Why does sodium alginate gel?

sodium alginateIt is the sodium salt of alginic acid, a chemical substance, which is generally used for thickening/anti-sedimentation/molding of food/beverage/beer, etc., and textile printing and dyeing is used for thickening with reactive dyes.

　　Water-soluble sodium alginate is a widely used food additive because it can form colloid after contacting with calcium ions. When the sodium alginate polymer was crosslinked by calcium ions and lost its rheology, the flow of water molecules was inhibited, resulting in a gel with extremely high water content. Different from other food gums, the colloid of sodium alginate is not thermally reversible and has good stability.

　　Trehalose is the name of a candy, which may contain a small amount of seaweed fragments, or candy with colloidal shape made of alginic acid (not necessarily sodium alginate), and gelatin (pure animal glue) may be used without any mixture of alginic acid, because sodium alginate is quite expensive at present. Water-soluble sodium alginate forms cross-linking bonds between macromolecules after reacting with multivalent cations (except magnesium). When the content of multivalent cations increases, the sodium alginate solution becomes more and more viscous, and finally forms gel and produces precipitate. Alginate colloid can be prepared by extruding sodium alginate solution into multivalent cation solution such as Ca2+, Sr2+ or Ba2+. Monovalent cation and Mg 2+ can't form gel, but the gel formed by Ba2+ and Sr2+ has stronger performance than that formed by Ca2+. Other multivalent cations, such as Pb2+, Cu2+, Cd2+, Co2+, Ni2+, Zn2+ and Mn2+, can also form sodium alginate cross-linked gel, but its application is limited because of its toxicity.

　　All divalent or multivalent metal ions except magnesium can form colloid after reacting with sodium alginate aqueous solution. Calcium ion is the most commonly used gelling agent. Because the reaction between calcium ion and sodium alginate is rapid, the method of adding calcium ion into sodium alginate system has great influence on the properties of the gel finally obtained. If calcium ion is added too quickly, the colloid produced is flaky gel and discontinuous gel structure. The speed of adding calcium ions can be controlled by slowly dissolving calcium salts or by adding multivalent chelating agents, such as tetrasodium pyrophosphate or sodium hexametaphosphate.

　　The basic methods of forming sodium alginate colloid in industry include the following methods:

　　1, dialysis/diffusion method

　　This is one of the most commonly used methods. When using this method, the aqueous solution of sodium alginate forms colloid after contacting with calcium ions from outside. The colloid formed in this way is generally uneven, because a part of sodium alginate that comes into contact with calcium ions early solidifies and shrinks after gel formation, and its solid content is higher than that of the colloid formed later. The lower the concentration of calcium ion, the smaller the relative molecular weight of sodium alginate, the higher the concentration and the higher the content of G acid, the stronger the heterogeneity. Because the gelation rate is limited by the diffusion rate of calcium ions, this method has limited application and can only be used to prepare thin sheet materials.

　　2. In situ method

　　This method generally uses calcium salts with low solubility or calcium ions coordinated with other materials. After fully mixing with sodium alginate, weak acid with slow release effect is added. Calcium ions are released under the action of acid and then combine with alginic acid to form colloid. The colloid formed in this way is very uniform, and the colloid which is not fully crosslinked, that is, the mixed colloid of calcium and sodium alginate, can also be prepared.

　　3. Cooling method

　　Because calcium ions in the solution can't combine with alginic acid at high temperature, the colloid of alginic acid can be prepared by mixing calcium ions with sodium alginate at high temperature and cooling.

　　4. Crosslinking method

　　This method uses epichlorohydrin, which is crosslinked by Xiang group in alginic acid molecule. Alginate loses its solubility after crosslinking and forms colloid. The colloid thus formed has stable structure and high water content, and can absorb 50-200 times of dry weight.

　　The properties of colloids formed under different processing conditions are quite different. The following are some basic rules about alginate colloid, which can be used to change the compactness and setting time of the gel:

　　(1) When the content of calcium chelate increases, the rigidity of gel decreases. On the contrary, when the content of calcium chelate decreases, the rigidity of gel increases. However, if the amount of chelating agent is too small, a gel with small particles is formed.

　　(2) The amount of calcium added is reduced, and the generated gel is softer; With the increase of calcium content, the gel formed is stronger. However, too much calcium will produce granular gel and even calcium alginate precipitate.

　　(3) In the acidic gel system, adding some slow-soluble acid can accelerate the solidification of the gel when its acid content increases, but it may produce granular gel.

　　(4) When the concentration of soluble sodium alginate increases, the generated gel becomes firm, but its structure may be uneven. The higher the concentration of sodium alginate, the more brittle the gel obtained and the sharper the edge of its fragments.

　　(5) As the amount of calcium added approaches the stoichiometric amount required for complete reaction with sodium alginate, the dehydration shrinkage tendency of the gel obtained increases.