Analyzing Alginate Functionality

Alginates are gel-forming gums that provide a wide range of functional properties in foods and beverages—from providing gelling for desserts and dairy products to thickening sauces and serving as gelatin replacements.

Alginate is a natural product found in a wide variety of brown seaweed. Chemically, it is a linear polymer consisting of alpha-L-guluronic acid and beta-D-mannuronic acid. Segments consisting only of D-mannuronic acid are called M blocks, while segments consisting of L-guluronic acid are called G blocks. MG blocks consist of alternating units of D-mannuronic acid and L-guluronic acid.

The ratio of these blocks in the alginate polymer impacts its functional properties. When gel strength is important, food designers should look for an alginate with a higher level of guluronic acid, since this fraction of the molecule forms more preferentially with calcium ions than mannuronic acid. Conversely, alginate with a higher level of mannuronic acid is frequently used to provide additional viscosity in the presence of calcium ions.

Food-grade alginate is extracted from seaweed by first washing the seaweed in a mild acid, then dissolving the seaweed in an alkaline solution to convert the alginate to a soluble form. Filtering the resulting solution removes any remaining insoluble residue, and then calcium alginate or alginic acid is obtained through precipitation. Either of these forms can be converted to sodium alginate, the most-common salt of alginic acid used by the food industry.

Sodium alginate is available in low-, medium- and high-viscosity grades. The degree of viscosity is determined by the molecular weight or degree of polymerization (DP), and by the conditions of the extraction process. Gels form when a soluble calcium source is added to a solution of sodium alginate in water. The calcium displaces the sodium from the alginate and holds the long alginate molecules together, thereby creating a gel. This is commonly referred to as the “egg box” model.

There are two main ways to gel an alginate. The first method is called an externally set alginate gel. This type of gel is most commonly prepared by surrounding the alginate solution in a highly soluble calcium bath. This allows the calcium to set the alginate from the outside in, and it allows for the gel to be molded into the desired shape. The second method is an internally set alginate gel. This requires that the calcium source be sequestered before it has a chance to mix with the alginate. The alginate needs time to hydrate first before the calcium is released from the sequestering agent. This can be accomplished by either choosing a minimally soluble calcium source or by adjusting the pH once the solution has hydrated. This is another advantage of sodium alginate, because gel formation can be delayed by adding a sequestrant, like phosphate.