The diastatic hydrolysis of starch has already been described ([179]). It is best secured at a somewhat higher temperature than that of the human stomach.
546. Aliphalytic Ferments.—In the hydrolysis of glycerids in the process of digestion the fat acids and glycerol are set free. Whether the glycerids be completely hydrolyzed before absorption is not definitely known. In certain cases where large quantities of oil have been exhibited for remedial purposes, the fat acids and soaps have been found in spherical masses in the dejecta[551] and have been mistaken for gall stones.
The fat which enters the chyle appears to be mostly unchanged, except that it is emulsified.[552] The aliphalytic ferment can be prepared from the fresh pancreas, preferably from animals that have not been fed for forty hours before killing. It is important to prepare the ferment entirely free of any trace of acid. The fresh glands are rubbed to a fine paste with powdered glass and extracted for four days with pure glycerol, to which one part of one per cent soda solution has been added. The filtered liquor contains aliphalytic, proteolytic and amylytic ferments, and is employed for saponification by shaking with the fat to form an emulsion and keeping the mixture, with occasional shaking, at a temperature of from 40° to 60°. The free acids can be titrated or separated from the unsaponified fats by solution in alcohol.[553]
Heretofore it has not been possible to separate a pure aliphalytic ferment from any of the digestive glands. The digestion of carbohydrates and that of fats are intimately associated, and these two classes of foods seem to play nearly the same rôle in the animal economy.
The aliphalytic ferments, prepared from the fresh pancreas, act also on the glucosids and other ester-like carbohydrate bodies. Since the fats may be regarded as ethers, the double action indicates the similarity of composition in the two classes of bodies.[554] The aliphalytic ferments exist also in plants and have been isolated from rape seed.[555]
547. Proteolytic Ferments.—The most important process in artificial digestion is the one relating to the action of the ferments on proteid matters. The hydrolysis of fats and carbohydrates by natural ferments takes place best in an alkaline medium, while in the case of proteids when pepsin is used an acid medium is preferred. Since the acidity of the stomach is due chiefly to hydrochloric, that acid is employed in artificial digestion. The hydrolyte used is uniformly the natural ferment of the gastric secretions, viz., pepsin; but this is often followed by the pancreatic ferment, (pancreatin, trypsin) in an alkaline medium. During the digestion, the proteids are changed into peptones, and the measurement of this change determines the degree of digestion. The total proteid matter is determined in the sample, and after the digestion is completed, the soluble peptones are removed by washing and the residual insoluble proteid matter determined by moist combustion. The difference in the two determinations shows the quantity of proteid matter digested. The investigations of Kühn on the digestion of proteids may be profitably consulted.[556] For a summary of digestion experiments in this country the résumé prepared by Gordon may be consulted.[557] The method followed in this laboratory is fully described by Bigelow and Hamilton.[558]
548. Ferments Employed.—Both the pepsins of commerce and those prepared directly from the stomachs of pigs may be used. The commercial scale pepsin is found, as a rule, entirely satisfactory, and more uniform results are secured by its use than from pepsin solutions made from time to time from pig stomachs. In the preparation of the pepsin solution one gram of the best scale pepsin is dissolved in one liter of 0.33 per cent hydrochloric acid. Two grams of the sample of food products, in fine powder, are suspended in 100 cubic centimeters of the solution and kept, with frequent shaking, at a temperature of 40° for twelve hours. The contents of the flask are poured on a wet filter, the residue on the filter well washed with water not above 40°, the filter paper and its contents transferred to a kjeldahl flask and the residual nitrogen determined and multiplied by 6.25 to get the undigested proteid matter. A large number of digestions can be conducted at once in a bath shown in [Fig. 117].[559] The quantity of water in the bath should be as large as possible.
549. Digestion in Pepsin and Pancreatin.—The digestion of the proteids is not as a rule wholly accomplished by the stomach juices, and, therefore, in order to secure in artificial digestion results approximating those produced in the living organism, it is necessary to follow the treatment with pepsin by a similar one with the pancreas juices. The method employed in this laboratory is essentially that of Stutzer modified by Wilson.[560]
Fig. 117. Bath for Artificial Digestion.