Wonderful food additives? Physicochemical properties and detailed explanation of sodium alginate

Sodium alginate (SA), lauded as a "miraculous food additive" in the US and a "longevity food" in Japan, is a naturally occurring linear polysaccharide. Its biodegradability, biocompatibility, and safety for the body provide strength and flexibility to tissues. Its gelling, adhesive, and stabilizing properties, along with its water retention capacity, allow for industrial applications. It is now widely used in the chemical, biological, pharmaceutical, and food industries.

The wide range of applications for sodium alginate stems from its excellent physicochemical properties. Sodium alginate is a polyanionic copolymer, a linear polymer composed of 1,4-poly-β-D-mannuronic acid and α-L-guluronic acid, with the chemical formula (C6H7O6Na)n. It exists as a white or slightly yellow solid powder, is hydrophilic, readily dissolves in water, and forms gels in the presence of divalent ions. All these properties make it a useful material for drug delivery and cell immobilization.

(Chemical structure of sodium alginate)

Physicochemical properties of sodium alginate:

1. Physicochemical properties

(1) Molecular weight

Sodium alginate has a high molecular weight, ranging from 32,000 to 400,000, with long M and G chains in its structure. Studies show that the viscosity of alginate solutions is affected by the molecular weight and pH of the reactants. Viscosity increases with decreasing pH, reaching a maximum around pH 3-3.5, as the carboxyl groups are protonated and can form hydrogen bonds at this pH. Increasing the molecular weight of alginate increases the gelation rate and the physical properties (tensile strength, elasticity, viscosity) of the gel.

(2) Solubility

Sodium alginate dissolves slowly in cold water, resulting in a viscous solution. It is insoluble in ethanol, water-alcohol solutions with more than 30% alcohol content, chloroform, and ether. Studies show that its solubility depends on pH, molecular weight, ionic strength, the nature and concentration of ions present in the structure.

(3) Stability

Sodium alginate is most stable at a pH of 6-11. Below pH 6, alginic acid precipitates and becomes insoluble in water; above pH 11, it coagulates. Viscosity is highest at pH 7 and decreases significantly with increasing temperature.

2. Mechanical properties

(1) Viscosity

The viscosity of alginate depends on the molecular weight and concentration of the polymer, while gelation (affinity for cations) depends on the amount of uronic acid in the structure. Therefore, the higher the uronic acid content found in the structure, the greater the solubility of alginate in water and the greater the gelling ability, resulting in a firmer, more viscous, and more stable gel.

(2) Mucoadhesive properties

Due to the presence of free carboxyl and hydroxyl groups in its structure, alginate exhibits good mucoadhesive properties. In a physiological environment, electrostatic repulsion occurs between alginate and mucin due to the sialic acid, sulfate groups in the mucus structure, and the negatively charged carboxyl anions of alginate. This property is an advantage for mucosal drug delivery, as it increases the contact time and adhesion of the drug to the site of action, improving drug bioavailability.

3. Biological properties

Due to its biological properties—non-toxicity and non-immunogenicity, biocompatibility, and biodegradability—sodium alginate has broad application prospects in the food, biomedical, and pharmaceutical fields.

4. Other properties

Alginate also possesses other properties, such as the ability to form a rigid, ordered, and strong three-dimensional gel with multivalent cations; and the ability to form composite gels with chitosan.