METHODS OF DIGESTION.
544. Artificial Digestion.—The nutrient values of cereals and other foods are determined both by chemical analysis and by digestion experiments. The heat forming properties of foods are disclosed by combustion in a calorimeter, but the quantity of heat produced is not in every case a guide to the ascertainment of the nutritive value. This is more certainly shown, especially in the case of proteid bodies, by the action of the natural digestive ferments.
It is probable that the digestion, which is secured by the action of these ferments without the digestive organs, is not always the same as the natural process, but when the conditions which prevail in natural digestion are imitated as closely as possible the effects produced can be considered as approximately those of the alimentary canal in healthy action.
Three classes of ferments are active in artificial digestion, viz., amylolytic ferments, serving to hydrolyze starch and sugars and to convert them into dextrose, maltose and levulose, aliphalytic ferments, which decompose the glycerids and proteolytic ferments, which act on the nitrogenous constituents of foods. When these ferments are made to act on foods under proper conditions of acidity and temperature, artificial digestion ensues, and by the measurement of the extent of the action an approximate estimate of their digestibility can be secured. In artificial digestion, the temperature should be kept near that of the body, viz., at about 40°.
The soluble ferments which are active in the digestion of foods, as has been intimated, comprise three great classes. Among the first class, viz., the amylolytic ferments, are included not only those which convert starch into dextrose, but also those which cause the hydrolysis of sugars in general. Among these may be mentioned ptyalin, invertase, trehalase, maltase, lactase, diastase, inulase, pectase and cyto-hydrolytic ferments which act upon the celluloses and other fibers.
Among the aliphalytic ferments, in addition to those which act also upon proteid matter, may be mentioned a special one, lipase.
In the third class of ferments are found pepsin, trypsin or pancreatin and papain.
For the latest information in regard to the nature of the soluble ferments and their nomenclature, the work of Bourquelot may be consulted.[550]
545. Amylytic Ferments.—A very active ferment of this kind is found in the saliva. Saliva may be easily collected from school boys, who will be found willing to engage in its production if supplied with a chewing gum. A gum free of sugar is to be used, or if the chewing gum of commerce is employed, the saliva should not be collected until the sugar has disappeared. A dozen boys with vigorous chewing will soon provide a sufficient quantity of saliva for practical use. The amylolytic digestion is conducted in the apparatus hereinafter described for digestion with pepsin and pancreatin. The starch or sugar in fine powder is mixed with ten parts of water and one part of saliva and kept at about 37°.5 for a definite time. The product is then examined for starch, sucrose, maltose, dextrose, dextrin and levulose by the processes already described. In natural digestion the hydrolysis of the carbohydrates is not completed in the mouth. The action of the ferment is somewhat diminished in the stomach, but not perhaps until half an hour after eating. The dilute hydrochloric acid in the stomach, which accumulates some time after eating, is not active in this hydrolysis. On the contrary the amylolytic ferment of the saliva is somewhat enfeebled by the presence of an acid. The active principle of the saliva is ptyalin.
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.
