Alginic acid and alginate are mainly polysaccharides extracted from brown algae of the genus Kombu, as well as Ecklonia maxima, Macrocystis pyrifera, Ascophyllum nodosum, Fucus serratus and other varieties of seaweeds in the genus. Alginic acid and alginate is a main product of seaweed industry in China. According to its nature, it can be mainly divided into water-soluble gum and insoluble gum two categories. Water-soluble alginate includes monovalent salt of alginate (sodium, potassium, ammonium alginate, etc.), two divalent salts of alginate (magnesium alginate and mercury alginate) and alginate derivatives; water-insoluble seaweed gum includes alginate, divalent salts of alginate (except magnesium and mercury salts) and trivalent salts of alginate (aluminum alginate, iron, chromium, etc.). The most widely used of these are sodium alginate, calcium alginate and propylene glycol alginate.

This type of alginate is found in the cell walls of seaweeds, and in its natural state it is a mixture of insoluble alginate (calcium, magnesium, sodium, potassium) salts. When extracted commercially, it is first treated with acid to convert it into insoluble alginate, then treated with alkali to form a soluble alginate solution, and then through a number of processes such as purification and filtration, it can be obtained through the addition of different substances to obtain different commercial alginate gum. Alginate is obtained through acid treatment, calcium alginate is obtained through CaCl2/CaCO3 treatment, sodium alginate is produced through Na2CO3 treatment, and ammonium alginate is produced through neutralization with carbonic acid. Alginate is reacted with propylene oxide to produce another important chemically modified derivative of alginate, propylene glycol alginate (PGA). Alginate is widely used in food, pharmaceutical and other industries due to its unique gel properties and its ability to thicken, stabilize, emulsify, disperse and form films.

1. Chemical composition and structure of alginate

Seaweed gum or alginate is the main polysaccharide structural component of brown seaweed. Alginate polymer consists of two monomers: β(1→4)-D-mannuronic acid unit and α(1→4)-L-guluronic acid unit, these two monomers alternately combined with each other to become three different structural chain segments, whose structure is: by the chain segment composed of mannuronic acid (-M-M-M-M-); by the chain segment composed of guluronic acid (-G-G-G-G-); by the alternation of the two monomer (M-G-M-M-G); a chain segment composed of guluronic acid (-G-G-G-G-); and a chain segment composed of two alternating monomers (M-G-M-G). The polymer molecule of seaweed gum consists of these three chain segments. The molecular weight can be as high as 200,000 molecules. The ratio of monomers to chain segments varies and depends on the raw material of the alginate. Different sources contain different ratios of mannuronic acid (M) to guluronic acid (G), resulting in different uses and properties. In a molecule, it may contain a continuous chain segment made up of only one of the glyoxylates, or it may be a block copolymer made up of two glyoxylate links. Variations in the proportions of the two glucuronic acids in the molecule, as well as differences in their location, can lead directly to differences in the properties of alginate, such as viscosity, gelling properties, and ion selectivity.

The polyguluronic acid chain segments are more rigid than the polymannuronic acid chain segments and have a larger nematic volume in solution, while the chain segments consisting of different kinds of glycoaldehydic acid links have better flexibility and a smaller nematic volume in solution than those consisting of the two glycoaldehydic acids mentioned above alone. All other things being equal, the greater the rigidity of the chain segments of the alginate molecule, the greater the viscosity of the solution prepared and the greater the brittleness of the gel formed．

Each kind of seaweed contains its different structure of seaweed gel, the special structure of seaweed gel has a great influence on its properties, especially on the presence of calcium ions when the gelling effect. The polyguluronic acid chain segments bind very strongly to calcium ions and form a fully polymerized reticular structure. The polymannuronic acid chain segments, while also binding to calcium, are not as strong. The calcium ion binds preferentially to the guluronic acid and also binds well to the guluronic acid residues between the two different chain segments. Complex binding between many chain segments on different molecules together form a complete mesh structure and form a gel. High molecular weight, low calcium content or high glucuronic acid composition of the chain segments of the seaweed gum formed a hard gel, has good gelling properties, generally used in food as a gelling agent. On the contrary, alginate gum with low molecular weight, high calcium content or containing chain segments composed of high mannuronic acid is often used as a thickener in food.

