=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]
[Illustration: 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.
[Illustration: FIG. 118.—BAG FOR COLLECTING FECES.]
[Illustration: 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.
Water, Ash, Proteid, Fiber, Fat, Undetermined,
per per per per per per
Food. cent. cent. cent. cent. cent. cent.
Sweet maize 83.85 1.13 2.18 4.14 0.62 8.08
Feces 72.01 ... ... ... ... ...
DRY.
Ash, Proteid, Fiber, Fat, Undetermined,
per per per per per
Food. cent. cent. cent. cent. 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.
Green, Dry,
Food. grams. 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]