Some Physico-Chemical Properties of Lactose

Lactose contains a free aldehyde group, and therefore it is capable of existing in two different isomeric modifications.When either of these is dissolved in water, it is partially transformed into the other until an equilibrium is reached.The transformation is slow at low temperatures.This fact is of practical importance because the rate of mutarotation sets an upper limit to the rate of solution, or of crystallization of lactose.The fact that the two forms have quite different solubilities is responsible for the recent production of beta lactose on a commercial scale.Erdmann (11) was the first to observe that the optical rotation of ordinary lactose solutions decreased on standing.He also prepared a new modification of lactose whose optical rotation increased with time.Dubrunfaut (10) also observed the fall in rotation of freshly prepared solutions, and reported that the fall was accelerated by heat.Later, Schmoeger ( 42) rediscovered what is now known as beta lactose.Although Erdmann's results had been published much earlier, they apparently had not received much recognition (12,43).During the latter part of the nineteenth century, a number of papers were published dealing with the mutarotation of sugars.Tanret (46) described three forms of lactose which he called alpha, beta, and gamma lactose.His gamma sugar is now known as beta lactose.The form which he named beta lactose was a mixture of the other two sugars.For a time the equilibrium mixture of mutarotating sugars was quite generally accepted as a distinct modification.Even as late as 1903, we find Roux (41) asserting that the aldoses can exist in three forms.At the beginning of the twentieth century, several papers appeared which revolutionized ideas regarding mutarotation.Previous to that time, Fischer (13) had suggested that the mutarotation was due to a hydration reaction, and that constant rotation was established only when the sugar was completely hydrated.Trey (47) attributed the mutarotation to some unknown change in configuration.Tanret apparently considered his beta lactose (equilibrium mixture) to be the aldehyde form.Then in 1899, Lowry (25) suggested that the mutarotation was due to the establishment of a dynamie equilibrium between structural isomers, and that the beta