FEEDS 145
until the weight is constant. After the sample has cooled in a desiccator
ifc should be weighed as rapidly as possible in order to avoid undue exposure
to moist air. Preserve the dried sample for the crude fat determination
(page 147), or for the ash determination, described below.
At Room Temperature.—Place 2-gm samples in separate 6-inch vacuum
desiccators (see Fig. 8, page 28) containing 200 cc of fresh concentrated
sulphuric acid and exhaust to a pressure of 1 mm by means of a
pump. It will require about four to six days drying to secure constant
weights. The desiccators should be rotated gently several times a day in
order to mix the lower, more concentrated sulphuric acid and the upper
layers that have become diluted by absorbed moisture. After 24 hours
drying, carefully open the desiccator and weigh the sample. Place in a
desiccator containing fresh sulphuric acid and repeat the process of drying
and weighing until the weight becomes constant. Calculate the total loss
as moisture. Preserve the dried sample for the crude fat determination
(page 147), or for the ash determination, described below.
Ash.—The ash determination requires much patience. The
high carbon contained in oily seeds is very hard to oxidize so as
to secure a white or gray ash but too high a temperature will
cause volatilization of certain ash constituents, such as chlorides
of the alkali metals. The ash should contain the mineral
compounds (such as calcium phosphate, potassium or sodium
chloride and some silicon compounds) of the plant tissues and
sap. Some phosphorus and sulphur may be present as part of
the protein molecule and these may be volatilized but they are
not properly to be considered as part of the ash unless they
are normally left upon burning (as phosphates or sulphates).
This is usually the case in grasses and leaves but not in seeds.
Determination of Ash.—Either the dried sample obtained in the moisture
test or a new undried sample may be used for this determination. Ignite,
cool and weigh a porcelain crucible, brush in the weighed sample and burn
at a low temperature, using a burner, or place the uncovered crucible in
a muffle furnace heated to about 700°. The crucible should be kept at
dull redness until the carbon is all consumed and the ash becomes nearly
white. A gray or black appearance of the residue indicates the presence of
unburned carbon but a red tint may be given by iron oxide normally present.
Cool in a desiccator, weigh and calculate the per cent of ash.
Mineral Analysis.—The solution of the ash in hydrochloric
acid is diluted to 250 cc and the mineral constituents determined
as described under soil analysis, beginning on page 256. This
analysis involves a considerable expenditure of time and it is
rarely useful, except in the solution of certain research problems.