The residue from the pepsin digestion, after washing, is treated for six hours at near 40° with 100 cubic centimeters of pancreas solution, prepared as follows:
Free the pancreas of a healthy steer of fat, pass it through a sausage grinder, rub one kilogram in a mortar with fine sand and allow to stand for a day or longer. Add three liters of lime water, one of glycerol, of 1.23 specific gravity, and a little chloroform and set aside for six days. Separate the liquor by pressure in a bag and filter it through paper. Before using, mix a quarter of a liter of the filtrate with three-quarters of a liter of water and five grams of dry sodium carbonate, or its equivalent crystallized, heat from 38° to 40° for two hours and filter.[561] In order to avoid the trouble of preparing the pancreas solution pure active pancreatin may be used.[562] One and a half grams of pure pancreatin and three grams of sodium carbonate are dissolved in one liter of water and 100 cubic centimeters of this solution are used for each two grams of the sample. In all cases where commercial pepsin and pancreatin are used, their activity should be tested with bodies such as boiled whites of eggs, whose coefficient of digestibility is well known and those samples be rejected which do not prove to have the required activity.[563]
550. Digestion in Pancreas Extract.—In order to save the time required for successive digestions in pepsin and pancreatin Niebling has proposed to make the digestion in the pancreas extract alone.[564] This process and also a slight modification of it have been used with success by Bigelow and McElroy.[565] Two grams of the sample are washed with ether and placed in a digestion flask with 100 cubic centimeters of two-tenths per cent hydrochloric acid. The contents of the flask are boiled for fifteen minutes, cooled, and made slightly alkaline with sodium carbonate. One hundred cubic centimeters of the unfiltered pancreas solution, prepared as directed above, are added and the digestion continued at 40° for six hours. The residue is thrown on a filter, washed, and the nitrogen determined. The method is simplified by the substitution of active commercial pancreatin for pancreas extract. The solution of the ferment is made of the same strength as is specified above.
551. Artificial Digestion of Cheese.—The artificial digestion of cheese is conducted by Stutzer as follows:[566]
The digestive liquor is prepared from the fresh stomachs of pigs by cutting them into fine pieces and mixing with five liters of water and 100 cubic centimeters of hydrochloric acid for each stomach. To prevent decomposition, two and a half grams of thymol, previously dissolved in alcohol, are added to each 600 cubic centimeters of the mixture. The mixture is allowed to stand for a day with occasional shaking, poured into a flannel bag and the liquid portion allowed to drain without pressing. The liquor obtained in this way is filtered, first through coarse and then through fine paper, and when thus prepared will keep several months without change. It is advisable to determine the content of hydrochloric acid in the liquor by titration and this content should be two-tenths of a per cent. The cheese to be digested is mixed with sand as previously described, freed of fat by extraction with ether, and a quantity corresponding to five grams of cheese placed in a beaker, covered with half a liter of the digestive liquor and kept at a temperature of 40° for forty-eight hours. At intervals of two hours the flasks are well shaken and five cubic centimeters of a ten per cent solution of hydrochloric acid added and this treatment continued until the quantity of hydrochloric acid amounts to one per cent. After the digestion is finished, the contents of the beaker are thrown on a filter, washed with water and the nitrogen determined in the usual way in the residue. By allowing the pepsin solution to act for two days as described above, the subsequent digestion with pancreas solution is superfluous.
552. Suggestions Regarding Manipulation.—The filter papers should be as quick working as possible to secure the separation of all undissolved particles. They should be of sufficient size to hold the whole contents of the digestion flask at once, since if allowed to become empty and partially dry, filtration is greatly impeded. The residue should be dried at once if not submitted immediately to moist combustion. After drying, the determination of the nitrogen can be made at any convenient time. Beaker flasks, i. e., lip erlenmeyers with a wide mouth are most convenient for holding the materials during digestion. The flasks are most conveniently held by a crossed rubber band attached at either end to pins in the wooden slats extending across the digestive bath. The bath should be suspended by cords from supports on the ceiling and a gentle rotatory motion imparted to it resembling the peristaltic action attending natural digestion.
553. Natural Digestion.—The digestion of foods by natural processes is determined chiefly by the classes of ferments already noted. The principle underlying digestive experiments with the animal organism may be stated as follows: A given weight of food of known composition is fed to a healthy animal under the conditions of careful control and preparation already mentioned. The solid dejecta of the animal during a given period are collected and weighed daily, being received directly from the animal in an appropriate bag, safely secured, as is shown in the accompanying [figure]. The dejecta are weighed, dried, ground to a fine powder, mixed and a representative part analyzed. The difference between the solid bodies in the dejecta and those given in the food during the period of experiment represents those nutrients which have been digested and absorbed during the passage of the food through the alimentary canal. The urine, containing solid bodies representing the waste of the animal organism, does not require to be analyzed for the simple control of digestive activity outlined above. In a complete determination of this kind the exhalations from the surface of the body and from the lungs are also determined. In the latter case the human animal is selected for the experiment; in the former it is more convenient to employ the lower animals, such as the sheep and cow.
