Lactose exists in two anomeric forms and can mutarotate from the a- to the b- form and vice versa by changing the configuration around the anomeric carbon. If either a- or b-lactose is dissolved in water, there is a gradual change from one form to the other until an equilibrium is established. In water at 20°C, the equilibrium mixture is composed of 62.7% b- and 37.3% a-lactose. The proportion of a-lactose increases with increasing temperature. The final proportions of a- and b-lactose in the mixture are not influenced by pH, but the rate at which the mixture reaches equilibrium is slowest at pH 5.0. The a- and b- forms of lactose differ with respect to:

· Solubility

· Crystal size and shape

· Hydration of crystal form (and hence hygroscopicity)

· Specific rotation

· Sweetness.