(Preliminary article: will be updated very soon)
The cell wall of seaweeds contains about 30 to 40% polysaccharide of plant’s dry matter. And these polysaccharides are commonly known as phycocolloids (= algal colloids).
By definition, the cell walls of red and brown algae consist of cellulose micro fibrils which contain amorphous polysaccharides called phycocolloids. The three major phycocolloids derived from seaweeds are agar-agar, carrageenans and alginate (also called alginic acid). The first two are procured from red algae, while the last one (alginate) is from brown algae.
Extraction of Phycocolloids
Extraction of Agar
Agar is a mixture of polymers.
Species of Gelidium, Gracilaria, Pterocladia and Acanthopeltis are the primary sources of agar in a world market that is valued at over U. S. $200 million.
Asian and South American species of Gracilaria now account for more than 70% of all agar produced because of the expansion of mariculture techniques during the past 20 years (Dawes 1987).
- Agar is extracted by dissolving dried plant powder in hot water.
- The water-soluble agar is separated by centrifugation, filtration, and chemically treated to remove color.
- The extract is then gelled by cooling and then squeezed to exude the water, impurities, and salts.
- The solution is dried as flakes or sheets and ground into a white powder.
- Aqueous concentrations of 1 to 2% agar produce strong to brittle gels at room temperature.
- The agar is decolorized and deodorized with activated charcoal, filtered under pressure, and evaporated under reduced pressure . Further purification by freezing is then undertaken.
Good quality agar is produced from Gelidium lingulatum, Gelidium anansii, Gracilaria chinensis. Food grade agar is extracted from Pterocladia.
Extraction of carrageenan
Of the 14 types of carrageenan, three types are most important commercially, lambda, kappa, and iota.
Carrageenan has annual sales of over $200 million and accounts for about 15% of the world use of food hydrocolloids (Bixler, 1996).
The primary sources of carrageenan are from Chondrus crispus and Mastocarpus stellatus in the North Atlantic, plus tropical species of Eucheuma and Kappaphycus in the Philippines and Hypnea in North Africa.
Philippine species are presently the source of about 50% of carrageenan used worldwide (40,000 tons).
Eucheuma and Kappaphycus farming in the Philippines are simple and highly productive (Bixler,1996). As described by Trono (1993), they are farmed using monolines tied between stakes or suspended on floating logs.
- Dried plants are washed to remove salts and debris and digested with hot water with alkali (KOH, CaOH).
- Alkaline extraction promotes swelling and removes sulfate groups, increasing 3,6-anhydro-D-galactose content. The latter substance increases gel strength and protein reactivity, an important use of carrageenan.
- Refined carrageenan is usually recovered by precipitation, washed with alcohol, dried, and powdered.
- Semirefined carrageenan is obtained by cooking the plants in aqueous KOH and then soaking in freshwater to extract most of the residual alkali, drying, and grinding to produce a flour.
Because of their different chemical and physical features, carrageenans can be specifically tailored for a wide variety of uses, especially for dairy products, as well as desserts and medicinal and industrial applications.
Carrageenan is extracted commercially from Chondrus crispus and Gigartina stellata. Eucheuma and Kappaphycus are also used to produce carrageenan in some countries.
Commercial extraction is similar to that for agar although carrageenan can not be purified by freezing.
Extraction of alginic acid
Alginic acid is a polyuronic (polysugar) polymer consisting of varying proportions of manuronic and glucuronic acid residues with a carboxyl group on C-6 (Fig. B-6). The former is a beta 1,4 linked D-manuronic acid, and the latter is alpha 1,4 linked L-glutaronic acid.
The annual value of alginate production is over U. S. $100 million.
- The brown algae most commonly harvested for alginate are species of Macrocystis, Ecklonia, Ascophyllum, Eisenia, Laminaria and Sargassum.
- The harvesting of different seaweeds varies, with Macrocystis pyrifera being cut by harvesting ships (Fig. B-7) and other species are hand harvested.
- The freshly cut down seaweeds are milled and washed at the factory.
- The insoluble alginate salts are then converted to soluble sodium alginate via heating in an alkali bath.
- Sedimentation, screening, filtration, and centrifugation separate the solid from the liquid phase, which is then clarified (acidified or electrolysis) and dried to a white powder.
Uses of phycocolloids
Uses of agar
- Agar is used in food processing (baked goods, candies, juices, wines, vinegar, Cheeses, Candies, Mayonnaise, Yogurt, Frozen foods),
- industrial applications (adhesives, cosmetics, toolmaking, electrophoretic gels, in paper sizing and coatings, Textile printing and dyeing, Cosmetic skin preparation, Lipstick),
- diverse medicinal uses (dental casts, bulk laxative, formatve structure in pills and capsules), and as a substratum for microbiological research.
Uses of carrageenan
- Originally, each of these plants was used as a sea vegetable, making jellies, puddings and thickened boiled dishes. Because of their different chemical and physical features, carrageenans can be specifically tailored for a wide variety of uses, especially for dairy products, as well as desserts and medicinal and industrial applications.
- Carrageenan is used in Ice cream, Salad dressings, Sauces and gravies, Cheeses, Chocolate milk, Whipped toppings, Frozen foods, Paints, Rubber, Polishes, Shampoos, Toothpastes, Lotions and creams and time release Capsules and coating tablets.
Uses of alginate
- The food industry (30%) uses alginates in production of frozen desserts, salad dressings, dairy , Sauces and gravies, Candies, Ice cream, and bakery products.
- Alginates are primarily used in Lotions and creams and Air-freshner gels, Textile printing and dyeing, the paper and textile industries (50%) , with the phycocolloid improving ink holdout and sheet smoothness in paper. The textile industry uses the compound with fiber-reactive dye pastes to print sharp lines and conserve dyes.
- Medicinal uses for alginates include protective coatings, suspension agents, and dental impression materials.
(This article is completely based on the Lecture sheet provided by respected Dr. Mohammad Azmal Hossain Bhuiyan, Professor, Department of Botany, University of Dhaka.
Some pictures and info have been added by the author. Any mistake, error, misinformation and other related things found in this article is only author’s to blame)