=1. Introduction.=—The analyst may approach the examination of
agricultural products from various directions. In the first place
he may desire to know their proximate and ultimate constitution
irrespective of their relations to the soil or to the food of man and
beast. Secondly, his study of these products may have reference solely
to the determination of the more valuable plant foods which they have
extracted from the soil and air. Lastly, he may approach his task from
a hygienic or economic standpoint for the purposes of determining the
wholesomeness or the nutritive and economic values of the products
of the field, orchard, or garden. In each case the object of the
investigation will have a considerable influence on the method of the
examination.
It will be the purpose of the present volume to discuss fully the
principles of all the standard processes of analysis and the best
practice thereof, to the end that the investigator or analyst, whatever
may be the design of his work, may find satisfactory directions for
prosecuting it. As in the previous volumes, it should be understood
that these pages are written largely for the teacher and the analyst
already skilled in the principles of analytical chemistry. Much is
therefore left to the individual judgment and experience of the worker,
to whom it is hoped a judicious choice of approved processes may be
made possible.
=2. Scope Of the Work.=—Under the term agricultural products is
included a large number of classes of bodies of most different
constitution. In general they are the products of vegetable and animal
metabolism. First of all come the vegetable products, fruits, grains
and grasses. These may be presented in their natural state, as cereals,
green fruits and fodders, or after a certain preparation, as starches,
sugars and flours. They may also be met with in even more advanced
stages of change, as cooked foods, alcohols and secondary organic
acids, such as vinegar. In general, by the term agricultural products
is meant not only the direct products of the farm, orchard and forest,
but also the modified products thereof and the results of manufacture
applied to the raw materials. Thus, not only the grain and straw of
wheat are proper materials for agricultural analysis, but also flour
and bran, bread and cakes made therefrom. In the case of maize and
barley, the manufactured products may extend much further, for not only
do we find starch and malt, but also alcohol and beer falling within
the scope of our work. In respect of animal products, the agricultural
analyst may be called on to investigate the subject of leather and
tanning; to determine the composition of meat, milk and butter; to
pass upon the character of lard, oleomargarine, and, in general, to
determine as fully as possible the course of animal food in all its
changes between the field, the packing house and the kitchen.
=3. Limitations of Work.=—It is evident from the preceding paragraph,
that in order to keep the magnitude of this volume within the limits
fixed for a single volume the text must be rigidly confined to the
fundamental principles and practice of agricultural analysis. The
interesting region of pharmacy and allied branches, in respect of
plant analysis, can find no description here, and in those branches
of technical chemistry, where the materials of elaboration are the
products of the field only a superficial view can be given. The main
purpose and motive of this volume must relate closely to the more
purely agricultural processes.
=4. General Manipulations.=—There are certain analytical operations
which are more or less of a general nature, that is, they are of
general application without reference to the character of the material
at hand. Among these may be mentioned the determination of moisture
and of ash, and the estimation of matters soluble in ether, alcohol and
other solvents. These processes will be first described. Preliminary to
these analytical steps it is of the utmost importance that the material
be properly prepared for examination. In general, this is accomplished
by drying the samples until they can be ground or crushed to a fine
powder, the attrition being continued until all the particles are made
to pass a sieve of a given fineness. The best sieve for this purpose
is one having circular apertures half a millimeter in diameter. Some
products, both vegetable and animal, require to be reduced to as fine
a state as possible without drying. In such instances, passing the
product through a sieve is obviously impracticable. Special grinding
and disintegrating machines are made for these purposes and they will
be described further on.
There are some agricultural products which have to be prepared for
examination in special ways and these methods will be given in
connection with the processes for analyzing the bodies referred
to. Nearly all the bodies, however, with which the analyst will be
concerned, can be prepared for examination by the general methods about
to be described.
=5. Preparation of the Sample.= (_a_) _Vegetable Substances._—For all
processes of analysis not executed on the fresh sample, substances of
a vegetable nature should, if in a fresh state, be dried as rapidly
as possible to prevent fermentative changes. It is often of interest
to determine the percentage of moisture in the fresh sample. For this
purpose a representative portion of the sample should be rapidly
reduced to as fine a condition as possible. To accomplish this it
should be passed through a shredding machine, or cut by scissors or a
knife into fine pieces. A few grams of the shredded material are dried
in a flat-bottomed dish at progressively increasing temperatures,
beginning at about 60° and ending at from 100° to 110°. The latter
temperature should be continued for only a short time. The principle of
this process is based upon the fact that if the temperature be raised
too high at first, some of the moisture in the interior cells of the
vegetable substance can be occluded by the too rapid desiccation of
the exterior layers which would take place at a high temperature. The
special processes for determining moisture will be given in another
place.
