THE CHEMISTRY OF THE SUGARS 95 



is either crushed in presses and the juice thus forced out, or 

 the sugar is systematically extracted by water ; the extract 

 is clarified with lime, decolourised with animal char- 

 coal, filtered and evaporated in vacuum pans till the sugar 

 crystallises. The dark mother liquor is known as molasses 

 or treacle, the sugar can be obtained from it by precipitation 

 with strontium hydroxide ; from this precipitate the sugar 

 is recovered by suspending in water, passing carbon 

 dioxide through, filtering from the strontium carbonate and 

 evaporating. 



Cane sugar crystallises from water in hard four-sided prisms ; 

 it is generally purified for purposes of scientific investigation 

 by recrystallisation from hot alcohol. It melts at about 

 160 C. and is dextro-rotatory ; it does not reduce Fehling 

 solution ; it also does not combine with phenyl-hydrazine. 



Maltose is obtained from starch by the action of the 

 enzyme known as amylase ; this reaction forms the subject 

 of Chapters V and VI of this book. 



Maltose is more soluble in water than is cane sugar, and 

 is more strongly dextro-rotatory ; it also reduces Fehling 

 solution and combines with phenyl hydrazine. A further 

 distinction from cane sugar lies in the fact that it directly 

 ferments with yeast, which is not the case with cane sugar. 



Milk sugar or lactose occurs in the milk of all mammals 

 to the extent of about 4 per cent. 



In the manufacture of cheese, milk is treated with a 

 clotting enzyme known as ' rennet ' which coagulates the 

 casein, milk sugar remaining in solution ; it can be readily 

 crystallised from this solution on evaporation, the crystals 

 containing one molecule of water of crystallisation. It is 

 much less sweet than cane sugar and is dextro-rotatory, 

 though to a less extent than cane sugar ; it reduces 

 Fehling solution slowly and forms a phenyl-hydrasine : 



