The formation of gels from sodium alginate is influenced by five factors: the type of sodium alginate, the type of calcium source, the calcium chelator, the temperature and the pH [1, 2].

Type of sodium alginate

Guluronic acid (G-unit) content and viscosity of sodium alginate affect the gelation effect. The content of G unit in different types of sodium alginate is different, and it can be known from the principle of gel formation between sodium alginate and Ca2+ that the G unit plays a major role in gelation, so the proportion of G unit in sodium alginate molecule is an important factor affecting the gelation effect.

The gels prepared with high G-type sodium alginate are rigid and brittle with good thermal stability, while the gels formed with high M-type sodium alginate have poor mechanical strength but good elasticity.

The viscosity of the sodium alginate solution varied with the molecular weight of sodium alginate at the same concentration. The higher the polymerization degree of sodium alginate, the higher the molecular weight, and the higher the viscosity exhibited at the same concentration. The degree of polymerization of sodium alginate can be controlled by controlling the extraction process to produce sodium alginate with different viscosities [3].

The longer the molecular chain of sodium alginate molecules in the formation of the gel will be more closely combined, reflecting the gel mechanical strength is also greater.

Calcium source type

Calcium alginate gels are prepared in different ways depending on the type of calcium source. The first is calcium salts with good solubility at neutral pH, such as calcium chloride and calcium lactate.

Traditional Ca2 + crosslinking sodium alginate to form a gel selected crosslinking agent is generally calcium chloride, the use of calcium chloride’s biggest drawback is that the gel rate is too fast, and Ca2 + contact to the sodium alginate solution immediately formed a gel, blocking the subsequent gel reaction occurs, resulting in sodium alginate conversion rate is a gradient, can not get a good three-dimensional structure of the uniform gel [2].

Another type of calcium salt with poor solubility is calcium sulfate, such as calcium sulfate, which has low solubility in water. Calcium sulfate is insoluble in water and dissolves slowly in water, which can provide some time for the dissolution of sodium alginate molecules, but the dissolution speed of calcium sulfate is still faster than sodium alginate in comparison, if sodium alginate and calcium sulfate powders are dissolved together, the Ca2+ and sodium alginate that are dissolved first will form a gel to block the subsequent gelation reaction. The gel produced by this method is not uniform and the gel strength is not high.

In addition, the amount of calcium source added also affects the effect of gel. As the concentration of sodium alginate increases, its water-holding capacity, elastic toughness and gel strength are gradually enhanced. Mainly when the calcium ion concentration is certain, low concentration of sodium alginate, sodium alginate and calcium ion exchange is incomplete, the formation of the gel body is weak, strength is small; concentration increases, there is sufficient sodium alginate and calcium ion exchange, the formation of calcium alginate space network structure dense, and sodium alginate macromolecules interact with each other to strengthen the water-holding ability to enhance the gel strength, elastic toughness and also better.

Calcium ion chelating agent

Calcium ion chelator can chelate the Ca2+ in the water, so that it can’t combine with sodium alginate, and control the rate of gel formation by competing with sodium alginate for Ca2+, so that the sodium alginate has sufficient time to dissolve, which can effectively improve the quality of the gel. The disadvantage of this method is that a chelating agent has to be introduced and it results in a waste of Ca2 + , which increases the dosage of additives and reduces the efficiency.

Temperature

Temperature has an indirect effect on the process of gel formation by sodium alginate. Increasing the reaction temperature of the system can accelerate the dissolution rate of calcium salt and sodium alginate, increase the intensity of molecular movement within the system, and accelerate the combination of Ca2 + with sodium alginate molecules, thus accelerating the gelation process. However, when the system temperature exceeds a certain limit, the gelation process will not occur [2].

Alginate has a method of gel formation by cooling and condensation, which involves dissolving the sodium alginate, acid, calcium salt and chelating agent used to prepare the gel together in hot water, and then cooling the solution to form a gel. When dissolved together in hot water, although the sodium alginate solution has come into contact with Ca2 +, it cannot be combined with Ca2 + when there is more heat energy on the chain of sodium alginate molecules, and the sodium alginate molecules can only react with Ca2 + to form a gel when the solution is cooled to a certain temperature.

Although sodium alginate is unable to form a gel at high temperatures, when an alginate gel is formed, the gel is thermally irreversible and can maintain its physical form at high temperatures.

pH

Adding acidic substances to reduce the pH value of the system can make some calcium salts that are insoluble under neutral and alkaline conditions be transformed into calcium salts with better solubility, such as mixing calcium hydrogen phosphate with sodium alginate in water and then adding in lactic acid, the insoluble calcium carbonate can react to produce calcium bicarbonate or calcium lactate with relatively good solubility, thus releasing Ca2 + and forming a gel with sodium alginate solution [2].