Pancreatic juice also has the power of coagulating milk, and is believed
to contain some rennet.
[Sidenote: Power of amylopsin]
Amylopsin, the starch-digesting enzym, appears to be very similar to
ptyalin in its power to digest carbohydrates. Amylopsin completes the
digestion of starch that was begun by the saliva. It acts on starch
with great activity. One part of amylopsin can change forty thousand
times its bulk of starch to glucose. This can act only in an alkaline
solution, and if any abnormal fermentation takes place in the digestive
tract, producing a large quantity of acids, the digestion of starch is
stopped. It is interesting to note that this enzym is entirely absent
from the pancreatic juice of infants. This explains why infants cannot
digest starch.
[Sidenote: Comparison of trypsin and pepsin]
The second enzym to be considered in the pancreatic juice is trypsin.
This is a substance distinct from pepsin, but its action is the same.
The chief distinction is that trypsin acts in an alkaline solution,
while pepsin acts in an acid solution. Trypsin is much more energetic in
its digestive power than the pepsin of the gastric juice. It completes
the digestion of proteids that is begun in the stomach, and converts
all proteids into soluble forms. A number of forms of proteid that are
not acted on at all by the gastric juice are readily digested by the
trypsin of the pancreatic juice.
[Sidenote: Fat digestion and absorption]
The fat-digesting enzym of the pancreatic juice is steapsin. This is the
principal fat-digesting enzym of the body. This substance has power to
split fats; that is, to convert them into fatty acids and glycerin of
which they were originally composed. This fatty acid then combines with
the alkalis of the bile and of the pancreatic juice to form soap. Soap
is soluble, and passes through the walls of the small intestines in this
form. Having passed through the walls of the intestines, soap is again
changed into fat. The probable reason for which Nature adopts such a
complex process for the absorption of fat, is because fat is insoluble.
If the intestinal walls were so constructed that fat-globules could be
taken directly through them, they would also be open for the entrance of
germs and other foreign substances.
[Sidenote: Frying fat unwholesome]
Fat is not acted on by the gastric juice. This explains why the process
of frying is so unwholesome. Frying causes a thin film of melted fat to
spread over the surface of the starch and of the proteid atoms, with the
result that these atoms cannot then be properly acted on by the saliva
and the gastric juice, and therefore cannot undergo the preliminary
changes necessary to normal digestion. Fat, taken in its natural form,
does not interfere with other digestive processes.