Polysaccharides. Группа авторов

      5

      Agars: Properties and Applications

       Sudhakar Padmesh and Aditi Singh*

       Amity Institute of Biotechnology, Amity University Uttar Pradesh, Lucknow Campus, Lucknow, India

       Abstract

      The roots of origination of agar belong to Japan. Agar was firstly known to come into the use in 1658. It was extracted from Gracilaria, an agarophyte seaweed which was used in food stuff back then. Agar was firstly introduced in far-east countries and later it was taken to the rest of the agarophyte producing countries. After so many uses of agar were introduced in Japan, it started to attract European countries and was introduced there in 1859 and then its use in microbiology study started in 1882. This comprehensive review describes the history of agar and uses of its seaweed worldwide, the production and origination of agar, industrial, biological uses as well as uses of agar in food industry. A substance made up of replicated molecules of repeated agarobiose units, is called agar. However agarose is a least substituted agar molecule with the highest gelling potential. The family of agar algae and its polysaccharide is dynamic. Physico-chemical and chemical properties defines the agar population through-out the algal families, and changes with the change in physicological, cellular and environmental grounds. Thus modifications in biochemistry of this molecule should be explicated. The chemical and molecular structure of agar, its fractions like agarose and agaropectin, the hydrogen bridges formed in its gelling formation, as well as a comparative study of types of agar derived from different seaweeds along with the different types of commercially produced agar food products will be discussed in this chapter.

      Keywords: Gracillaria, agar, agaropectin, agarophyte, seaweed, hydrogen bridges

5.1 History and Origin of Agar

The first known use of agar was in Japan in 1658, when it was used as a food gelling material. Japanese used to extract agar from Gracilaria, by freezing and thawing method which was totally dependent upon the weather back then. Tarazaemon Minoya of Japan is generally considered as the discoverer of agar in 1963. Later its use extended during the seventeenth and eighteenth centuries. Agar was being used a lot before than other phycocolloids, like alginates or carrageens. These phycocolloids came into use almost 200 years later. In year 1859 Payen introduced agar in Europe as a food stuff and Koch introduced its microbiological uses in 1882 right before he introduced Koch Pastulates in 1884. Still western countries became familiar with agar in the nineteenth century. In 1905, a lot broader use of agar was established with its introduction in raw materials and manufacturing processes in Japan [1]. Later in 1949, some articles were published describing more details on various characteristics of agar associated with its raw materials and development, years before when its chemical structure was described, spectroscopy like infrared spectroscopy was developed, even much more refined techniques like Nuclear Magnetic Resonance (NMR) proton and ¹³C spectroscopy became feasible. Though, later NMR proton and ¹³C spectroscopy helped understanding its molecular structure thoroughly. Mantell [1] provided the international commercial data of agar about quality of agar worldwide and various production processes. It covered most of the data during the period of world war two and post war years until 1950s. Two research articles by M. Glicksman in 1969 [2] and 1983 [3] were published, revising in-depth knowledge about hydrocolloids. These papers provided a good understanding of chemical structures and applications of agar. In 1995 Norman Stanley [4] published a book chapter defining molecular structure as well as properties of agar gel.

      Agar was approved as GRAS (Generally Recognized as Safe) in 1972 by US Food and Drug Administration (FDA) [5] due to its past usage as food in East Asian countries for more than three hundred years. Agar cleared all the three standard tests done by FDA to safeguard its consumptions as food product. These tests were toxicology test [6], teratology test [7] and test for mutagenesis [8].

      5.1.1 Agarophytes Used in Agar Manufacturing

      Gelidium amansii (Rhodophyceae phylum), a red seaweed was exclusively used in Japan in the seventeenth century. Later this tradition was followed by China and Korea. It was the most abundant seaweed found on their seashores. When shortage of G. amansii came, they started to use other seaweeds like Gracilaria. These agars with poor gelling quality were named agaroids. But it was overcome with alkali hydrolyzing the sulfates. After that it became possible to increase the gelling property of Gracilaria and producing stronger agars.

      Pterocladia, Gelidium and Gelidiella seaweeds have a natural internal transformation property of maturing the polysaccharide in the weed through an enzymatic process. Gracilaria doesn’t have the natural property of maturing the polysaccharide so it is amplified industrially using different chemical methods before extracting agar from it.

5.2 Physical Properties of Agar Producing Seaweeds

      Agar, a natural polysaccharide, builds up in the cell wall of agarophytic algae in crystallized cellulose fibrous form. They work like a polysaccharide reserve in there. That being said the agar content varies season to season. Initially Golgi complex secretes a sulfated intermediate agar of low molecular weight, which later gets deposited into cell wall and gets enzymatically polymerized desulfates. The left over product is called agropectin. The electron microscopic imaging showed that the very thick cell wall of Gelidium sesquipedale contains agar and the rhizoids contain high MW agarose molecules. This might be possible because of the smaller rhizoidal cells are more abundant in higher water movement areas, thus supporting the fact that the water current plays very important role in the abundance of agar and agarose in weed’s cell wall. Table 5.1 describes the taxonomic classification of agar.

      Table 5.1 Classification of the taxa of agarophytes [9].