68 THE CARBOHYDRATES 



tained in the beet has been raised from iO'6 per cent in the 

 period 1880-90 to about 15 per cent in the period 1900-10, 

 and the beetroot is gradually displacing the sugar cane as a 

 source of sucrose. Owing to exceptionally favourable weather 

 conditions the yield in the year 1908-9 rose to about 18 '5 per 

 cent, but individual beets have been known to contain up to 

 27 per cent of sugar. 



Properties. 



Cane sugar crystallizes from water in monoclinic crystals 

 which do not contain water of crystallization ; it is readily soluble 

 in water and only slightly soluble in alcohol ; it is dextro- 

 rotatory, its specific rotation being a^ = -^66-5. 



When heated to 160° it melts to a glassy mass known as 

 barley sugar, which gradually becomes crystalline again ; if 

 heated to 190-200° it is converted into an uncrystallizable 

 brown substance known as caramel, which is used for colouring 

 beer and wine. 



Reactions. 



1. Solutions of cane sugar heated with concentrated 

 hydrochloric acid turn reddish pink. 



2. If warmed with concentrated hydrochloric acid and a few 

 crystals of resorcin a deep red colour is produced owing to the 

 liberation of levulose. 



3. Cane sugar does not react with phenylhydrazine. 



4. Cane sugar does not reduce Nylander's reagent. 



5. Solutions of cane sugar do not reduce Fehling's solution 

 until they have been inverted by boiling for a short time with 

 a few drops of dilute sulphuric acid ; if then made alkaline and 

 boiled with Fehling's solution reduction ensues. 



If a solution in water is boiled with a few drops of mineral 

 acid, the sign of the optical activity of the solution changes 

 from 4- to - . This change, which is known as inversion^ is 

 due to the fact that the mineral acid hydrolyses the cane sugar, 

 converting it into equal molecular proportions of the two 

 sugars dextrose and levulose, 



CiaHjaOn + H^O = C^W^^^, + CgHiaOg 

 and since the optical activity of levulose is greater than that 

 of dextrose the resulting invert sugar is laevo-rotatory. 



