Dairy Chemistry And Biochemistry

LACTOSE 25

Glucose- 1 -phoSPhE

UDP gliiccisr-4-rpinier.osr

gnlncros~llrr~~l~?.\:fr,.cl.vr

cr-I~/ctnlDu/ttil? *LACTOSE

Glucose

Figure 2.3 Pathway for lactose synthesis.

2.2.3 Lactose equilibrium in solution
The configuration around the C, of glucose (i.e. the anomeric C) is not
stable and can readily change (mutarotate) from the x- to the /?-form and
vice versa when the sugar is in solution as a consequence of the fact that the
hemiacetal form is in equilibrium with the open chain aldehyde form which
can be converted into either of the two isomeric forms (Figure 2.2).
When either isomer is dissolved in water, there is a gradual change from
one form to the other until equilibrium is established, i.e. mutarotation.
These changes may be followed by measuring the change in optical rotation
with time until, at equilibrium, the specific rotation is + 55.4".
The composition of the mixture at equilibrium may be calculated as
follows:

Specific rotation [NIP

a-form + 89.4"

p-form + 35.0"

Equilibrium mixture + 55.4"

Let equilibrium mixture = 100

Let x% of the lactose be in the cr-form

Then (100 - x)% is the p-form