In the estimation of cattlefoods and of plant substances in general the residue insoluble in dilute boiling acid and alkali is called crude or indigestible fiber.
The principle on which the determination depends rests on the assumption that all the protein, starch and other digestible carbohydrates will be removed from the sample by successive digestion at a boiling temperature with acid and alkali solutions of a given strength. It is evident that the complex body obtained by the treatment outlined above is not in any sense a definite chemical compound, but it may be considered as being composed partly of cellulose.
272. Official Method of Determining Crude Fiber.—The method of estimating crude fiber, adopted by the Association of Official Agricultural Chemists, is as follows: [222]
Extract two grams of the substance with ordinary ether, at least almost completely, or use the residue from the determination of the ether extract. To this residue, in a half liter flask, add 200 cubic centimeters of boiling 1.25 per cent sulfuric acid; connect the flask with an inverted condenser, the tube of which passes only a short distance beyond the rubber stopper into the flask. Boil at once, and continue the boiling for thirty minutes. A blast of air conducted into the flask may serve to reduce the frothing of the liquid. Filter, wash thoroughly with boiling water until the washings are no longer acid, rinse the substance back into the same flask with 200 cubic centimeters of a boiling 1.25 per cent solution of sodium hydroxid, free or nearly free of sodium carbonate, boil at once and continue the boiling for thirty minutes in the same manner as directed above for the treatment with acid. Filter into a gooch, and wash with boiling water until the washings are neutral, dry at 110°, weigh and incinerate completely. The loss of weight is crude fiber.
The filter used for the first filtration may be linen, one of the forms of glass wool or asbestos filters, or any other form that secures clear and reasonably rapid filtration. The solutions of sulfuric acid and sodium hydroxid are to be made up of the specified strength, determined accurately by titration and not merely from specific gravity.
The experience of this laboratory has shown that results practically identical with those got as above, are obtained by conducting the digestions in hard glass beakers covered with watch glasses. The ease of manipulation in the modification of the process just mentioned is a sufficient justification for its use.
273. Separation of Cellulose.—Hoppe-Seyler observed that cellulose, when melted with the alkalies at a temperature as high as 200°, was not sensibly attacked.[223]
Lange has based a process for determining cellulose on this observation.[224]
The process, as improved by him, is carried out as follows:
From five to ten grams of the substance are moistened with water and placed in a porcelain dish with about three times their weight of caustic alkali free of nitrates and about twenty cubic centimeters of water. The porcelain dish should be deep and crucible shaped and should be placed in an oil-bath, the temperature of which is easily controlled. The contents of the dish are stirred with the thermometer bulb until all foaming ceases and the temperature of the mixture is then kept at from 175° to 180° for an hour. After the melt has cooled to 80° about seventy-five cubic centimeters of hot water are added to bring it into solution and it is then allowed to cool. The solution is acidified with sulfuric and placed in large centrifugal tubes. After being made slightly alkaline with soda lye, the tubes are subjected to continued energetic centrifugal action until the cellulose is separated. The supernatant liquid can be nearly all poured off and the separated cellulose is broken up, treated with hot water and again separated by centrifugal action. The cellulose is finally collected upon the asbestos felt, washed with hot water, alcohol and ether, dried and weighed. With a little practice it is possible to complete the separation of cellulose in two and one-half hours.