V] CARBOHYDRATES 65 



{a) Add a few drops to a little boiling Fehling solution. Reduction immediately 

 takes place. 



(6) Make the phloroglucinol, orcinol and furfural tests for pentoses, using a small 

 quantity only of the hydrolysis mixture for the tests. A positive result will be given 

 in each case. The pentoses, arabinose and xylose, are responsible for these reactions. 



(c) Add to some of the solution phenylhydrazine hydrochloride, sodium acetate 

 and a little acetic acid, and leave in boiling water for half an hour for the osazone test 

 [see Expt. 41 {d)]. A mixture of osazones will separate out, among which glucosazone 

 can be identified. 



{d) Concentrate the remainder of the solution and then add some nitric acid of 

 sp. gr. 1"I5 (see Expt. 43). Evaporate down on a water-bath to one-third ol the bulk 

 of the liquid and then pour into about 100 c.c. of water. A white microcrystalline 

 precipitate of mucic acid will separate out, either at once or in the course of a day or 

 two. This demonstrates the presence of galactose. 



^ PeCTIO SUBSTANCES. 



These substances are considered at this point since they are said to 

 constitute, in more or less intimate connexion with cellulose, the middle 

 lamella of cell-walls in many tissues. The pectic substances are frequently 

 found in the juices of succulent fruits in which the tissues have dis- 

 integrated, such as red currants and gooseberries. They have been iso- 

 lated chiefly from fleshy roots, stems and fruits, as, for instance, from 

 turnips, beetroot, rhubarb stems, oranges, apples, cherries and straw- 

 berries ; quite recently, also, from cabbage, onions and pea-pods. Recent 

 investigations point to the fact that all these tissues contain the same 

 pectic material, and it is possible that all such substances may be 

 identical. 



The chief pectic compound occurring in the cell-wall, probably in 

 combination with cellulose, is of aii acidic nature and has been provision- 

 ally termed pectinogen (Schryver and Haynes, 81). It is extracted, in 

 the form of the ammonium salt, by treating the tissue residue (after 

 expressing the juice) with warm dilute ammonium oxalate solution. 

 From this solution, either the salt, or, after acidification, pectinogen 

 itself, can be precipitated as a very bulky gelatinous mass by adding 

 alcohol. Pectinogen is an acid and is soluble in water giving a thick 

 opalescent solution ; its sodium, potassium, ammonium and calcium salts 

 are also soluble. Pectinogen solution, therefore, is not precipitated either 

 by acid or by dilute solutions of calcium salts. 



In the case of juicy fruits, such as currants and gooseberries, the 

 pectinogen can be precipitated as a gelatinous precipitate by adding 

 alcohol to the expressed juice. In the case of fleshy fruits, stems and 

 roots, the juice, as a rule, contains but little pectinogen and the 



o. 5 