The arrangement of the stalls and of the apparatus for collecting the excreta should be such as is both convenient and effective.[567]
The method of constructing a bag for attachment to a sheep is shown in [Fig. 118]. It is made according to the directions given by Gay, of heavy cloth and in such a way as to fit closely the posterior parts of the animal.[568] When attached, its appearance is shown in [Fig. 119].
Fig. 118.—Bag for
Collecting Feces.
Fig. 119.—Fecal Bag Attachment.
Healthy animals in the prime of life are used, and the feeding experiments are conducted with as large a number of animals as possible, in order to eliminate the effects of idiosyncrasy. The food used is previously prepared in abundant quantity and its composition determined by the analysis of an average sample.
The feeding period is divided into two parts. In the first part the animal is fed for a few days with the selected food until it is certain that all the excreta are derived from the nutrients used. In the second part the same food is continued and the excreta collected, weighed, the moisture determined, and the total weight of the water-free excreta ascertained. The first part should be of at least seven and the second of at least five days duration. The urine and dung are analyzed separately. Males are preferred for the digestion experiments because of the greater ease of collecting the urine and feces without mixing. For ordinary purposes the feces only are collected. The methods of analysis do not differ from those described for the determination of the usual ingredients of a food.
Example.—The following data taken from the results of digestive experiments, obtained at the Maine Station, will illustrate the method of comparing the composition of the food with that of the feces and of determining the degree of digestion which the proteids and other constituents of the food have undergone.
Composition of Maize Fodder and of Feces
Therefrom after Feeding to Sheep.
| Before Drying. | ||||||
|---|---|---|---|---|---|---|
| Food | Water, per cent. | Ash, per cent. | Proteid, per cent. | Fiber, per cent. | Fat, per cent. | Undetermined, per cent. |
| Sweet maize | 83.85 | 1.13 | 2.18 | 4.14 | 0.62 | 8.08 |
| Feces | 72.01 | ... | ... | ... | ... | ... |
| Dry. | ||||||
| Food | Ash, per cent. | Proteid, per cent. | Fiber, per cent. | Fat, per cent. | Undetermined, per cent. | |
| Sweet maize | 7.01 | 13.52 | 25.63 | 3.86 | 49.98 | |
| Feces | 14.42 | 17.52 | 19.34 | 2.68 | 46.04 | |
| Daily Weights. | ||||||
| Food | Green, grams. | Dry, grams. | ||||
| Sweet maize | 2521 | 407 | ||||
| Feces | 445 | 125 | ||||
| Per Cent Digested. | ||||||
| Food | Ash, | Proteid, | Fiber, | Undetermined, | Fat, | |
| Sweet maize | 37.0 | 60.2 | 76.9 | 71.8 | 78.3 | |
In the above instance it is seen that the coefficient of digestibility extended from 37.0 per cent in the case of the mineral components of the food, to 78.3 per cent in the case of the fats. These data are taken only from the results obtained from a single sheep and one article of food. The mean data secured from two animals and three kinds of maize fodder show the following per cents of digestibility: Ash 39.4, proteid 61.8, fiber 76.7, undetermined matters 72.1, fat 76.4. The undetermined matters are those usually known as nitrogen free extract and composed chiefly of pentosans and other carbohydrates.[569]
554. Natural Digestibility of Pentosans.—The digestibility of pentosan bodies in foods under the influence of natural ferments has been investigated by Lindsey and Holland.[570] The feeding and collection of the feces is carried on as described above and the relative proportions of pentosan bodies in the foods and feces determined by estimating the furfuraldehyd as prescribed in paragraph [150].[571]