Agar

The title of this article is ambiguous. Further meanings are listed under Agar (disambiguation) .

Agar (from the Indonesian / Malay ), and agar-agar , Agartang , Japanese isinglass , edges ( Japanese 寒天 ), or Chinese or Japanese gelatin called, is a galactose - polymer (a polysaccharide ), the curd to form. The basic units of agar are agarose and sulfated agaropectin .

Agar is made from the cell walls of some types of algae (especially red algae such as Gracilaria , Gelidiopsis , Gelidium , Hypnea and Sphaerococcus species), mainly from East Asia .

properties

Agar is tasteless and indigestible. It is a very good gelling agent that is independent of the surrounding medium. Even a concentration of 1%, dissolved in hot water, is sufficient for an elastic, sufficiently mechanically stable, but not too firm gel with a very thin layer of aqueous liquid on the surface (formed by syneresis ). Half a teaspoon is about as effective as four sheets of gelatin .

use

Agar plates in Petri dishes as a culture medium for microbiology

Pure culture of the plant Physcomitrella patens on an agar plate (in a Petri dish , 9 cm diameter)

In food

Agar is used as a thickener in soups, confectionery, and ice cream. In the EU it is approved as a food additive under number E 406 . Due to the relatively high price, however, it is used relatively rarely. In the household it can be used as a vegan substitute for gelatine (obtained from animal body residues). Since syneresis is usually extremely undesirable, the addition of locust bean gum is recommended . In Japan and China, agar has been used for the preparation of food since the 17th century. B. Tokoroten noodles. In Southeast Asia it is an integral part of the kitchen. It is used regionally for the preparation of various desserts because gelatine is unknown.

In the life sciences

The German microbiologist Walther Hesse discovered in 1881 the advantages of agar culture media over those with gelatine for the cultivation of bacteria. He published this in 1884. The proposal came from his wife, Fanny Angelina Hesse , who used agar to make fruit jelly and vegetable jelly. Robert Koch , whom Hesse informed immediately of his discovery, then succeeded in discovering the tubercle bacillus , which he reported on March 24, 1882.

In microbiology , nutrient media for microorganisms are almost always solidified with agar. This is because agar is more resistant to high temperature sterilization than gelatine gels, which become liquid at higher incubation temperatures. Agar gel becomes liquid at 95 ° C and solidifies at 45 ° C. This means that it is possible to add thermally unstable substances before they solidify. Since some microorganisms can digest gelatin, this is another reason to use agar.

The thin layer of aqueous liquid ( syneresis ) that forms on the gel surface makes it easier for solid objects to slide on the surface. This also facilitates the even distribution of material with a spatula or an inoculation loop .

In molecular biology , agarose is used as a matrix for the separation of nucleic acids by means of gel electrophoresis .

Agar in gel form is also used as a substrate for various plants to be grown in laboratories. In comparison to other gelling agents such as Gelrite, for example, these agents are by no means inert during cultivation , but rather influence the physiology of plant cell cultures, for example .

In the medicine

Due to its indigestibility, agar is used as a laxative (in a higher dose than in food) . In Indonesia, agar-agar is said to have positive effects against diabetes mellitus and heart diseases .

For packaging

In April 2016 the designer collective AMAM received the Lexus Design Award for the Agar Plasticity project at the Salone del Mobile in Milan . Porous, freeze-dried agar gel is used as padding - in pieces or as a molded part - in shipping packaging for fragile goods and can thus replace plastics such as bubble film, which are undesirable in the “ocean biosphere”.

Sources of supply

Agar is available in large supermarkets , health food stores , health food stores and Asian grocery stores. Agar in its pure form can be purchased in pharmacies .

Alternatives

Other gelling agents obtained from red algae are carrageenan and furcellaran . The latter is also known as Danish agar or Baltic agar.

Individual evidence

1. ↑ General Overview of Food Hydrocolloids . In: Wüstenberg, T. (Ed.): Cellulose and Cellulose Derivatives in the Food Industry: Fundamentals and Applications . Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2014, p. 4 .
2. ↑ Heavenly Sweets: Tokoroten Recipe for Tokoroten, accessed July 20, 2014.
3. ↑ W. Hesse: About the quantitative determination of the microorganisms contained in the air. In: Messages from the Imperial Health Office . Vol. 2, 1884, pp. 182-207.
4. ↑ AP Hitchens, MC Leikind (1939): The Introduction of agar-agar into Bacteriology. In: J. Bacteriol. Vol. 37, No. 5, 1939, pp. 485-493, PMID 16560221 PDF .
5. ↑ Georg Räschemeyer Mrs. Hesse's secret recipe in mare no. 140, June / July 2020, p. 84 ff
6. ↑ Birgit Hadeler, Sirkka Scholz, Ralf Reski : Gelrite and agar differently influence cytokinin-sensitivity of a moss . In: Journal of Plant Physiology Vol. 146, 1995, pp. 369-371.
7. ↑ http://orf.at/stories/2334147/2334146/ Competition for the pop film: Algae conquer the packaging industry, orf.at, April 30, 2016, accessed May 1, 2016.