Dairy Chemistry And Biochemistry

26 DAIRY CHEMISTRY AND BIOCHEMISTRY

oL I

I I I

2 4 6 8

PH

Figure 2.4 Effect of pH on the rate of mutarotation of lactose.

At equilibrium:
89.4~ + 35(100 - X) = 55.4 x 100
x = 37.3
100-x = 62.7
Thus, the equilibrium mixture at 20°C is composed of 62.7% 8- and
37.3% a-lactose. The equilibrium constant, P/a, is 1.68 at 20°C. The
proportion of lactose in the @-form increases as the temperature is increased
and the equilibrium constant consequently decreases. The equilibrium
constant is not influenced by pH, but the rate of mutarotation is dependent
on both temperature and pH. The change from m- to p-lactose is 51.1, 17.5
and 3.4% complete at 25, 15 and O"C, respectively, in 1 h and is almost
instantaneous at about 75°C.
The rate of mutarotation is slowest at pH 5.0, increasing rapidly at more
acid or alkaline values; equilibrium is established in a few minutes at pH 9.0
(Figure 2.4).