Agar or agar-agar is a gelatinous substance, obtained from algae and discovered in the
late 1650s or early 1660s by Minoya Tarozaemon in Japan, where it is called Kanten.
Agar is derived from the polysaccharide agarose, which forms the supporting structure in the
cell walls of certain species of algae, and which is released on boiling. These algae
are known as agarophytes and belong to the Rhodophyta phylum. Agar is actually the resulting
mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller
molecules called agaropectin. Throughout history into modern times, agar
has been chiefly used as an ingredient in desserts throughout Asia and also as a solid
substrate to contain culture media for microbiological work. Agar can be used as a laxative, an appetite
suppressant, vegetarian gelatin substitute, a thickener for soups, in fruit preserves,
ice cream, and other desserts, as a clarifying agent in brewing, and for sizing paper and
fabrics. The gelling agent in agar is an unbranched
polysaccharide obtained from the cell walls of some species of red algae, primarily from
the genera Gelidium and Gracilaria. For commercial purposes, it is derived primarily from Gelidium
amansii. In chemical terms, agar is a polymer made up of subunits of the sugar galactose.
Background
Agar was discovered around 1658 by Minoya Tarozaemon in Japan), an innkeeper who was
said to have discarded some surplus seaweed soup and noticed that it gelled later after
a winter night's freezing. Agar was first used in microbiology in 1882
by the German microbiologist Walther Hesse, an assistant working in Robert Koch's laboratory,
on the suggestion of his wife Angelina Fannie Eilshemius Hesse. He discovered that it was
more useful as a solidifying agent than gelatin, due to its better solidifying temperature.
Agar was first subjected to chemical analysis in 1859 by the French chemist Anselme Payen,
who had obtained agar from the marine algae Gelidium corneum.
Agar consists of a mixture of agarose and agaropectin. Agarose, the predominant component
of agar, is a linear polymer, made up of the repeating monomeric unit of agarobiose. Agarobiose
is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agaropectin
is a heterogeneous mixture of smaller molecules that occur in lesser amounts, and is made
up of alternating units of D-galactose and L-galactose heavily modified with acidic side-groups,
such as sulfate and pyruvate. Agar exhibits hysteresis, melting at 85 °C
and solidifying from 32-40 °C. This property lends a suitable balance between easy melting
and good gel stability at relatively high temperatures. Since many scientific applications
require incubation at temperatures close to human body temperature, agar is more appropriate
than other solidifying agents that melt at this temperature, such as gelatin.
The word "agar" comes from agar-agar, the Malay/Indonesian name for red algae from which
the jelly is produced. It is also known as Kanten, Japanese isinglass, Ceylon moss or
Jaffna moss. Gracilaria lichenoides is specifically referred to as agal-agal or Ceylon agar.
Uses Microbiology
An agar plate or Petri dish is used to provide a growth medium using a mix of agar and other
nutrients in which microorganisms, including bacteria and fungi, can be cultured and observed
under the microscope. Agar is indigestible for many organisms so that microbial growth
does not affect the gel used and it remains stable. Agar is typically sold commercially
as a powder that can be mixed with water and prepared similarly to gelatin before use as
a growth medium. Other ingredients are added to the agar to meet the nutritional needs
of the microbes. Many specific formulations are available, because some microbes prefer
certain environmental conditions over others. Agar is often dispensed using a sterile media
dispenser. Motility assays
As a gel, an agar or agarose medium is porous and therefore can be used to measure microorganism
motility and mobility. The gel's porosity is directly related to the concentration of
agarose in the medium, so various levels of effective viscosity can be selected, depending
on the experimental objectives. A common identification assay involves culturing
a sample of the organism deep within a block of nutrient agar. Cells will attempt to grow
within the gel structure. Motile species will be able to migrate, albeit slowly, throughout
the gel and infiltration rates can then be visualized, whereas non-motile species will
show growth only along the now-empty path introduced by the invasive initial sample
deposition. Another setup commonly used for measuring
chemotaxis and chemokinesis utilizes the under-agarose cell migration assay, whereby a layer of agarose
gel is placed between a cell population and a chemoattractant. As a concentration gradient
develops from the diffusion of the chemoattractant into the gel, various cell populations requiring
different stimulation levels to migrate can then be visualized over time using microphotography
as they tunnel upward through the gel against gravity along the gradient.
