[Sidenote: Alternation of digestive juices]
The digestive juices of the human body are five in number, namely:
Saliva, gastric juice, bile, pancreatic juice, and the several
intestinal juices. Beginning with the saliva these juices alternate,
first an alkali, then an acid. It is the opinion of the writer that this
alternating plan is carried on throughout the entire intestinal tract,
as the final dissolution of food matter takes place in the intestinal
canal. These five juices are secreted from the blood by special cells
or glands. Each of these juices contain one or more enzyms or digestive
principles. These enzyms are highly organized chemical compounds which
have the property of changing other chemical compounds without being
destroyed or used up themselves except in minute quantities.
[Sidenote: Malt and yeast-cells]
Malt, which was studied in the last lesson, and which is produced by
the sprouting of barley, is a true digestive enzym of the barley.
Yeast-cells are minute plants which secrete an enzym that causes the
fermentation of bread. It was formerly thought that the fermentation of
yeast could not take place except in the presence of a living cell. This
has now been disproved, as a German scientist has succeeded in grinding
up yeast-cells and filtering off the chemical compound or true enzym
which causes the fermentation of sugar.
[Sidenote: Fermentation due to enzyms]
It is now recognized by scientists that all processes of fermentation
and digestion found in plant and animal life are due to definite
chemical compounds known as enzyms. The action of digestion is truly a
chemical one, and could take place without the body as well as within,
if we could manufacture the proper enzym and could produce the exact
conditions of temperature, moisture, etc., that are found in the human
digestive economy.
[Sidenote: Predigested foods]
The manufacture of predigested foods depends upon various processes of
fermentation, or upon the digestion that may be carried on by inorganic
chemical agents, such as acids, or by the ferments of bacteria, or other
forms of life. The following are illustrations of these processes of
predigestion:
1 The manufacture of glucose from starch by the action of
sulfuric acid
2 The malting of starch for the production of malt-sugar or of
fermented liquors
3 The making of cheese by the action of the enzym rennet which
has been extracted from the stomach of a calf
A great amount of discussion, pro and con, has been raised over the
subject of predigested food. The foregoing examples will show that the
subject of predigested food, taken in its broadest sense, cannot be
dismissed summarily with either approbation or disapproval. We must
consider the particular chemical process involved in each case and the
final chemical products, as well as its mechanical condition. These
things must be taken into consideration when we pass an opinion upon the
wholesomeness of a so-called predigested food.
With this diversion to illustrate the breadth and the importance of the
action of enzyms, I will now return to the consideration of the chemical
action of the human digestive organs.