Agar-Agar

Gracilaria

Another name that is used for agar is agar-agar. Agar is a natural polysaccharide, present in the different seaweeds of the family Rhodophyceae (Gelidium spp. and Gracilaria spp.).
These seaweeds are harvested mainly in the USA and Japan. After harvesting, the agar is extracted from the seaweed and purified.

Production Process of Agar
The basic extraction of agar from seaweeds, such as Gelidium and Gracilaria, starts with washing the seaweed to remove foreign matter. There are some differences in the processing of the various seaweeds prior to extraction, depending on the type of seaweed used.

Gelidium is simply washed to remove sand, salts, shells and other foreign matter and is then placed in tanks for extraction with hot water.

Gracilaria is also washed, but it must be treated with alkali before extraction; this alkaline pre-treatment causes a chemical change in the agar from Gracilaria, resulting in a product with an increased gel strength. Without this alkaline pre-treatment, most Gracilaria species yield an agar with a gel strength that is too low for commercial use.

The alkali treatment is heating the seaweed in a 2-5 percent sodium hydroxide at 85-90°C for 1 hour; the strength of the alkali varies with the seaweed species and is determined by testing on a small scale. After removal of the alkali, the seaweed is washed with water, and sometimes with a very weak acid to neutralize any residual alkali.

In the next processing step, the seaweed is heated with water for several hours during which the agar dissolves in the water. Gelidium seaweed is more resistant and extraction under pressure (105-110°C for 2-4 hours) is faster and gives higher yields. Gracilaria seaweed is usually treated with water at 95-100°C for 2-4 hours. The remainder of the process is the same for both types of raw material. The hot extract is given a coarse filtration to remove the seaweed residue, filter aid is added and the extract is pumped through a filter press. The extract is thick and will gel if allowed to cool, so it must be kept hot during the filtration processes. The hot filtrate is cooled and forms a gel which contains about 1 percent agar. The remaining 99 percent is water containing salts, proteins, carbohydrates and color molecules. The gel is broken into pieces, and, if necessary, it is bleached to reduce the color. Then it might be washed again to remove the bleach and remaining salts.

The next step is removing the remainder of the water from the gel. There are two methods commonly used: either a freeze-thaw process or by using pressure. In the freeze-thaw process, the gel is slowly frozen so that large ice crystals form. The structure of the gel is broken by the ice crystals and when it thaws again a lot of the water drains away, leaving a concentrated gel which contains about 10-12 percent agar. Sometimes, this gel is placed between porous filter cloths and squeezed in a hydraulic press to remove more water. However, this is a slow process, and usually the thawed material is simply drained and placed in a hot-air dryer. After drying, it is milled to the required particle size. Because of the refrigeration costs, this freeze-thaw process is relatively expensive, compared to the alternative described next.

The alternative process relies on syneresis. To seperate the water from the agar gel, pressure is used. The 1 percent agar gel is placed in a press. Pressure is applied and very slowly increased over about 24 hours, forcing liquid out of the gel. The remaining gel, now containing about 20 percent agar, is removed from the press, shredded and dried in a hot-air oven before being milled to the required particle size. With no refrigeration required, the energy consumption is much lower than for the freeze-thaw method, and, since more water has been removed, less soluble matter remains, so the agar is more pure. Less energy is also needed in the drying process since less water needs to be removed.

This process based on syneresis has been widely adopted by large agar producers who can afford the higher capital costs for this equipment.

Chemical Structure and Properties
Agar is a combination of neutral (agarose) and electrically charged (agaropectine) molecules. Agarose is a linear polymer,made up of the repeating monomeric unit of agarobiose. Agarobiose is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose.

Agar is soluble in hot water. It forms a reversible gel when it cools down. The temperature at which agar gels is between 32 and 43°C. The gel can be melted again when heated to 85ºC. Due to its high melting point, it does not melt in the mouth. It also does not need refrigeration, making it very suitable for warm climates.

Agar has a high gel strength. It can be used in an broad pH range. It can used alone or mixed with other hydrocolloids, carbohydrates and proteins.

Some agars are sugar reactive, combining the agar with high sucrose solutions increases the gel strength.

Agar does not have a flavor or odor.
Agar has a slight yellow – off white color. It is available in a powder or strip form.

Functionality in Food
Agar is used as a thickening agent and stabilizer. It can also be used as a non-caloric fiber as it is not digested by the human body.

Applications
About 90 percent of the agar produced is used in foods. In the baked goods industry, the ability of agar gels to withstand high temperatures makes it very suitable as a stabilizer and thickener in pie fillings and meringues. Adding agar to icing creates a more stable, non stick and smooth icing.

Some agars, especially those extracted from Gracilaria chilensis, can be used in confectionery with a very high sugar content, such as fruit candies.

Agar is traditionally used in Asian countries, as the gelling component of jellies. A popular Japanese sweet dish is mitsumame: cubes of agar gel containing fruit and added colors. It can be canned and sterilized without the cubes melting.

Agar is also used in gelled meat and fish products, and is often preferred to gelatin because of its higher melting temperature and gel strength.

In combination with other gums, agar has been used to stabilize sherbets and icecreams. It can improves the texture of dairy products like cream cheese and yoghurt. It has been used to clarify wines, especially plum wine, which is difficult to clarify by traditional methods.

Agar is used in vegetarian foods such as meat substitutes.