2. Chemical derivatives of alginic acid

Alginic acid can be made into a number of derivatives through the later chemical modification process. Propylene glycol alginate (PGA) is one of the most typical derivatives, but also has realized the industrial production and a large number of alginate derivatives have been applied.PGA has acid stability, and can prevent the precipitation caused by calcium and other high-valent metal ions, which has obvious advantages in the application of some acidic food.

In addition, alginate can be reacted with organic amines to produce ammonium alginate salts. Organic amines that can be used include: triethanolamine, triisopropylamine, butylamine, dibutylamine and dipentylamine. Ammonium alginate can also be produced by reacting PGA with primary amines such as ammonia, ethanolamine, ethylenediamine, ethylamine, propylamine, isobutylamine and butylamine, but it is not easy to react with secondary amines. Industrial production of ammonium alginate is generally produced by neutralizing alginic acid with ammonia or ammonium carbonate. At present, although it has been able to synthesize alginate acetate and alginate sulfate, but it has not yet been applied in practice. Carboxymethyl alginate can be made by treating sodium alginate with chloroacetic acid and alkali, and a series of hydrocarbon-based diol esters of alginate can also be synthesized. Ethylene oxide and alginate reaction can generate 2-hydroxyethyl alginate.

3. physical properties of alginate

Commercially useful water-soluble seaweeds include monovalent salts of alginate (sodium alginate, potassium alginate, ammonium alginate), calcium alginate, ammonium-calcium mixed salts of alginate, alginic acid, and propylene glycol ester of alginate.

Alginate, as a hydrophilic polysaccharide substance, readily absorbs water from the atmosphere and thus the equilibrium moisture content is related to the relative humidity. Alginate has good dry storage stability at room temperature or lower, so alginate products should be stored in a cool, dry place.

Alginate is a kind of hydrophilic polymer, when it is put into the water, if not stirred, the gel particles may be agglomerated, and its center part is not easy to be wet by the water, resulting in slow dissolution, which brings trouble to the use. In the production of the general use of high shear dissolution method, that is, in the non-stop high-speed stirring, slowly add the glue powder to the water, continue to stir until it becomes a thick glue. Appropriate heating during the dissolution process, or adding appropriate amount of sugar and other dry powder mixing and dispersing before adding to the water will also help the dissolution of alginate.

• Alginic acid

Alginate, molecular formula (C6H7O6H)n, white or light yellow powder, insoluble in cold water, soluble in alkaline solution, insoluble in organic solvents. It is odorless and tasteless, or has a slight special odor. pH value of 3% water suspension is 2.0-3.4, and it is precipitated by calcium salt. Alginic acid is a kind of polyglucuronic acid extracted from seaweeds (e.g. kelp, macroalgae, etc.), which can be used as stabilizer, thickener, emulsifier and gel-forming agent in the food industry, and it can be used as thickening stabilizer for ice cream, sauce, jam, bread, noodles, whipping cream, soup, etc.; defrosting adjusting agent for frozen food; suspending agent for soft drinks; coating agent for baked food; emulsifier for pudding and spray-dried cream powder. Emulsifier for pudding and spray-dried cream powder. Alginic acid can also be used in the pharmaceutical and health care industry, as an anti-obesity agent and a new type of agent for the treatment of gastric diseases have a greater medical value, at the same time, it is also an important raw material for the production of propyleneglycol alginate, alginic acid triethylamine, alginic acid bis(sodium salt) (PSS) and so on.

