196 THE CARBOHYDRATES 



hydrolysis of this soluble pectin yields galacturonic and acetic 

 acids as well as methyl alcohol, arabinose, and galactose 

 according to the equation — 



C43H62O37 + 10H2O = 4C6Hio07 + 3CH3COOH +,2CH30H + C5H10O5 + QHuOe 

 Galacturonic Acetic Methyl Arabinose Galactose 



acid acid alcohol 



On the other hand, alkaline hydrolysis by means of calcium 

 or barium hydroxides yields methyl alcohol, acetic acid, a 

 galacto-araban, and a tetragalacturonic acid ; the latter acid 

 being regarded as a condensation product arising from the 

 ehmination of 3 molecules of water from 4 molecules of 

 galacturonic acid as follows : — 



4CHO(CHOH)4 . COOH— 3H2O = C^JiaiO,^ 



Galacturonic acid Tetragalacturonic acid 



The galacto-araban set free by alkaline hydrolysis must be 

 distinguished from the araban separated from the original 

 plant material by boiling with water. 



Combining the information obtained from acid and alkaline 

 hydrolysis, Ehrlich concluded that the original pectin was a 

 triacetyl-arabino-galacto-dimethoxy-tetragalacturonic acid. 



The pectins of currants and strawberries and also of 

 orange peel are generally similar but show a rather higher 

 content of galacturonic acid, i.e. 73-78 per cent, as compared 

 with 68 per cent for sugar beet and 61 per cent for flax. 

 Whereas the fruits mentioned contain about 40-50 per cent 

 of the total pectin dissolved in the juice, the beet and orange 

 peel contain only about 5-10 per cent in soluble form. As 

 has been stated above, some difficulty is experienced in recon- 

 ciling the various results obtained by different investigators. 



Thus von Fellenberg,* who first established the presence of 

 methylated carboxyl groups in the pectins from various fruits, 

 came to the conclusion that the completely demethylated acid 

 corresponding to pectin was a pectic acid of the formula 

 C70H104O68, and that the formula of the neutral pectin was — 



(C^HsOJa (CeHjoOs)^ (C,H,0, . €00^3)3 . 2H2O 

 Arabinose Methyl Galacturonic acid 



pentose and 

 Galactose 



i.e. a condensation product formed from 2 molecules of 

 * V. Fellenberg : " Biochem. Zeit.," 1918, 85, 141. 