The rest of the sample should be partly dried at a lower temperature
or air-dried. In the case of fodders and most cattle foods the samples
come to the analyst in a naturally air-dried state. When grasses are
harvested at a time near their maturity they are sun-dried in the
meadows before placing in the stack or barn. Such sun-dried samples are
already in a state fit for grinding. Green grasses and fodders should
be dried in the sun, or in a bath at a low temperature from 50° to
60° until all danger of fermentative action is over, and then air- or
sun-dried in the usual way.
Seeds and cereals usually reach the analyst in a condition suited to
grinding without further preliminary preparation. Fruits and vegetables
present greater difficulties. Containing larger quantities of water,
and often considerable amounts of sugar, they are dried with greater
difficulty. The principles which should guide all processes of drying
are those already mentioned, _viz._, to secure a sufficient degree of
desiccation to permit of fine grinding and at a temperature high enough
to prevent fermentative action, and yet not sufficiently high to cause
any marked changes in the constituents of the vegetable organism.
(_b_) _Animal Substances._—The difficulties connected with the
preliminary treatment of animal substances are far greater than those
just mentioned. Such samples are composed of widely differing tissues,
blood, bone, tendon, muscle and adipose matters, and all the complex
components of the animal organism are to be considered. The whole
animal may be presented for analysis, in which case the different parts
composing it should be separated and weighed as exactly as possible.
Where only definite parts are to be examined it is best to separate the
muscle, bone, and fat as well as may be, before attempting to reduce
the whole to a fine powder. The soft portions of the sample are to be
ground as finely as possible in a meat or sausage cutter. The bones
are crushed in some appropriate manner, and thus prepared for further
examination. Where the flesh and softer portions are to be dried and
finely ground, the presence of fat often renders the process almost
impossible. In such cases the fat must be at least partially removed
by petroleum or other solvent. In practically fat-free samples the
material, after grinding in a meat cutter, can be partially dried at
low temperatures from 60° to 75°, and afterwards ground in much the
same manner as is practiced with vegetable substances.
As is the case with the preliminary treatment of vegetable matters,
it is impossible to give any general directions of universal
applicability. The tact and experience of the analyst in all these
cases are better than any dicta of the books. In some instances, as
will appear further on, definite directions for given substances can be
given, but in all cases the general principles of procedure are on the
lines already indicated.
=6. Preserving Samples.=—In most cases, as is indicated in the
foregoing paragraphs, the sample may be dried before grinding to such a
degree as to prevent danger from fermentation or decay. The fine-ground
samples are usually preserved in glass-stoppered bottles, carefully
marked or numbered. In some cases it is advisable to sterilize the
bottles after stoppering, by subjecting them to a temperature of 100°
for some time. In the case of cereals assurance should be had that the
samples do not contain the eggs of any of the pests that often destroy
these products. As a rule, samples should be kept for a time after the
completion of the analytical work, and this is especially true in all
cases where there is any prospect of dispute or litigation. In general
it may be said, that samples should be destroyed only when they are
spoiled, or when storage room is exhausted.
=7. Collecting Samples.=—When possible, the analyst should be his
own collector. There is often as much danger from data obtained on
non-representative samples as from imperfect manipulation. When
personal supervision is not possible, the sample when received,
should be accompanied by an intelligible description of the method of
taking it, and of what it represents. In all cases the object of the
examination must be kept steadily in view. Where comparisons are to be
made the methods of collecting must be rigidly the same.
The processes of analysis, as conducted with agricultural products,
are tedious and difficult. The absolutely definite conditions that
attend the analysis of mineral substances, are mostly lacking. The
simple determinations of carbon, hydrogen, nitrogen and sulfur,
which are required in the usual processes of organic analysis, are
simplicity itself when compared with the operations which have to be
performed on agricultural products to determine their character and
their value as food and raiment. We have to do here with matters on
which the sustenance, health and prosperity of the human race are more
intimately concerned than with any other of the sciences. This fact
also emphasizes the necessity for care in collecting the materials on
which the work is to be performed.
=8. Grinding Samples.=—In order to properly conduct the processes of
agricultural analysis it is important to have the sample finely ground.
This arises both from the fact that such a sample is apt to contain an
average content of the various complex substances of which the material
under examination is composed, and because the analytical processes can
be conducted with greater success upon the finely divided matter. In
mineral analysis it is customary to grind the sample to an impalpable
powder in an agate mortar. With agricultural products, however, such
a degree of fineness is difficult to attain, and moreover, is not
necessary. There is a great difference of opinion among analysts
respecting the degree of fineness desirable. In some cases we must be
content with a sample which will pass a sieve with a millimeter mesh;
in fact it may be found impossible, on account of the stickiness of the
material, to sift it at all. In such cases a thorough trituration, so
as to form a homogeneous mass will have to be accepted as sufficient.