• Sodium alginate

Sodium alginate, also known as sodium fucoidan, kelp gum, brown algae gum, alginate, is a white or light yellow powder or particles, odorless, tasteless, soluble in water, its aqueous solution is a viscous colloid, insoluble in alcohol and other organic solvents. The molecular formula is C5H7O4COONa)n. It is widely used in food, medicine, textile, printing and dyeing, papermaking, daily-use chemical industry, etc. It is mainly used in the food industry as stabilizer, thickener, emulsifier, dispersing agent and coagulant in the processing of cold drinks, pastries, candies, instant beverages and foodstuffs, etc. Especially since the 1980s, seaweed has been used in the processing of foodstuffs. Especially since the 1980s, sodium alginate has been continuously expanded in food applications. Sodium alginate is not only a safe food additive, but also can be used as the base material of bionic food or therapeutic food. Since it is actually a natural dietary fiber, it has been reported to slow down the absorption of fatty acids and bile salts, and has the effect of lowering serum cholesterol, blood triglycerides, and blood glucose, which can prevent modern diseases such as hypertension, diabetes, and obesity. It can inhibit the accumulation of harmful metals such as strontium, cadmium and lead in the body in the intestinal tract. It is because of these important roles of sodium fucoidan that it has been increasingly emphasized at home and abroad.

• Potassium alginate

Potassium alginate molecular formula: (C6H7O6K)n, properties: white to light yellow irregular powder, odorless, tasteless, easily soluble in water to form a viscous solution, insoluble in ethanol or ethanol content higher than 30% (wt) of the hydroalcohol solution, insoluble in chloroform, ether and pH less than 3 acid. Potassium alginate can be generally obtained by reacting alginate with potassium carbonate or potassium hydroxide.

It can be used as stabilizer and thickener in canned food, ice cream, noodles and other food according to GB2760 of China. Uses: Mainly used in medicine and food industry. Potassium alginate is a kind of natural polysaccharide carbohydrates extracted from seaweeds, which is reported to have the effect of lowering blood fat, blood sugar, cholesterol, etc. It is mainly used in pharmaceuticals and health food.

• Ammonium Alginate

Ammonium alginate is white to light yellow fibrous powder or coarse powder, almost odorless and tasteless, slowly dissolved in water to form a viscous colloidal solution, insoluble in ethanol and ethanol content higher than 30% (wt) of the hydroalcohol solution, insoluble in chloroform, ether and pH value of less than 3 acid solution. Its industrial production method is generally obtained by neutralizing alginate with ammonia or ammonium carbonate.

• Calcium alginate

Calcium alginate, molecular formula: [(C6H7O6)2Ca]n, white powder to light yellow indefinite powder, odorless, tasteless, insoluble in water and organic solvents, insoluble in ethanol. Slowly soluble in sodium polyphosphate, sodium carbonate solutions and solutions of calcium compounds. Its industrial system is generally obtained by the reaction between alginate and calcium hydroxide or calcium carbonate.

4. Rheological properties of alginate and influencing factors

There is no correlation between the viscosity of alginate and the ability to gel, in practice, there is no clear boundary between thickening and weak gel, the presence of a small amount of calcium ions can make the viscosity increase, while a large number of calcium ions make the solution into a gel. Pure alginate dissolved in distilled water makes a homogeneous solution with high fluidity. Physical factors that affect the fluid properties of alginate solutions include temperature, shear rate, polymer particle size, concentration, and solvents miscible with distilled water. Chemical factors affecting alginate solutions are: pH, chelates, various cations and quaternary amine compounds.

• Rheological properties of alginate solutions

The concentration of alginate solution is an important factor affecting the rheological properties of alginate solution. For example, the medium viscosity of sodium alginate solution, when the concentration of 0.5%, in the low shear rate range for the Newtonian fluid characteristics, in the high shear rate on the performance of non-Newtonian fluid characteristics; but when the concentration of 2.5%, in both the low and high shear rate are shown as non-Newtonian fluid characteristics. Similarly, a 3% solution of propylene glycol alginate exhibits shear thinning over a wide range of shear rates; whereas at a concentration of l% or less, the solution has an almost stable viscosity and does not exhibit shear thinning at shear rates below lOO s-1.

Sodium alginate has a high molecular weight and molecular rigidity, and high apparent viscosity solutions can be obtained even at low concentrations.

