Chemistry Reference
In-Depth Information
Lactose is important in the manufacture of fermented dairy products
where it serves as a carbon source for lactic acid bacteria (LAB)
which produce lactic acid.
1.2.1.
Modification of the Concentration of Lactose in Milk Through
Genetic Engineering
There has been considerable interest in modifying the lactose content of
milk by genetic engineering (see Leaver and Law, 2003). Since the concentra-
tion of lactose is controlled by the concentration of
-La in the secretory cells,
the approach to changing the concentration of lactose involves altering the
level of this protein. There is interest in reducing the level of lactose for at least
three reasons:
Lactose is the least valuable constituent in milk but it costs energy on
the part of the animal to synthesize it; therefore, it would be econom-
ically advantageous to reduce the lactose content of milk.
Since lactose effectively controls the water content of milk and most
dairy processes require the removal of water, it would be advanta-
geous to reduce the amount of water in milk by reducing the level of
lactose. However, if the level of lactose is reduced too much, the
viscosity of the milk will be too high for easy expression of milk; the
viscosity of mouse milk genetically engineered to contain no lactose
was so high that the pups were unable to suckle and died (see Leaver
and Law, 2003). Obviously, this problem could be overcome by
reducing the level of lactose rather than eliminating it. Alternatively,
it may be possible to modify the milk secretory mechanism to pro-
duce a more useful, or at least a less problematic, sugar than lactose,
e.g. glucose, maltose or lactulose (which is a laxative and prebiotic),
or it might be possible to increase the concentration of salts in milk.
As discussed below, most adult humans are unable to digest lactose.
If the problems arising from high viscosity were resolved, lactose-free
or -reduced milk would be nutritionally desirable.
However, in some cases it would be advantageous to increase the lactose
content of milk. The economic benefits of increasing the milk output of sows
by increasing its lactose content were discussed by Wheeler (2003).
1.2.2.
Nutritional Problems Associated with Lactose
Mammals cannot absorb disaccharides from the intestine; they are first
hydrolysed in the small intestine to monosaccharides, which are absorbed.
Lactose is hydrolysed by
-galactosidase (
-gal; also referred to as lactase)
