction titration;


3. colorimetry;


4. chromatography;


5. enzymatically.

2.7.1 Polarimetry

The specific rotation, [a]?, of lactose in solution at equilibrium is + 55.4"
expressed on an anhydrous basis (+52.6" on a monohydrate basis). The
specific rotation is defined as the optical rotation of a solution containing
1 gml-' in a 1 dm polarimeter tube; it is affected by temperature (20°C is
usually used; indicated by superscript) and wavelength (usually the sodium
D line (589.3 nm) is used; indicated by subscript).

where a is the measured optical rotation; 1, the light path in dm; and c, the
concentration as g m1-I. It is usually expressed as:

where c is in g perlOOml.
The milk sample must first be defatted and deproteinated, usually by
treatment with mercuric nitrate (Hg(NO,),). In calculating the concentra-
tion of lactose, a correction should be used for the concentration of fat and
protein in the precipitate.