FEEDS

149

of boiling sodium hydroxide, make the volume up to 425 cc and boil for
30 minutes. Filter through an alundum cup and wash with hot water
until neutral to phenolphthalein. Dry the residue in an oven for 3 hours at
110°, place in a desiccator, cool and weigh. Ignite as in the ash deter-
mination, cool and weigh again. The loss in weight represents fiber. Cal-
culate the per cent of crude fiber in the sample.
Crude Protein.—"Crude protein/' is a conventional term,
embracing all forms of plant nitrogenous bodies except nitrates.
The latter are not usually found in feeding stuffs. There is no
good direct method for determining protein and either pure or
" crude" protein is calculated from the per cent of nitrogen.
Since plant proteins contain about 16 per cent of nitrogen, the
nitrogen per cent is multiplied by 6.25 (•= ~r^~) to convert it to
the approximate corresponding per cent of protein. The nitrogen
content varies for proteins of different classes. In the deter-
mination of milk protein 6.38 is the factor used, but nitrogen
found in grasses and fruits is partly in the form of amids and has a
lower conversion factor because of somewhat higher per cent.
Nitrogen.—From the above discussion it will be seen that the
determination of protein rests upon the nitrogen determination.
The Kjeldahl process for this determination consists in digesting
the organic material with boiling concentrated sulphuric acid
until complete decomposition has been effected. The exact
course of the reactions cannot be traced but the carbon and
hydrogen are completely oxidized and nitrogen is converted into
ammonia, which immediately combines with sulphuric acid and
remains as ammonium sulphate. The completion of decom-
position is insured by the final addition of a small amount of
potassium permanganate. The solution is then diluted with
water, an excess of sodium hydroxide is added and the resultant"
ammonia is distilled into a measured quantity of standard acid
solution, the excess of which is then titrated by a standard base.
Digestion.—The digestion with sulphuric acid is best accom-
plished in a pear-shaped flask with a long neck, like that shown in
Fig. 43. The concentrated sulphuric acid of commerce boils at
temperatures ranging from 210° to 340°, according to the per-
cent of water contained in it. Such a temperature is high enough
above that of the surrounding air to permit condensation of