Source of gelatin. Gelatin is obtained from collagen by hydrolysis.
Collagen is a protein and is found in connective tissue, tendons, cartilage,
in the organic part of bone tissue, in the inner layer of skin of animals,
and in fish skins, scales, and ''sounds" or fish bladders. Collagen is not found
as a pure substance in these tissues, but in combination with inorganic ma-
terials and with other proteins like elastin, mucin, mucoid, etc. White con-
nective tissue is found between the muscle fibers, in tendons, and in the
inner layers of skin. It contains a high percentage of collagen. The jelly-
ing of meat broth after cooling is due to the gelatin, which has been formed
from the collagen of the connective tissue. Sometimes the concentration
of the gelatin in the broth is not great enough to bring about the formation
of a gel. When pieces of baked fish skins are allowed to cool on the baking
pan, they adhere to it quite tenaciously, because of the adhesive quality
of the gelatin or glue formed from the collagen of the fish skin.
Distinction between Gelatin and Glue
Commercially there is little distinction between gelatins and glues, ex-
cept that edible gelatin is a high-grade product and complies with the Pure
Food Laws. It contains only traces of harmful ingredients, and Bogue
states that it should be made from ''such stock and by such sanitary
methods, as not to be objectionable from the ethical point of view." Glue
may be made from the same type of stock as the gelatin, but usually no
effort is made to remove impurities.
Manufacture of gelatin. In making edible gelatin the stock is or
should be selected with greater care than for inedible gelatin or glue. Some
edible gelatins are made entirely from bones. Others are made from calf-
skin and others from pork skin. The bones of young animals contain more
collagen than bones from older animals. For edible gelatin the bones are
178
ACIDS 179
washed to free them from dirt, then degreased by extraction of fat by fat
solvents. Next the soluble lime salts are removed by leaching the bones
with acid. The principal salts in bone are the calcium phosphates, but some
calcium carbonate and magnesium phosphate are also found. It is difficult
to free the bones from all the traces of inorganic matter, and the elec-
trolytes left influence the pH of the resulting gelatin. The degreased bone
with the inorganic salts removed is known as ossein and contains the col-
lagen from which the gelatin is made. The ossein is soaked in lime water.
In the lime water and in acid solution the collagen swells. This swelling
makes it possible to hydrolyze the collagen to gelatin at lower temperatures
than would otherwise be possible. This is important in the manufacture of
gelatin, for with higher temperatures or long heating the jellying strength
of the gelatin is lessened. The gelatin is then dried. The gelatin thus
obtained has some ash and is not at the isoelectric point, but on either the
acid or alkaline side of the isoelectric point. Putrefaction by bacteria must
be prevented during the manufacturing process, as the bacterial action
lowers the grade of gelatin obtained and odors develop that remain in
the finished gelatin even if putrefaction is later stopped.
Market types of gelatin. Gelatin is put upon the market in three
forms: granulated, pulverized, and sheet. The granulated is the form most
commonly obtained.
Page 144
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Source of gelatin. Gelatin is obtained from collagen by hydrolysis.
Collagen is a protein and is found in connective tissue, tendons, cartilage,
in the organic part of bone tissue, in the inner layer of skin of animals,
and in fish skins, scales, and ''sounds" or fish bladders. Collagen is not found
as a pure substance in these tissues, but in combination with inorganic ma-
terials and with other proteins like elastin, mucin, mucoid, etc. White con-
nective tissue is found between the muscle fibers, in tendons, and in the
inner layers of skin. It contains a high percentage of collagen. The jelly-
ing of meat broth after cooling is due to the gelatin, which has been formed
from the collagen of the connective tissue. Sometimes the concentration
of the gelatin in the broth is not great enough to bring about the formation
of a gel. When pieces of baked fish skins are allowed to cool on the baking
pan, they adhere to it quite tenaciously, because of the adhesive quality
of the gelatin or glue formed from the collagen of the fish skin.
Distinction between Gelatin and Glue
Commercially there is little distinction between gelatins and glues, ex-
cept that edible gelatin is a high-grade product and complies with the Pure
Food Laws. It contains only traces of harmful ingredients, and Bogue
states that it should be made from ''such stock and by such sanitary
methods, as not to be objectionable from the ethical point of view." Glue
may be made from the same type of stock as the gelatin, but usually no
effort is made to remove impurities.
Manufacture of gelatin. In making edible gelatin the stock is or
should be selected with greater care than for inedible gelatin or glue. Some
edible gelatins are made entirely from bones. Others are made from calf-
skin and others from pork skin. The bones of young animals contain more
collagen than bones from older animals. For edible gelatin the bones are
178
ACIDS 179
washed to free them from dirt, then degreased by extraction of fat by fat
solvents. Next the soluble lime salts are removed by leaching the bones
with acid. The principal salts in bone are the calcium phosphates, but some
calcium carbonate and magnesium phosphate are also found. It is difficult
to free the bones from all the traces of inorganic matter, and the elec-
trolytes left influence the pH of the resulting gelatin. The degreased bone
with the inorganic salts removed is known as ossein and contains the col-
lagen from which the gelatin is made. The ossein is soaked in lime water.
In the lime water and in acid solution the collagen swells. This swelling
makes it possible to hydrolyze the collagen to gelatin at lower temperatures
than would otherwise be possible. This is important in the manufacture of
gelatin, for with higher temperatures or long heating the jellying strength
of the gelatin is lessened. The gelatin is then dried. The gelatin thus
obtained has some ash and is not at the isoelectric point, but on either the
acid or alkaline side of the isoelectric point. Putrefaction by bacteria must
be prevented during the manufacturing process, as the bacterial action
lowers the grade of gelatin obtained and odors develop that remain in
the finished gelatin even if putrefaction is later stopped.
Market types of gelatin. Gelatin is put upon the market in three
forms: granulated, pulverized, and sheet. The granulated is the form most
commonly obtained.