What is Alginic Acid? - Structure, Solubility & Products

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Alginic acid is a product derived from brown algae. In this lesson, we will learn the structure of alginic acid and how it is used in food and drug products.

What is Alginic Acid?

Chances are that you frequently eat alginic acid. It can be found in some ice cream, cakes, salad dressings, and many other products. It is a great thickener, so it is frequently used in food. Alginic acid comes from brown algae.

Alginic acid comes from brown algae such as this
Brown algae

Alginic acid is made up of D-mannuronic acid and L-guluronic acid structures connected with an alpha 1,4 bonds. These two structures do not necessarily alternate from one to the other. For example, a guluronate can be connected to another guluronate with an alpha 1,4 bond. The exact number of mannuronates or guluronates in alginic acid isn't consistent. It naturally changes from one plant to another. But, most often they occur in sets of two, so there will be two mannuronic acids connected with an alpha 1,4 bond, then connected to a guluronic acid with an alpha 1,4 bond which is connected to another guluronic acid with an alpha 1,4 bond. This pattern will then repeat.

D-mannuronic acid is similar to D-mannose, only it has a COOH instead of a CH2 OH on carbon 6. And L-guluronic acid is similar to L-glucose, only, once again, it has a COOH instead of a CH2 OH on carbon 6.

Basic structure of alginic acid, this unit will repeat over and over in alginic acid
Alginic acid structure

Alginic Acid Solubility

Alginic acid has very low solubility in water and most organic solvents. But, most food products are water based. For example, ice cream is typically at least 50% water. So, how does alginic acid act as a thickener in these products if it can't dissolve in water? By adding a cation such as sodium or potassium to alginic acid, to form sodium alginate or potassium alginate, it becomes highly soluble in water.

The negative charges on alginic acid cause it to form a long chain, trying to get the negative charges as far away from each other as possible. Water has both partial positive charges and partial negative charges. The negative charges on alginic acid will readily be attracted to the partial positive charges of the hydrogen atoms in water. But, if there are no positive ions with the alginic acid, the partial negative charges on the oxygen atoms in water will be required to interact with each other as alginic acid is taking away the partial positive charges of the hydrogen. So, without positive ions with the alginic acid in the form of potassium or sodium, the alginic acid cannot dissolve in the water. With the positive ions it can readily dissolve into the water, forming a viscous (thick) solution.

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