Where bodies can be reduced to a powder however, it is best to pass
them through a sieve with circular perforations half a millimeter in
diameter. A finer degree of subdivision than this is rarely necessary.
=9. The Grinding Apparatus.=—The simplest form of apparatus for
reducing samples for analysis to a condition suited to passing a fine
sieve is a mortar. Where only a few samples are to be prepared and in
small quantities, it will not be necessary to provide anything further.
After the sample is well disintegrated it is poured on the sieve and
all that can pass is shaken or brushed through. The sieve is provided
with a receptacle, into which it fits closely, to avoid loss of any
particles which may be reduced to a dust. The top of the sieve, when
shaken, may also be covered if there be any tendency to loss from dust.
Any residue failing to pass the sieve is returned to the mortar and
the process thus repeated until all the material has been secured in
the receiver. The particles more difficult of pulverization are often
different in structure from the more easily pulverized portions, and
the sifted matter must always be carefully mixed before the subsample
is taken for examination. Often the materials, or portions thereof,
will contain particles tough and resistant to the pestle, but the
operator must have patience and persistence, for it is highly necessary
to accurate work that the whole sample be reduced to proper size.
[Illustration: FIGURE 1. MILL FOR GRINDING DRY SAMPLES.]
Where many samples are to be prepared, or in large quantities, mills
should take the place of mortars. For properly air-dried vegetable
substances, some form of mill used in grinding drugs may be employed.
Grinding surfaces of chilled corrugated steel are to be preferred.
The essential features of such a mill are that it be made of the best
material, properly tempered, and that the parts be easily separated
for convenience in cleaning. The grinding surfaces must also be so
constructed and adjusted as to secure the proper degree of fineness.
In fig. 1 is shown a mill of rather simple construction, which has
long been in satisfactory use in this laboratory. Small mills may
be operated by hand power, but when they are to be used constantly
steam power should be provided. In addition to the removal of nearly
all the moisture by air-drying there are many oleaginous seeds which
cannot be finely ground until their oil has been removed. For this
purpose the grinding surfaces of the mill are opened so that the seeds
are only coarsely broken in passing through. The fragments are then
digested with light petroleum in a large flask, furnished with a reflux
condenser. After digestion the fragments are again passed through the
mill adjusted to break them into finer particles.
The alternate grinding and digestion are thus continued until the
pulverization is complete. On a small specially prepared sample the
total content of oil is separately determined.
Fresh animal tissues are best prepared for preliminary treatment by
passing through a sausage mill. The partially homogeneous mass thus
secured should be dried at a low temperature and reground as finely as
possible. Where much fat is present it may be necessary to extract it
as mentioned above, in the case of oleaginous seeds. In such cases both
the moisture and fat in the original material should be determined on
small specially prepared samples with as great accuracy as possible.
Bones, hoofs, horns, hair and hides present special difficulties in
preparation, which the analyst will have to overcome with such skill
and ingenuity as he may possess.
The analyst will find many specially prepared animal foods already in
a fairly homogeneous form, such as potted and canned meats, infant
and invalid foods, and the like. Even with these substances, however,
a preliminary grinding and mixing will be found of advantage before
undertaking the analytical work. Many cases will arise which are
apparently entirely without the classification given above. But even in
such instances the analyst should not be without resources. Frequently
some dry inert substance may be mixed with the material in definite
quantities, whereby it is rendered more easily prepared. Perhaps no
case will be presented where persistent and judicious efforts to secure
a fairly homogeneous sample for analysis will be wholly unavailing.
[Illustration: FIGURE 2. COMMINUTOR FOR GREEN SAMPLES.]
In the case of green vegetable matters which require to be reduced
rapidly to a fine state of subdivision in order to secure even a
fairly good sample some special provision must be made. This is
the case with stalks of maize and sugar cane, root crops, such as
potatoes and beets, and green fodders, such as clover and grasses.
The chopping of these bodies into fine fodders by hand is slow and
often impracticable. The particles rapidly lose moisture and it is
important to secure them promptly as in the preparation of beet pulp
for polarization. For general use we have found the apparatus shown
in fig. 2 quite satisfactory in this laboratory. It consists of a
series of staggered circular saws carried on an axis and geared to be
driven at a high velocity, in the case mentioned, 1,400 revolutions
per minute. The green material is fed against the revolving saws by
the toothed gear-work shown, and is thus reduced to a very fine pulp,
which is received in the box below. Stalks of maize, green fodders,
sugar canes, beets and other fresh vegetable matters are by this
process reduced to a fine homogeneous pulp, suited for sampling and for
analytical operations. Such pulped material can also be spread in a
fine layer and dried rapidly at a low temperature, thus avoiding danger
of fermentative changes when it is desired to secure the materials in
a dry condition or to preserve them for future examination. Samples of
sorghum cane, thus pulped and dried, have been preserved for many years
with their sugar content unchanged.
