Gelatin is a protein derivative obtained by evaporating an aqueous extract made from bones, skins, and tendons of various domestic animals. Some important sources are: Ox, Bos taurus, and Sheep, Ovis aries belonging to family Bovidae
GELATIN
Synonyms
Gelfoam; puragel; gelatinum.
Biological Source
Gelatin is a protein derivative obtained by evaporating an
aqueous extract made from bones, skins, and tendons of various domestic
animals. Some important sources are: Ox, Bos
taurus, and Sheep, Ovis aries
belonging to family Bovidae
Preparation
The process of manufacture of gelatin vary from factory to
factory. However, the general outline of the process is given below.
Raw
material
Bones, skins, and tendons of Bovideans is collected and
subjected to liming operation.
Liming
Process
The raw material is first subjected to the treatment known
as ‘liming’. In this process, the skins and tendons are steeped for fifteen to
twenty and sometimes for 40 days in a dilute milk of lime. During this, fleshy
matter gets dis-solved, chondroproteins of connective tissues gets removed and
fatty matter is saponified. The animal skin is further thoroughly washed in
running water.
Defattying
In case of bones, the material is properly ground and
defatted in close iron cylinders by treatment with organic solvents such as
benzene. The mineral and inorganic part of the bone is removed by treatment
with hydrochloric acid.
Extraction
The treated material from bones, skins and tendons is boiled
with water in open pans with perforated false bottom. This process can also be
carried out under reduced pressure. The clear liquid runs of again and again
and is evaporated until it reaches to above 45 per cent gelatin content.
Setting
The concentrated gelatin extract is transferred to shallow
metal trays or trays with glass bottom. It is allowed to set as a semisolid
jelly.
Drying
The jelly is transferred to trays with a perforated wire
netting bottom and passed through series of drying compartments of 30–60°C
increasing each time with 10°C. About a month is taken for complete drying.
Bleaching
In case of darker colour, finished product is subjected to
bleaching by sulphur dioxide. Bleaching affords a light coloured gelatin.
Characteristics
Gelatin occurs as a colourless or slightly yellow,
transparent, brittle, practically odourless, tasteless sheet, flakes or course
granular powder. In water it swells and absorbs 5–10 times its weight of water
to form a gel in solutions below 35–40°C. It is insoluble in cold water and
organic solvents, soluble in hot water, glycerol, acetic acid; and is
amphoteric. In dry condition it is stable in air, but when moist or in
solution, it is attacked by bacteria. The gelatinizing property of Gelatin is
reduced by boiling for long time. The quality of gelatin is determined on the
basis of its jelly strength (Bloom strength) with the help of a Bloom
gelometer. Jelly strength is used in the preparation of suppositories and
pessaries.
Commercially two types of gelatin, A and B, are avail-able.
Type A has an isoelectric point between pH 7 and 9. It is incompatible with
anionic compounds such as Acacia, Agar and Tragacanth. Type B has an
isoelectric point between 4.7 and 5, and it is used with anionic mixtures.
Gelatin is coloured with a certified colour for manufacturing capsules or for
coating of tablets. It may contain various additives.
Chemical Constituents
Gelatin consists of the protein glutin which on hydrolysis
gives a mixture of amino acids. The approximate amino-acid contents are:
glycine (25.5%), alanine (8.7%), valine (2.5%), leucine (3.2%), isoleucine
(1.4%), cystine and cysteine (0.1%), methionine (1.0%), tyrosine (0.5%),
aspartic acid (6.6%), glutamic acid (11.4%), arginine (8.1%), lysine (4.1%),
and histidine (0.8%). Nutritionally, gelatin is an incomplete protein lacking
tryptophan. The gelatinizing compound is known as chondrin and the adhesive
nature of gelatin is due to the presence of glutin.
Chemical Tests
1. Biuret
reaction: To alkaline
solution of a protein (2 ml), a
dilute solution of copper sulphate is added. A red or violet colour is formed
with peptides containing at least two peptide linkages. A dipeptide does not
give this test.
2. Xanthoproteic
reaction: Proteins
usually form a yellow colour when
warmed with concentrated nitric acid. This colour becomes orange when the
solution is made alkaline.
3. Millon’s
reaction: Millon’s
reagent (mercuric nitrate in nitric
acid containing a trace of nitrous acid) usually yields a white precipitate on
addition to a protein solution which turns red on heating.
4. Ninhydrin
test: To an
aqueous solution of a protein an
alcoholic solution of ninhydrin is added and then heated. Red to violet colour
is formed.
5. On heating gelatin (1 g) with soda
lime, smell of ammonia is produced.
6. A solution of gelatin (0.5 g) in
water (10 ml) is precipitated to white buff coloured precipitate on addition
of few drops of tannic acid (10%).
7. With picric acid gelatin forms
yellow precipitate.
Uses
Gelatin is used to prepare pastilles, pastes, suppositories,
capsules, pill-coatings, gelatin sponge; as suspending agent, tablet binder,
coating agent, as stabilizer, thickener and texturizer in food; for
manufacturing rubber substitutes, adhesives, cements, lithographic and printing
inks, plastic compounds, artificial silk, photographic plates and films, light
filters for mercury lamps, clarifying agent, in hectographic matters, sizing
paper and textiles, for inhibiting crystallization in bacteriology, for preparing
cultures and as a nutrient.
It forms glycerinated gelatin with glycerin which is used as
vehicle and for manufacture of suppositories. Combined with zinc, it forms zinc
gelatin which is employed as a topical protectant. As a nutrient, Gelatin is used
as commercial food products and bacteriologic culture media.
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