=558. Nutritive Value of Foods.=—The value of a food as a nutrient
depends on the amount of heat it gives on combustion in the tissues of
the body, _i. e._ oxidation, and in its fitness to nourish the tissues
of the body, to promote growth and repair waste. The foods which
supply heat to the body are organic in their nature and are typically
represented by fats and carbohydrates. The foods which promote growth
and supply waste are not only those which preeminently supply heat,
but also include the inorganic bodies and organic nitrogenous matters
represented typically by the proteids. It is not proper to say that
one class of food is definitely devoted to heat forming and another to
tissue building, inasmuch as the same substance may play an important
rôle in both directions. As heat formers, carbohydrates and proteids
have an almost equal value, as measured by combustion in oxygen, while
fat has a double value for this purpose. The assumption that combustion
in oxygen forms a just criterion for determining the value of a food
must not be taken too literally. There are only a few bodies of the
vast number which burn in oxygen that are capable of assimilation
and oxidation by the animal organism. Only those parts of the food
that become soluble and assimilable under the action of the digestive
ferments, take part in nutrition and the percentage of food materials
digested varies within wide limits but rarely approaches 100. It may
be safely said that less than two-thirds of the total food materials
ingested are dissolved, absorbed, decomposed and assimilated in the
animal system. We have no means of knowing how far the decomposition
(oxidation) extends before assimilation, and therefore no theoretical
means of calculating the quantity of heat which is produced during the
progress of digestion. The vital thermostat is far more delicate than
any mechanical contrivance for regulating temperature and the quantity
of food, in a state of health, converted into heat, is just sufficient
to maintain the temperature of the body at a normal degree. Any excess
of heat produced, as by violent muscular exertion, is dissipated
through the lungs, the perspiration and other secretions of the body.
Pure cellulose or undigestible fiber, when burned in oxygen, will
give a thermal value approximating that of sugar, but no illustration
is required to show that when taken into the system the bodily heat
afforded by it is insignificant in quantity.
Thermal values, therefore, have little comparative usefulness
in determining nutritive worth, except when applied to foods of
approximately the same digestive coefficient.
=559. Comparative Value of Food Constituents.=—It has already been
noted that, judged by combustion in oxygen, carbohydrates and proteids
have about half the thermal value possessed by fats. Commercially,
the values of foods depend in a far greater degree on their flavor
and cooking qualities than upon the amount of nutrition they
contain. Butter fat, which is scarcely more nutritious than tallow,
is worth twice as much in the market, while the prices paid for
vegetables and fruits are not based to any great extent on their food
properties.[574] In cereals, especially in wheat, the quantity of fat is
relatively small, and starch is the preponderating element. In meats,
carbohydrates are practically eliminated and fats and proteids are the
predominating constituents.
In the markets, fats and proteids command far higher prices than
sugars and starches. The relative commercial food value of a cereal
may be roughly approximated by multiplying the percentages of fat and
protein by two and a half and adding the products to the percentage of
carbohydrates less insoluble fiber. This method was adopted in valuing
the cereals at the World’s Columbian Exposition.[575]
=560. Nutritive Ratio.=—In solid foods the nutritive ratio is that
existing between the percentage of proteids and that of carbohydrates,
increased by multiplying the fat by two and a half and adding the
product. In a cereal containing twelve per cent of protein, seventy-two
of carbohydrates, exclusive of fiber, and three of fat, the ratio is
12: 72 + 3 × 2.5 = 6.5. Instead of calculating the nutritive ratio
directly from the data obtained by analysis, it may be reckoned from
the per cents of the three substances in the sample multiplied by their
digestive coefficient. Since the relative amounts of proteids, fats and
carbohydrates digested do not greatly differ, the numerical expression
of the nutritive ratio is nearly the same when obtained by each of
these methods of calculation.
Where the proportion of protein is relatively large the ratio is called
narrow, 1: 4 ... 6. When the proportion of protein is relatively small
the ratio is called broad 1: 8 ... 12. In feeding, the nutritive ratio
is varied in harmony with the purpose in view, a narrow ratio favoring
the development of muscular energy, and a wide one promoting the
deposition of fat and the development of heat. These principles guide
the scientific farmer in mixing rations for his stock, the work horses
receiving a comparatively narrow and the beeves a relatively wide ratio
in their food.
=561. Calorimetric Analyses of Foods.=—The general principles of
calorimetry have been already noticed. The theoretical and chemical
relations of calorimetry have been fully discussed by Berthelot,
Thomsen, Ostwald and Muir.[576] In the analyses of foods the values
as determined by calculation or combustion are of importance in
determining the nutritive relations.
Atwater has presented a résumé of the history and importance of
the calorimetric investigations of foods to which the analyst is
referred.[577]
In the computation of food values the percentages of proteids,
carbohydrates and fats are determined and the required data obtained
by applying the factors 4100, 5500 and 9300 calories for one gram of
carbohydrates, proteids and fats respectively.
For most purposes the computed values are sufficient, but it is well to
check them from time to time by actual combustions in a calorimeter.