The viscosity-shear curves of medium-viscosity sodium alginate and potassium alginate are consistent over the entire range of shear rates. The viscosity-shear curves of low-viscosity PGA and sodium alginate essentially overlap over the range of shear rates higher than 10,000 s-1, and bifurcate only at lower shear rates.

• Factors affecting the rheological properties of alginate solution

Temperature

When the temperature increases, the viscosity of alginate solution decreases, and the viscosity decreases by about 12% for every 5.6℃ increase in temperature. If it is not under high temperature for a long time, the viscosity can be recovered when the temperature is lowered. Heating causes thermal degradation of alginate, the degree of which is temperature and time dependent. Although lowering the temperature of the alginate solution will increase the viscosity, but will not generate a gel, the alginate solution will be frozen, and then thawed and thawed again, its viscosity will not change.

Solvent

The addition of small amounts of non-aqueous solvents that are miscible with water, such as ethanol, ethylene glycol or acetone, will increase the viscosity of alginate solutions and ultimately lead to the precipitation of alginate. The permissible limits of alginate solutions for these solvents are influenced by the source of the alginate, the degree of polymerization, the type of cation present, and the concentration of the solution.

Concentration

Similar to most other food gels, the viscosity of alginates such as sodium alginate, ammonium alginate, potassium alginate and PGA increases with their concentration in aqueous solutions. Of course, there are large differences in the viscosity increase for various viscosity grades of alginates.

pH

Generally speaking, alginate is more stable under acidic conditions, especially for PGA. pH value should be lowered to 3.0 when PGA may gel, higher than 7.0 will be saponification and decomposition, while the pH value of 3.0-7.0 is quite stable, so PGA is very suitable for the application of acidic food.

Gelation

Alginate can react with many high-valent cations (except magnesium) to produce cross-linking. When the content of multivalent cations increases, the alginate solution thickens and forms a gel, which eventually precipitates.

All alginate gels are the result of interactions between alginate molecules, and they are thermally irreversible. The structure and strength of the gel can be adjusted by choosing the appropriate gelling agent.

Multivalent metal ions, such as zinc, aluminum, copper, in the presence of excess ammonia, can generate complexes with alginate. When ammonia is removed from this system, insoluble alginate is produced. Calcium is most commonly used to change the fluid properties of alginate solutions and gelling properties of polyvalent cations, calcium can also be used to prepare insoluble alginate fibers and films.

Adding calcium to an alginate system can significantly change its gelling properties. However, it must be noted that if the calcium is added too fast, it may lead to local reaction too fast, affecting the homogeneity of the whole system, generating discontinuous gel. Therefore, try to use calcium salts can be slowly dissolved, or add such as sodium tripolyphosphate or sodium hexametaphosphate such as integrators, in order to control the rate of calcium.

5. The role of alginate and protein

Alginate and other water-soluble gum similar to the role of proteins. The main use of this action can be used for precipitation recovery of proteins. It is generally believed that in the controlled action of alginate and protein, hydrogen bonding and van der Waals forces are important factors leading to this action. It also depends on the charge carried by the macromolecule, with the maximum interaction occurring at the smallest point of charge. Measurements of the viscosity of alginate-protein systems at different pH show that when the pH is lowered close to the protein iso-point, the viscosity of the system increases due to the formation of soluble complexes. If the pH is further reduced, precipitation of the complex occurs due to the loss of all the charge carried. In addition to being used to precipitate proteins, alginate can also be used to inhibit protein precipitation under appropriate conditions. Under the isoelectric point of proteins, the addition of an appropriate amount of alginate can lower the isoelectric point and inhibit the precipitation of proteins in order to maintain the proteins in solution. At lower pH (pH 3.5 to 4.0), alginate has a greater ability to precipitate proteins than pectin and carboxymethyl cellulose, which is mainly due to the fact that in the chain of the alginate molecule, the charge carried by the end group of each unit is higher than that of both pectin and carboxymethyl cellulose. In addition, the spatial configuration is also an important factor.