Physical Properties

Gelation

The most notable characteristic of alginate is significant changes in physical properties depending on the type and concentration of coexisting cations. The physical state can instantly shift from a smoothly flowing aqueous solution to a firm gel.

Gelling Mechanism

Alginate is a polysaccharide composed of two types of uronic acids, mannuronic acid (M) and guluronic acid (G), which are polymerized in a linear chain. This chain structure includes "G-blocks," made up of G-G linkages, and "M-blocks," formed by M-M linkages.

When multivalent cations like Ca²⁺ are present, the negative charges of the carboxyl groups in the G-blocks are attracted to Ca²⁺ ions, creating cross-links. As two alginate molecules form these cross-links, the molecular chains lose their freedom, transitioning from a sol (a fluid aqueous solution) to a gel (an elastic body). This cross-linked structure forms with G-blocks encapsulating Ca²⁺ ions, resembling the way an egg fits into an egg carton; thus, it is known as the “Egg Box Junction.”

Instant Sol⇔Gel Transition

Cross-linking through ion exchange reactions occurs instantly. For example, when sodium alginate is dripped into a calcium solution, the surface in contact with Ca²⁺ ions gels immediately, creating a spherical jelly with a liquid interior and a gelled surface. This phenomenon is applied in the production of artificial salmon roe and similar products.
Adjustable Gel
Strength

The Egg Box Junction is a cross-linking structure unique to G-block chains, so alginate with a high ratio of guluronic acid (G) forms a rigid, high-strength gel. In contrast, alginate with a higher ratio of mannuronic acid (M) creates a more flexible, low-strength gel. This allows for the selection of alginates with various gel strengths, tailored to specific needs.
Controlled Gelation Speed

The gelation speed of alginate can be adjusted by controlling the state of Ca²⁺ ions. This can be achieved through methods such as (1) selecting different types of calcium salts, (2) using chelating agents to regulate the availability of calcium ions, and (3) adjusting the pH of the solution. These approaches enable customization of gelation speed according to specific applications.

Watch the Gelation on the video

Heat Stability

Unlike other gelling agents like agar, pectin, or gelatin, which gel upon cooling, alginate gels through an ion exchange reaction. As a result, alginate gels do not revert to a solution, even with heating, and retain their shape through processes like heat sterilization, cooking, and repeated freezing and thawing.

Viscosity

The viscosity of alginate can be freely controlled by adjusting conditions such as temperature, pH, molecular weight, and concentration. This section explains the factors that influence viscosity, using a sodium alginate solution as an example.

Molecular Weight

As the molecular weight increases (i.e., the molecular chains become longer), the chains entangle more strongly, causing the viscosity of the aqueous solution to rise exponentially.

Concentration

As the concentration increases, the interactions between molecules become stronger, and the viscosity rises logarithmically.

Lowering the pH of the aqueous solution reduces the dissociation of carboxyl groups, leading to insolubility. As the number of insoluble molecular chains increases, the viscosity (apparent viscosity) of the solution rises. However, if the pH is lowered too much, alginate precipitates, and the viscosity is lost.

Inorganic Electrolytes

When inorganic electrolytes containing monovalent cations, such as table salt, are added to an aqueous solution, the ionic strength of the solution increases, causing the molecules to contract. This results in a decrease in solubility and a reduction in viscosity.

Temperature

When the aqueous solution is heated, molecular motion increases, causing the viscosity to decrease. Conversely, cooling the solution raises the viscosity. However, freezing and thawing do not affect the viscosity.

Storage Duration

When sodium alginate is stored for long periods, its molecular weight decreases, resulting in reduced viscosity. This decrease occurs more rapidly at higher temperatures.

Wide Range of Options

KIMICA offers a diverse selection of alginates, ranging from ultra-low viscosity types with almost no viscosity to standard types that exhibit smooth viscosity, as well as ultra-high viscosity types that demonstrate thixotropic gel-like behavior.

Ultra-low Viscosity

An aqueous solution of alginate is the smoothest among natural water-soluble hydrocolloids and exhibits flow properties closest to a Newtonian fluid. In particular, ultra-low viscosity alginate demonstrates Newtonian flow, with viscosity remaining almost unchanged even when the concentration or shear force is increased.

Ultra-high Viscosity

Ultra-high viscosity alginate exhibits thixotropic flow, where its viscosity decreases when shear force is applied to the aqueous solution, then gradually returns to its original state when left undisturbed.
The aqueous solution is gel-like and brings a rich variation in texture.

Propylene Glycol Alginate (PGA)

Propylene Glycol Alginate (PGA) is excellent thickening and stabilizing agents with high acid and salt resistance. While alginate salts exhibit unique gelling and stabilizing properties due to the high ion-exchange capacity of their carboxyl groups, their high reactivity can lead to gelling or precipitation under acidic or high-salt conditions, limiting their use. PGA, however, achieve reduced reactivity through the esterification of the carboxyl groups in alginic acid, allowing them to provide superior thickening and stabilizing effects in fields where alginate salts are less suitable.（Learn more）

Concentration

As concentration increases, intermolecular interactions strengthen, causing viscosity to rise logarithmically.

Acid Resistance

It maintains stable viscosity within an acidic range of pH 3 to 5. Under alkaline conditions, however, the ester breaks down, leading to a decrease in viscosity.

Salt Resistance

It maintains stable viscosity even in solutions containing monovalent cations, such as sodium chloride.

Even in environments containing divalent or higher cations, such as Ca²⁺, it does not gel and provides thickening and stabilizing effects.

Emulsifying Stability・Protein Stability

Propylene Glycol Alginate, which possess both hydrophilic and lipophilic groups in their structure, exhibit emulsifying stability between oil and water. It also has the ability to bind with proteins and stabilize structures. This property is utilized in various applications, such as stabilizing beer foam, improving gluten formation in noodles and bread, enhancing the dispersion stability of milk proteins, and maintaining the shape of meringue.

Stabilizing Effect on
Lactic Acid Proteins

KIMICA Alginate Products

Examples of Alginate Uses and Applications