66 CARBOHYDRATES [ch. 



procedure is as follows. The tissues are thoroughly disintegrated in a 

 mincing machine and pressed free from all juice in a powerful press. 

 The residue is then dried, finely ground, washed with water and finally 

 extracted with dilute ammonium oxalate solution in which pectinogen 

 is soluble. The extract is concentrated and the pectinogen precipitated 

 by alcohol. It may be purified by reprecipitation. When dried it forms 

 an almost colourless granular powder. Put into water it absorbs large 

 quantities of liquid and dissolves slowly, giving an opalescent solution 

 with a distinctly acid reaction. 



When pectinogen solution is treated with normal caustic soda at 

 ordinary temperatures, the sodium salt is first formed and this is rapidly 

 changed into the salt of another substance termed pectin (cytopectic 

 acid) (Clayson, Norris and Schryver, 7). Cytopectic acid is insoluble in 

 water and is readily converted into a gel under certain conditions ; its 

 calcium salt is also insoluble. Thus, if a solution of pectinogen, made 

 alkaline with caustic soda, is allowed to stand for ten minutes, on adding 

 acid a gelatinous precipitate of cytopectic acid is formed, and on adding 

 calcium chloride solution, a gelatinous precipitate of the calcium salt of 

 cytopectic acid. A similar precipitate is also formed when lime water 

 is added in excess to a solution of pectin and it is allowed to stand. 



If the tissue residue is first treated with caustic soda solution, the 

 pectinogen is changed in situ into the cytopectic acid which, though not 

 itself extracted with caustic soda, can be subsequently extracted by 

 ammonium oxalate solution and separated as a gel by addition of acid. 



Analyses of pectin from various sources, i.e. apples, oranges, straw- 

 berries, cabbage, onions, pea-pods, rhubarb and turnips, have led to the 

 suggestion of C17H24O16 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 musHn 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 5% 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 4 7o 

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

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



