Zein freshly precipitated by pouring its alcoholic solution in water is wholly insoluble in water, and, on boiling therewith, is changed into the variety insoluble in dilute alcohol. Boiled with dilute sulfuric acid, six in 300 cubic centimeters of water, it melts, forming a gummy mass, which is very slowly attacked by the acid yielding proteoses and peptones. Heated with stronger sulfuric acid it undergoes decomposition, yielding leucin, tyrosin, and glutamic acid.
402. Solvent Action of Acids and Alkalies.—In the preceding paragraphs, a synopsis has been given of the methods of separating proteid matters in such a manner as to secure them in a pure state in the same conditions as they exist in the natural substances. A very large percentage of the proteid matter is still left undissolved after extraction with the solvents already mentioned.
Often important information may be gained concerning the nature of the residual proteid matters by fractional extraction with dilute acids and alkalies. When the strength of these solutions is such that they contain about one per cent of the acid or alkali, the whole of the proteid matter may be dissolved by boiling successively with acid and alkali for half an hour. The proteid matter passing into solution in these cases is usually changed in character, assuming the nature of proteoses or allied bodies, when treated with an acid, and becoming albuminates when boiled with an alkali. Easily soluble carbohydrate matter is also removed by this treatment so that the residue obtained consists largely of cellulose and is known as crude or insoluble fiber. The removal of all the bodies soluble in dilute boiling acid and alkali is accomplished by the method described in paragraph [272].
For research purposes, the solvent action of dilute alkali is of chief importance to the analyst, and the extraction of the proteid matter, after all that is soluble in water, common salt solution and alcohol has been removed, should commence with a solution of potassium or sodium hydroxid containing not over two-tenths per cent of the alkali.
It has been shown by Osborne that the solvent action of very dilute alkali, in the cold, may be exerted without changing the character of the dissolved proteid.[379]
403. Method of Extraction.—The solvent employed is usually a two-tenths per cent solution of potassium hydroxid. It may be added directly to the substance or may follow extraction with water, salt solution or alcohol. In the former case, the manipulation is illustrated by the following description of the treatment of oatmeal:[380]
One hundred grams of oatmeal are mixed with half a liter of a two-tenths per cent potassium hydroxid solution and allowed to stand for some time at room temperature. The mixture is strained through a cloth to remove the chaff and the residue is stirred with another small portion of the solvent, again strained in the same cloth and the residue squeezed dry. The strained liquids are united and enough more of the solvent added to make the volume 700 cubic centimeters. After standing for some time, the insoluble matter settles to the bottom of the vessel and the supernatant liquid is decanted. More solvent is added to the residue, well mixed therewith and treated as above. It is advisable to make a third extraction in the same way. The extracts are united, passed through a filter, the proteid matter in solution thrown out by acetic acid, washed with water, alcohol and ether and dried over sulfuric acid.
The methods of procedure, when the sample has been previously extracted with water, salt solution or alcohol, are essentially the same as that just described and the reader may consult the paper of Osborne for details.[381]
404. Methods of Drying Separated Proteids.—In the preceding paragraphs, the analyst has been directed, in most instances, to dry the proteid matter, after it is secured in as pure a form as possible, at room temperature, over sulfuric acid. By this treatment the preparation may be obtained in a form suited to the study of its physical properties, since its solubility has not been affected by subjecting it to a high temperature. When it is desired to use the sample only for chemical analysis it is not necessary to wait on the slow process above mentioned. In this case the sample may be dried in an inert atmosphere at the temperature of a steam-bath or even at 110°. It is better, however, to avoid so high a temperature and to conduct the desiccation in vacuo at a heat not above that of boiling water. The sample, before drying, should be reduced to the finest possible state of comminution, otherwise particles of aqueous vapor may be retained with great tenacity.
In many cases it is advisable to dry the sample pretty thoroughly, then grind to a fine powder and finish the desiccation with the pulverulent mass. This treatment can be followed when the quantity of the material is considerably in excess of that required for the analytical operations.