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234 ANIMAL FATS AND PHOSPHATIDES.
only after vigorous and prolonged shaking, but the emulsion is not per-
sistent. The presence of some soap causes a very fine and permanent
emulsion to form easily. Fat produces spots on paper which do not
disappear; it is not volatile; it boils at about 300° C. with partial decom-
position, and burns with a luminous and smoky flame. The fatty acids
have most of the above-mentioned properties in common with the neutral
fats, but differ from them in being soluble in alcohol-ether, in having
an acid reaction, and by not giving the acrolein test. The neutral fats
generate a strong irritating vapor of acrolein, due to the decomposition
of glycerin, C3H5 (OH)3—2H2 = C2H3 .CHO, when heated alone, or
more easily when heated with potassium bisulphate or with other dehy-
drating substances.
The neutral fats may be split by the addition of the constituents of
water according to the following equation:
C3 H5 (OR) 3 4-3H2 = CVH5 (OH) 3 4-3HOR.
This splitting may be produced by the pancreatic enzjine and other
enzymes occurring in the animal and vegetable kingdoms, ’for example,
the castor lipase. The reverse action, namely, the synthesis of fatty acid
esters, can be brought about by enzymes, such as pancreatic lipase (see
page 60). The cleavage of the neutral fats can also be accomplished
by superheated steam or by dilute acids. We most frequently decompose
the neutral fats by boiling them with not too concentrated caustic alkali,
or. still better (in biochemical researches), with an alcoholic potash solu-
tion or with sodium alcoholate. By this procedure, which is called sapon-
ification, the alkali salts of the fatty acids (soaps) are formed. If the
saponification is made with lead oxide, then lead plaster, the lead salt of
the fatty acids is produced. By saponification is to be understood not
only the cleavage of neutral fats by alkalies, but also the splitting of neutral
fats into fatty acids and glycerin in general.
On keeping fats for a long time in contact with air they undergo a
change, becoming yellow in color and acid in reaction, and they develop
an unpleasant odor and taste, becoming rancid. In this change a part
of the fat is split into fatty acids and glycerin, and then an oxidation
of the free fatty acids takes place, producing volatile bodies of an
unpleasant odor.
The three most important fats of the animal kingdom are stearin,
valmitin, and olein.
CH2 .O.Ci8 H35
Stearin, or tristearin, CsyHnoOe^CH.O.CisHssO, occurs especially in
CH2 .O.CisH35
the solid varieties of tallow but also in the vegetable fats. Stearic acid,
C 18 H:jf;02 , is found in the free state in decomposed pus, in the expectora-
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