Calcium alginate and uses

Calcium alginate is a water-insoluble gel-like cream-colored substance that can be made by adding an aqueous solution of calcium chloride to an aqueous solution of sodium alginate.Calcium alginate is also used to entrap enzymes and form artificial seeds in plant tissue culture."Alginate" generally refers to salts of alginic acid,but can also refer to alginic acid derivatives and alginic acid itself; Alginate is found in the cell walls of brown algae as calcium,magnesium and sodium salts of alginic acid.

Preparation

Extraction of alginate: 

To extract alginate,seaweed is broken into pieces and stirred with a hot solution of alkali (usually sodium carbonate).Over a period of about two hours,the alginate dissolves into sodium alginate,forming a very viscous slurry.This slurry also contains the insoluble part of the seaweed,mainly cellulose.This insoluble residue must be removed from solution.The solution is too thick (viscous) to filter and must be diluted with plenty of water.After dilution, the solution is forced through a filter cloth in a filter press.However,the undissolved residue fragments are very fine and can quickly clog filter cloths.Therefore,before starting to filter,filter aids such as diatomaceous earth must be added; this keeps most of the fine particles away from the surface of the filter cloth and facilitates filtration.However, filter aids are expensive and add significantly to the cost.To reduce the amount of filter aid needed,some processors force air into the extract when it is diluted with water (extract and dilution water are mixed in an in-line mixer and air is forced in).Fine air bubbles adhere to the residue particles.The diluted extract sits for hours while air rises to the top,carrying away residual particles.This frothy mixture of air and residue is removed from the top and the solution is drawn from the bottom and pumped into the filter.

Preparation of calcium alginate from sodium alginate

Calcium alginate can be produced from sodium alginate solution by adding calcium salts such as calcium chloride.This forms an insoluble calcium alginate salt,which precipitates out of solution.Calcium alginate can then be redissolved in various sodium carbonate solutions to produce alginate products containing specific ratios of sodium to calcium.This affects the physical and chemical properties of alginate.

Structure

Due to its technological importance,the structure of calcium alginate hydrogels has long been the subject of scientific investigation. The most famous theory of alginate-metal binding is the "egg-box" model proposed in the 1970s.The model is based on the unique arrangement of the hydroxyl groups of the polymerized α-L-guluronic acid (G) unit, which provides a cavity for the cation, like an egg in an egg carton.While the reality may be much more complex,the Egg-Box model is a useful explanatory tool and the original paper that proposed it now has over 1500 citations.

Use

The uses of calcium alginate are:

• Production of insoluble artificial seeds in plant tissue culture

• For immobilization of enzymes by embedding

• Production of edible substances

• Add wound dressing (alginate dressing) as a hemostatic agent

• As an alginate hydrogel,it can be used in controlled release drug delivery system

• For spheroidization of molecular gastronomy.