[Illustration: FIGURE 3. RASP FOR SUGAR BEETS.]
Such a machine is also useful in preparing vegetable matter for the
separation of its juices in presses. Samples of sugar cane, sugar
beets, apples and other bodies of like nature can thus be prepared to
secure their juices for chemical examination. Such an apparatus we
have found is fully as useful and indispensable in an agricultural
laboratory as a drug mill for air-dried materials.
It is often desirable in the preparation of roots for sugar analysis
to secure them in a completely disintegrated state, that is with the
cellular tissues practically all broken. Such a pulped material can
be treated with water and the sugar juices it contains thus at once
distributed to all parts of the liquid mass. The operation is known
as instantaneous diffusion. The pulp of the vegetable matter is thus
introduced into the measuring flask along with the juices and the
content of sugar can be easily determined. Several forms of apparatus
have been devised for this purpose, one of which is shown in fig.
3. This process, originally devised by Pellet, has come into quite
general use in the determination of the sugar content of beets.[1]
It is observed that it can be applied to other tubers, such as the
turnip, potato, artichoke, etc. It is desirable, therefore, that
an agricultural laboratory be equipped with at least three kinds
of grinding machines; _viz._, first, the common drug mill used for
grinding seeds, air-dried fodders, and the like; second, a pulping
machine like the system of staggered saws above described for the
purpose of reducing green vegetable matter to a fine state of
subdivision, or one like the pellet rasp for tubers; third, a mill for
general use such as is employed for making sausages from soft animal
tissues.
[Illustration: FIGURE 4. DREEF GRINDING APPARATUS.]
=10. Grinding Apparatus at Halle Station.=—The machine used at the
Halle station for grinding samples for analysis is shown in Fig. 4.[2]
It is so adjusted as to have both the upper and lower grinding surfaces
in motion. The power is transmitted through the pulley D, which is
fixed to an axis carrying also the inner grinding attachment B. Through
C₂, C₃, C₄, and C₁, the reverse motion is transmitted to the outer
grinder A. By means of the lever E the two grinding surfaces can be
separated when the mill is to be cleaned. The dree mill above described
is especially useful for grinding malt, dry brewers’ grains, cereals
for starch determinations and similar dry bodies. It is not suited to
grinding oily seeds and moist samples. These, according to the Halle
methods, are rubbed up in a mortar until of a size suited to analysis,
and samples such as moist residues, wet cereals, mashes, beet cuttings,
silage, etc., are dried before grinding. If it be desired to avoid the
loss of acids which may have been formed during fermentation, about ten
grams of magnesia should be thoroughly incorporated with each kilogram
of the material before drying.
=11. Preliminary Treatment of Fish.=—The method used by Atwater in
preparing fish for analysis is given below.[3] The same process may
also be found applicable in the preparation of other animal tissues.
The specimens, when received at the laboratory, are at once weighed.
The flesh is then separated from the refuse and both are weighed. There
is always a slight loss in the separation, due to evaporation and to
slimy and fatty matters and small fragments of the tissues which adhere
to the hands and the utensils employed in preparing the sample. Perfect
separation of the flesh from the other parts of the fish is difficult,
but the loss resulting from imperfect separation is small. The skin
of the fish, although it has considerable nutritive value, should be
separated with the other refuse.
The partial drying of the flesh for securing samples for analytical
work is accomplished by chopping it as finely as possible and
subjecting from fifty to one hundred grams of it for a day to a
temperature of 96° in an atmosphere of hydrogen. After cooling and
allowing to stand in the open air for twelve hours, the sample is again
weighed, and then ground to a fine powder and made to pass a sieve
with a half millimeter mesh. If the samples be very fat they cannot
be ground to pass so fine a sieve. In such a case a coarser sieve may
be used or the sample reduced to as fine and homogeneous a state as
possible, and bottled without sifting.
The reason for drying in hydrogen is to prevent oxidation of the fats.
As will be seen further on, however, such bodies can be quickly and
accurately dried at low temperatures in a vacuum, and thus all danger
of oxidation be avoided. In fact, the preliminary drying of all animal
and vegetable tissues, where oxidation is to be feared, can be safely
accomplished in a partial vacuum by methods to be described in another
place. In order to be able to calculate the data of the analysis to the
original fresh state of the substance, a portion of the fresh material
should have its water quantitively determined as accurately as possible.