Analyzing Alginate Functionality

Alginates are gel-forming gums that provide a wide range of functional properties in foods and beveragesfrom 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.

Alginate salts are cold-water-soluble. They can form strong gels without heat, and the gels will not melt under heated conditions. Propylene glycol alginate (PGA) is a chemically modified alginate that remains in solution at low pH and is often used in more-acidic products, such as salad dressings and fruit beverages.

Heat-stable gels

In bakery products, alginates are used to form soft gels in fruit preps, fruit-pie fillings and icings. For high-Brix fruit preps, alginate contributes to a smooth texture and sheen, without masking flavor. Its heat stability prevents filling from boil-over when heated.

Alginate is also fully compatible with all types of fruits, pH levels, solids contents and other ingredients typically used in pie fillings. Alginates also can provide a range of textures, from fully gelled to a more-viscous, nongelled structure.

In icing, alginates give improved texture and stability. They are generally effective for icings and frostings at levels between 0.05% and 0.20%.

In water-based dessert gels, alginates form a strong, stable gel with excellent flavor release. Heat is not required for gel formation, and alginate is effective at any pH level and solids content at a usage level of 0.8% or less. Alginates can replace gelatin in these desserts, which is especially important when formulating a vegetarian product.

Emulsion stability

Alginates stabilize both the emulsion and suspend solid particles in creamy salad dressing. In low-fat dressings, PGA can impart some fat-mimetic properties and also boost viscosity lost from fat reduction. PGA has excellent stability in a dressings acidic environment, is heat-stable and remains effective at levels of 0.05% or less.

PGA also works as a stabilizer in a variety of beverages. In orange juice, adding 0.1% alginate can stabilize the pulp and provide a subtle boost in body without adversely affecting flavor or other important attributes.

Another beverage application where PGA provides stabilization is whipped fruit drinks, which should have smooth body, stable foam and full flavor. PGA is suitable for these types of beverages since it is effective at any pH and Brix level. It can also be easily dissolved without heat and with moderate shear, although high shear is recommended to obtain maximum foam.

Maureen Akins is a food scientist in the applications section of R&D at TIC Gums, Inc., Belcamp, MD. She currently oversees areas of product development using gums to provide stabilization in meat, bakery, salad dressing, dairy and nonfood applications. Akins has a B.S. in biology and a M.S. in food science, both from Virginia Tech. For more information, e-mail [email protected] .