64 CARBOHYDRATES AND THEIR [CH. 



pectinogen precipitated by alcohol. It may be purified by reprecipita- 

 tion. 



Pectinogen is precipitated from aqueous solution by alcohol as a very 

 bulky gelatinous mass, but when dried it forms an almost colourless granu- 

 lar powder. Put into water it absorbs large quantities of liquid and dis- 

 solves slowly, giving an opalescent solution with a distinctly acid reaction. 



As mentioned above pectinogen in alkaline solution is rapidly con- 

 verted into pectin. A solution of pectinogen is not precipitated either 

 by acid or dilute solutions of calcium salts but, after treatment with 

 alkali and conversion into pectin, both the aforesaid reagents produce 

 gelatinous precipitates. A similar precipitate is also formed when lime 

 water is added in excess to a solution of pectinogen and it is allowed to 

 stand. There is little doubt that the pectinogen is converted by the 

 alkali into pectin. Pectin is also an acid substance and it is insoluble 

 in water, giving an insoluble salt with calcium. After treatment of 

 pectinogen with alkali the pectin can, as already stated, be precipitated 

 by adding acid. 



Analyses of pectin from various sources have led to the suggestion 

 of C^H^Oje as its formula. There is also evidence that it contains one 

 pentose group. This can be detected and estimated by the furfural 

 phloroglucide method (see Expt. 53). 



Expt. 66. Extraction and reactions of pectinogen. Take about half a pound of red 

 currants and squeeze out the juice through fine muslin into a large beaker. Then add 

 to the juice about 2-3 times its bulk of 96-98% alcohol. A bulky gelatinous precipi- 

 tate of pectinogen will separate out. Allow the precipitate to stand for a time in the 

 alcohol, and then filter off. Wash with alcohol and finally press free from liquid. 

 Dissolve the precipitate in as little water as will enable it to go into solution. To two 

 small portions of the solution add respectively (a) a few drops of strong hydrochloric 

 acid, (6) an excess of calcium chloride solution. Note that no precipitate is formed in 

 either case. 



Expt. 67. Conversion of pectinogen into pectin, and reactions of pectin. Take about 

 one-third of the pectinogen solution prepared in Expt. 66, make it alkaline with 

 caustic soda, and let it stand for about 10-15 minutes. Then divide the solution 

 into two parts and add respectively (a) sufficient hydrochloric acid to acidify, 

 (6) excess of calcium chloride solution. In the first case a gel of pectin is formed : 

 in the second case a gelatinous precipitate of the calcium salt of pectin. 



To a further quantity of the pectinogen add excess of lime water and let it stand. 

 The gelatinous calcium precipitate will separate out in a short time. 



Expt. 68. Detection of the pentose group in pectinogen. Filter off the pectin gel 

 obtained in the last experiment and allow it to dry. Then test for the pentose group 

 by the orcinol, phloroglucinol and furfural tests (see Expt. 38). All results will be 

 found to be positive. 



