ALGAL POLYSACHHARIDES IN FOOD INDUSTRY
The consumption of polysaccharides as dietary fibres has gained importance over the last few years mainly because of their ability to improve health. Marine algae have been used as dietary supplements by maritime societies since centuries ago . Seaweeds were also used as supplements in animal meals, fertilizers and soil conditioners . Marine algae used for consumption is now called "sea vegetables" and is consumed in Japan, Ireland, Canada, Norway and other developed countries as well . The major groups of seaweeds that are exploited commercially include Chlorophyta, Phaeophyta and Rhodophyta .
Algal biotechnology has played an important role in finding novel biochemical from algae and alleviating problems related to food scarcity due to the phenomenal increase in world population. Many valuable products have been commercially produced from different species of algae. These include beta-carotene production from Dunaliella (halophilic green algae), polyunsaturated fatty acids from flagellates and diatoms, astaxanthin from Haemotococcus and phycobiliproteins from red algae and cyanobacteria .
Most of the natural polysaccharides used in industry come from two main sources- algae and microbes. The usage of algal polysaccharides is more prevalent in the food industry due to the unique behaviour of algal polysaccharides . Microscopic algae are rich in proteins, vitamins and minerals and hence have been used as a food by humans since long back . The main uses of polysaccharides in the food industry have been as gelling agents, stabilizers, thickeners and emulsifiers. They are also used in paints, cosmetics and in paper industries.
Sources of Algal Polysaccharides
Agar and carrageenan are the two main polysaccharides extracted from red algae. Alginates are another type of polysaccharide extracted from brown algae (shown in figure 2). While carrageenan is used mostly in the food industry, agar and alginates are employed both in the food and pharmaceutical sectors. It is estimated that seaweed extracts contribute to an annual turnover of several hundred million dollars .
Agar is extracted from red seaweeds like Gracilariadeae and Gelideaceae. It is present as polysaccharide in the cell walls of these seaweeds and is generally extracted by boiling the weed with water and drying the extract. Agar is insoluble in cold water but becomes soluble in boiling water forming a gel on cooling down. It has been shown that low strength agar has a viscous nature rather than showing good gelling properties and has been added in honey to make honey melt in the mouth. This desirable quality is not achieved if pectin is added .
Alginates are composed of uronic acids, alpha-L-guluronic and B-(1,4) mannuronic acids. The process of extraction depends whether the wet or dry varieties of seaweeds are used .
Kapaphycus and Betaphycus are major sources of carragenen. The main skeletal framework of carrageenan consists of galactose polymers linked through alpha-(1,3) and beta-(1,4) linkages alternatively . Due to strong negative charges shown by carrageenans over normal pH ranges, they are used in "milk reactivity". The lambda and kappa types are used in cold milk and pasteurised products respectively . Carrageenan is used as an additive in yoghurt, chocolate milk and puddings .
The ability of these hydrocolloids to change the nature of aqueous solutions by acting as gelling and thickening agents have made them industrially valuable. The commercial application of these polysaccharide gums depend on their physical and chemical properties .
The other major uses of algae are the manufacture of Single Cell Proteins (SCP). SCP is defined as the "the dried biomass of microbe that can be used as a protein source in the diet." Cyanobacteria are being mass cultured and used as health foods and food colorants . The common species of algae used as single cell proteins are Spirulina maxima, Spirulina fusiformis, Chlorella pyrenoidosa and Dunaliella salina with spirulina and chlorella being the major ones. Apart from their nutritional values, the main reason these algae are being accepted as supplementary health food is because they are easily digested and accepted by the human body .
It is a spirally coiled, multicellular, filamentous blue green alga commonly found in fresh, marine and brackish waters. Biomass of spirulina consists of 67% protein and is rich in beta carotene. Spirulina is recommended as a supplementary food for under nourished children by medical bodies around the world. It is being used in the preparation of purce, bread sandwich and Indian dishes like dosa and enhances the colour, flavour and acceptability of these dishes. Spirulina has many therapeutic properties like promoting wound healing by stimulating skin metabolism, Beta carotene acts as an anticancer agent and presence of phycocyanin in spirulina
protects body from pathogenic disease . They are being marketed either in the form of powder or tablets as shown in figure 3.
Morphologically, it is spherical, non-motile, unicellular green algae found in fresh waters. Chlorella is consumed as a source of instant energy for sportsmen and people engaged in heavy work. Therapeutic advantages of consuming chlorella include cure of allergy and bronchial asthma, normal functioning of kidney and eye lenses .
Applications in Food Industry
With the increasing demand of food supply and raising food quality standards, it has become a necessity to add additives to food laying emphasis on their natural sources. These additives serve to improve the taste, colour, flavour and also act as potential preservatives. Algal polysaccharides are being employed in manufacture of soft drinks, confectionary, salad dressings, dairy products and some alcoholic beverages .
Carrageenan provides excellent purge control, slicing and freeze thaw stability and prevents the darkening of meat. Carrageenan improves shear stability and viscosity of fluid dairy products HTST chocolate milk and are ideal for frozen desserts like ice creams, puddings, cottage cheese dressing etc. Sodium alginate id used in fruit fillings, cheese sauces, icings and mousses. Algal polysaccharides are used in cheese and meat products where high gel strength and clarity are required. Products intended for food service steam trays e.g. Fajita strips are mixed with agar and carrageenan to provide good water binding and improve yield without visible gel formation  .
Many species of algae are used for extracting potentially useful and valuable phycocolloides and are exploited commercially by the food industry. For centuries, marine algae have been used as source of food in some parts of the world but the recent realization that seaweeds and their polysaccharidic extracts are superior sources of fibre, vitamins and minerals have created a huge demand in the food markets worldwide. The major sources of algal polysaccharides include alginate, agar and carrageenan and these are finding widespread applications in foods like frozen meat and poultry, dairy products, bakery and alcoholic beverages. A better understanding of the chemical and physical properties of these polysaccharides will enable production of desired quality and cheaper food products in the near future.
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