=562. Combustion in Oxygen.=—The author made a series of combustions
of carbonaceous materials in oxygen at the laboratory of Purdue
University in 1877, the ignition being secured by a platinum wire
rendered incandescent by the electric current. The data obtained were
unsatisfactory on account of the crudeness of the apparatus. The
discovery of the process of burning the samples in oxygen at a high
pressure has made it possible to get expressions of thermal data which
while not yet perfect, possess a working degree of accuracy. The best
form of bomb calorimeter heretofore employed is that of Hempel, as
modified by Atwater and Woods.[578]
A section of this calorimeter, with all the parts in place, is shown in
Fig. 120.
In the figure the steel cylinder _A_, about 12.5 centimeters deep and
6.2 in diameter, represents the chamber in which the combustion takes
place. Its walls are about half a centimeter thick and it weighs about
three kilograms. It is closed, when all the parts are ready and the
sample in place, by the collar _C_, which is secured gas tight by means
of a powerful spanner. The cover is provided with a neck _D_ carrying a
screw _E_ and a valve screw _F_. In the neck _D_, where the bottom of
the cylinder screw _E_ rests, is a shoulder fitted with a lead washer.
Through _G_ the oxygen used for combustion is introduced. The upper
edge of the cylinder _A_ is beveled and fits into a groove in the cover
_B_, carrying a soft metal washer. To facilitate the screwing on of the
cover, ball bearings _KK_, made of hard steel, are introduced between
the collar and the cover. The platinum wires _H_ and _I_ support the
platinum crucible holding the combustible bodies which are ignited by
raising the spiral iron wire connecting them to the temperature of
fusion by an electric current. The combustion apparatus when charged is
immersed in a metal cylinder _M_, containing water and resting on small
cylinders of cork. The water is stirred by the apparatus _LL_. The
cylinder _M_ is contained in two large concentric cylinders, _N_, _O_,
made of non-conducting materials and covered with disks of hard rubber.
The space between _O_ and _N_ may be filled with water. The temperature
is measured by the thermometer _P_, graduated to hundredths of a degree
and the reading is best accomplished by means of a cathetometer.
[Illustration: FIG. 120. HEMPEL AND ATWATER’S CALORIMETER.]
=563. The Williams Calorimeter.=—The calorimeter bomb has been
improved by Williams by making it of aluminum bronze of a spheroidal
shape. The interior of the bomb is plated with gold. By an ingenious
arrangement of contacts the firing is secured by means of a permanently
insulated electrode fixed in the side of the bomb. The calorimetric
water, as well as that in the insulating vessel, is stirred by means
of an electrical screw so regulated as to produce no appreciable
degree of heat mechanically. The combustion is started by fusing a
fine platinum wire of definite length and thickness by means of an
electric current. The heat value of this fusion is determined and the
calories produced deducted from the total calories of the combustion.
The valve admitting the oxygen is sealed automatically on breaking
connection with the oxygen cylinder. The effluent gases, at the end of
the combustion, may be withdrawn through an alkaline solution and any
nitric acid therein thus be fixed and determined.[579]
=564. Manipulation and Calculation.=—The material to be burned is
conveniently prepared by pressing it into tablets. The oxygen is
supplied from cylinders, of which two should be used, one at a pressure
of more than twenty atmospheres. By this arrangement a pump is not
required.
In practical use, a known weight of the substance to be burned is
placed in the platinum capsule, the cover of the bomb screwed on, after
all adjustments have been made, and the apparatus immersed in the water
contained in _M_, which should be about 2° below room temperature. All
the covers are placed in position and the temperature, of the water in
_M_ begins to rise. Readings of the thermometer are taken at intervals
of about one minute for six minutes, at which time the temperature of
the bomb and calorimetric water may be regarded as sensibly the same.
The electric current is turned on, the iron wire at once melts, ignites
the substance and the combustion rapidly takes place. In the case of
bodies which do not burn readily Atwater adds to them some naphthalene,
the thermal value of which is previously determined. The calories due
to the combustion of the added naphthalene are deducted from the total
calories obtained.
The temperature of the water in _M_ rises rapidly at first, and
readings are made at intervals of one minute for five minutes, and then
again after ten minutes. The first of the initial readings, the one at
the moment of turning on the current, and the last one mentioned above
are the data from which the correction, made necessary by the influence
of the temperature of the room, is calculated by the following
formulas.[580]
The preliminary readings of the thermometer at one minute intervals
are represented by _t_₁, _t_₂, _t_₃ ... _t_ₙ₁. The last observation
tₙ₁ is taken as the beginning temperature of the combustion and is
represented in the formulas for calculations by Θ₁. The readings after
combustion are also made at intervals of one minute, and are designated
by Θ₂, Θ₃ ... Θₙ. The readings are continued until there is no observed
change between the last two. Generally this is secured by five or six
readings.
The third period of observations begins with the last reading Θₙ, which
in the next series is represented by _tʹ_₁, _tʹ_₂ ... _tʹ_ₙ₂.