Sodium alginate is a naturally occurring polysaccharide found in brown seaweed weeds. It is a sodium alginate salt of formula (C6H7O6Na). Sodium alginate is soluble in hot and cold water and can be cured and bonded.
To extract the alginate, the seaweed is broken into pieces and stirred with a hot solution of an alkali. The alginate dissolves as sodium alginate to give a very thick insoluble slurry. This insoluble residue must be removed from the solution. The solution is diluted with a very large quantity of water. After dilution, the solution is filtered through a filter press. A filter aid, such as diatomaceous earth, must be added to prevent clogging. The diluted extract is left for several hours while the air rises to the top, taking the residue particles with it. This frothy mix of air and residue is removed from the top and the solution is withdrawn from the bottom.
The next step is precipitation of the alginate from the filtered solution, either by alginic acid or calcium alginate method.
Alginic acid process
Acid is added to the extract and alginic acid is formed as gelatinous pieces. This is removed by floatation facilitated by carbon dioxide which is obtained when excess carbonate is added initially. The remaining extract contains very little alginic acid, 1-2%, which is concentrated to 7-8% by centrifuging. To this alcohol solution is added and then solid sodium carbonate is added till the required pH is reached. The sodium alginate is extruded in the form of pellets, oven dried and milled.
Calcium Alginate process
In this method, calcium chloride is added to the extract to obtain solid calcium alginate in the form of fibres. The fibrous material can be removed by sieving and excess calcium is removed by using water. It is stirred with dilute acid to give alginic acid and this is screw pressed to form a paste. To this solid sodium carbonate is added till the required pH is reached. The sodium alginate is extruded in the form of pellets, oven dried and milled.
Sodium alginate is used in the food industry to increase the viscosity and as an emulsifier. Sodium alginate is used as a stabilizer for ice cream, yogurt, butter and cheese. Sodium alginate can also be used as a thickener and emulsifier for salads, puddings, jams, tomato juice and canned products. It is used as a hydrating agent for noodles, bread, frozen and frozen products. Sodium alginate is widely used to produce gelatinous products if sauce.
Alginate swells in water and this property is used by the pharmaceutical industry as a disintegrant. It is used in liquid medicines to increase the viscosity and solid suspension. It is also used to fix cells and enzymes.
Sodium alginate has the ability to thicken in water, making it suitable for textile printing. It is used for surface sizing of paper. Its oil resistance to paper is also better. Alginates are widely used in the dental industry as impression materials. It is a good chelating agent for the extraction of radioactive substances from the body.