CH. V.] LACTOSE. 115 



solutions, it has, relatively, a very feeble reducing power towards the acid 

 Barfoed's reagent. It must be emphasised that useful information can only 

 be derived from Barfoed's test if the reducing power of the solution towards 

 alkaline reagents is known. 



122. Prepare the osazone as directed in Exercise 109. Malt- 

 osazone is much more soluble than glucosazone, and only separates 

 on cooling. It is important to allow the solution to cool slowly as 

 directed. It generally crystallises in clusters of broad plates, not in 

 needles. It can be recrystallised by dissolving the precipitate in 

 boiling water, filtering, and allowing the hot solution to cool slowly. 

 It melts at 206 C. 



Lactose is the sugar found in milk, and often in the 

 urine of women during lactation. It has reactions very 

 similar to those of maltose. It is hydrolysed by boiling 

 acids, and by the ferment lactase into equal parts of glucose 

 and galactose. 



Constitutionally it is glucose-/3-galactoside. 

 It is not fermented by ordinary yeast. 



Use a i per cent, solution for the following exercises : 



123. Repeat Exs. 95, 97, 99 and 100. The reactions are 

 indistinguishable from those of glucose. 



124. Repeat Ex. 98. A reduction is generally obtained. 

 (Distinction from glucose.) 



125. Repeat Ex. 102, using 14 drops of the 20 per cent, copper 

 sulphate for the first trial. Lactose has a reducing power about 

 70 per cent, of that of glucose. 



126. Repeat Ex. 103, using 18 drops of the copper sulphate 

 for the first tube. The reducing power of the solution has been 

 markedly increased, owing to the hydrolysis of the lactose to glucose 

 and galactose. (Distinction from glucose.) 



127. Repeat Ex. 101. A reduction is not usually obtained. 

 (Distinction from glucose.) 



128. Prepare the osazone (see Ex. 108). Lactosazone is much 

 more soluble in hot water than glucosazone. It crystallises in 

 irregular clusters of fine needles (" Hedge-hog crystals "). It can 

 be recrystallised from hot water (see Ex. 122). It melts at 200 C. 



