a firm, resilient gel that does not melt below
85 C. This ability to gel at a much lower
temperature than the melting temperature of
the gel, commonly called hysteresis lag, is
uniquely long in agar, and many of its uses
depend on this property. Agar gels also have
the property of shrinking and exuding water
from their surface (syneresis), particularly
when broken. The gel strength of agar can
be increased by addition of dextrose, sucrose,
and locust bean gum, while it tends to weak-
en with gelatin, algin, starch, and karaya
gum. The colorless, tasteless powder can
absorb up to 200 times its volume of water
when forming a gel.
Agar solutions have low viscosity; their
degree of clarity and color (yellowish to col-
orless) depend on the quality and source of the
agar, as do their gel strength, gelling tempera-
ture, and the degree of syneresis. Quality is
largely affected by extraction procedures.
Physical and rheological properties of agar
that provide the greatest determinations of
quality are the average molecular weight and
molecular weight distribution.^1
Agar is insoluble in organic solvents and is
precipitated from aqueous solution by alcohol
and tannin.
CHEMICAL COMPOSITION
The structure of agar is still not fully deter-
mined, the problem being complicated by the
large number of commercial sources of agar.
It is generally believed that all agars consist
of two major polysaccharides (neutral aga-
rose and charged agaropectin), although sev-
eral studies have indicated a much more
complicated structure.2–6Agarose is the gel-
ling fraction and agaropectin is the nongel-
ling fraction. Both are composed of a linear
chain of alternatingb-D-galactopyranose and
3,6-anhydro-a-L-galactopyranose residues,
with agaropectin having a higher proportion
of uronic acid, sulfate, and pyruvic acid resi-
dues.^6 Commercial agar may contain free ami-
no acids (arginine,aspartic acid, glutamic acid,
and threonine) and free sugars (galactose and
gluconic acid).^7 It may also contain other sugar
residues including 4-O-methyl-L-galactose,
6-O-methyl-D-galactose,8–10D-xylose, andO-
methylpentose, as well as boric acid (approxi-
mately 0.1%)^11 and various inorganic cations
(Naþ,Kþ,Ca^2 þ,Mg^2 þ, etc.).^12PHARMACOLOGY AND BIOLOGICAL
ACTIVITIESAgar is nontoxic and can be ingested in large
doses without much distress. It passes through
the intestinal tract mostly unabsorbed.^13 Agar
has shownin vitroantipeptic activities^14 and
the results of a study in rats suggested that it
may elevate serum or tissue cholesterol
levels.^15TOXICOLOGYMice fed agar showed significantly more
colon tumors per animal (twice as many) as
those fed diets without agar. Agar-fed animals
also showed decreased levels of fecal neutral
sterol and bile acid concentrations.^13 When
fed to rats at 5% and 15% levels of the diet,
agar impaired protein utilization.^16USESMedicinal, Pharmaceutical, and Cosmetic.
As a bulk laxative, particularly in chronic
constipation; in the manufacture of emulsions,
suspensions, gels, and hydrophilic supposito-
ries; in dentistry as basic constituent of revers-
ible impression and duplicating materials.Food. Used in canned meat and fish products
as gel filler or gel binder; in baked goods
(icings and glazes); and in confectionery,
dairy products, processed fruits, sweet sauces,
and reconstituted vegetables, among others.
Highest average maximum usage level usually
about 0.4% in baked goods.
Agar has been used as a food in the Far East
for centuries.10 Agar