Plant biology
Research grade agar is used extensively in plant biology as it is supplemented with a
nutrient and vitamin mixture that allows for seedling germination in Petri dishes under
sterile conditions. Nutrient and vitamin supplementation for Arabidopsis thaliana is standard across
most experimental conditions. Murashige & Skoog nutrient mix and Gamborg's B5 vitamin mix
in general are used. A 1.0% agar/0.44% MS+vitamin dH2O solution is suitable for growth media
between normal growth temps. The solidification of the agar within any
growth media is pH-dependent, with an optimal range between 5.4-5.7. Usually, the application
of KOH is needed to increase the pH to this range. A general guideline is about 600 µl
0.1M KOH per 250 ml GM. This entire mixture can be sterilized using the liquid cycle of
an autoclave. This medium nicely lends itself to the application
of specific concentrations of phytohormones etc. to induce specific growth patterns in
that one can easily prepare a solution containing the desired amount of hormone, add it to the
known volume of GM, and autoclave to both sterilize and evaporate off any solvent that
may have been used to dissolve the often-polar hormones. This hormone/GM solution can be
spread across the surface of Petri dishes sown with germinated and/or etiolated seedlings.
Experiments with the moss Physcomitrella patens, however, have shown that choice of the gelling
agent — agar or Gelrite - does influence phytohormone sensitivity of the plant cell
culture. Culinary
Agar-agar is a natural vegetable gelatin counterpart. White and semi-translucent, it is sold in
packages as washed and dried strips or in powdered form. It can be used to make jellies,
puddings, and custards. For making jelly, it is boiled in water until the solids dissolve.
Sweetener, flavouring, colouring, fruit or vegetables are then added and the liquid is
poured into molds to be served as desserts and vegetable aspics, or incorporated with
other desserts, such as a jelly layer in a cake.
Agar-agar is approximately 80% fiber, so it can serve as an intestinal regulator. Its
bulk quality is behind one of the latest fad diets in Asia, the kanten diet. Once ingested,
kanten triples in size and absorbs water. This results in the consumers feeling more
full. This diet has recently received some press coverage in the United States as well.
The diet has shown promise in obesity studies. One use of agar in Japanese cuisine is anmitsu,
a dessert made of small cubes of agar jelly and served in a bowl with various fruits or
other ingredients. It is also the main ingredient in mizu yōkan, another popular Japanese food.
In Philippine cuisine, it is used to make the jelly bars in the various gulaman refreshments
or desserts such as sago gulaman, buko pandan, agar flan, halo-halo, and the black and red
gulaman used in various fruit salads. In Vietnamese cuisine, jellies made of flavored
layers of agar agar, called thạch, are a popular dessert, and are often made in ornate
molds for special occasions. In Indian cuisine, agar agar is known as "China grass" and is
used for making desserts. In Burmese cuisine, a sweet jelly known as kyauk kyaw is made
from agar. In Russia, it is used in addition or as a
replacement to pectin in jams and marmalades, as a substitute to gelatin for its superior
gelling properties, and as a strengthening ingredient in souffles and custards. Another
use of agar-agar is in ptich'ye moloko, a rich jellified custard used as a cake filling
or chocolate-glazed as individual sweets. Agar-agar may also be used as the gelling
agent in gel clarification, a culinary technique used to clarify stocks, sauces, and other
liquids. Other uses
Agar is used: As an impression material in dentistry.
To make salt bridges for use in electrochemistry. In formicariums as a transparent substitute
for sand and a source of nutrition. As a natural ingredient to form modelling
clay for young children to play with. Gelidium agar is used primarily for bacteriological
plates. Gracilaria agar is used mainly in food applications.
See also
References
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