**2. History**

Alginic acid was first discovered and patented (patent date: 12 January 1881) by the British chemical scientist E. C. C. Stanford, and he continued the work on its discovery, contributing to the elucidation of the chemical structure of alginic acid [3]. The Stanford patent explains how the alginate can be extracted by soaking the algae with water or diluted acid, then extracting with sodium carbonate, and then precipitating the alginate present in the solution by addition of acid [4].

In the second decade of the twentieth century, some scientific groups working separately with alginate found that uronic acid was one of the constituents of alginic acid. Moreover, this discovery led to further study in the years to come. These investigations led to the discovery of D-mannuronic acid in hydrolyzed alginate samples. The nature of the bonds in the uronic acid residues in the alginate was identical to that in the cellulose, through the β 1, 4 bond.

It was only in the 1950s that with the work of Fischer and Dörfel [5], through a chromatographic study of uronic acids, the presence of a different uronic acid from what had been identified was discovered, identifying this new acid with L-guluronic acid. And that acid had a considerable quantity in the sample analyzed, and as such, a quantitative method was developed to determine the two acids present in alginate, mannuronic acid and guluronic acid.

Thereafter, alginate was identified as a binary copolymer composed of residues of guluronic and mannuronic acids, but in general, it was reported that alginic acid was chemically homogeneous and of equal chemical structure, independent of the raw material from which it was extracted.

This principle had to be scientifically proven, and the alginate had to be fractionated chemically from different sources to prove the theory. The development of fractionation techniques was done mainly by Haug [6], who helped the characterization alginate as a block copolymer and in the correlation between the block structure and the physical properties of an alginate with that composition.

## **3. Chemical structure**

The alginate is an unbranched biopolymer family. The alginates consist of 1,4-β-Dmannuronic acid (M) and 1,4 α-L-guluronic acid (G) monomers, with a homogeneous (poly-G, poly-M) or heterogeneous (MG) block composition, which was proven by partial acid hydrolysis. That is, each alginate-producing species may exhibit different alginate compositions and as such differences in the ratio of mannuronic and guluronic acid blocks, varying in composition and sequence [7]. The proportions of the two acids vary from species to species and from different parts of the same seaweed [8].

It was proven that the alginates do not have regular repeating units and that the distribution of the monomers along the polymer chain could not be described; hence the knowledge of the monomeric composition was not sufficient to determine the sequential structure of alginates from diverse species.

Alginate is found in seaweeds as salts of different metals, primarily sodium and calcium, in the intercellular regions and cell walls. Its biological functions in seaweeds are of structural and ion exchange type. Alginate enriched in polymannuronic acid is found in young cell wall tissue and/or intercellular regions, whereas polyguluronic-rich alginate is found in the cell wall having a high affinity for Ca2+, which is mainly responsible for gel strength [9]. Alginate polymer is synthesized in the cytoplasm and then transported to the cell surface [10].

The principal differentiation between algal and bacterial alginates at the molecular level is the presence of Ο-acetyl groups at C2 and/or C3 in the bacterial alginates [11].

## **4. Alginate extraction**

The extraction methodology of alginate is recurring on the transforming of the insoluble mixture of the alginic acid salts prevenient of the cell wall in a

**5**

**Figure 1.**

*Anderson.*

*Introductory Chapter: Alginates - A General Overview DOI: http://dx.doi.org/10.5772/intechopen.88381*

resources utilized.

alginate production.

soluble alginate salt, which is naturally recommended for aqueous high-affinity polysaccharide obtained from the main species of brown algae (e.g., Fucales and Laminariales). The industry uses brown seaweeds mainly from the genus *Macrocystis*, *Laminaria* (**Figure 1a**), *Lessonia*, *Ascophyllum*, *Alaria*, *Ecklonia*, *Eisenia*, *Nereocystis*, *Sargassum*, *Cystoseira*, and *Fucus* (**Figure 1b**), with *Macrocystis pyrifera* (**Figure 1c**) and *Ascophyllum nodosum* (**Figure 1d**) being the principal

Various species are harvested, and some are even cultivated offshore

(e.g., *Laminaria* and *Alaria* (**Figure 1e**)) for alginate production, between southern and northern hemispheres. The species used are generally harvested from natural resources. China and some northern hemisphere countries, however, cultivate these species, which is an expensive way of alginate production. In this case, *Laminaria* is mostly used as food, and if surplus amounts are cultivated, then it can be used for

Brown seaweeds that grow in cold water and those growing at a temperature up to 20°C are more used because they have more alginic acid content than the brown seaweeds found in more warm waters. And due to more turbulent waters, the seaweeds produce more content than the same species in calmer waters [12].

The alginic acid appears in the seaweeds as an insoluble mixed salt. To extract it, it is necessary to convert the alginic acid into its soluble salt forms such as sodium or potassium [12]. The alginate is made alternately insoluble and soluble in solvent by ion exchange reactions to separate out from the other constituents of algae. As large molecules must diffuse out from the plant tissues, the seaweed is preferably reduced to small particles as a preliminary step. Therefore, the first step is to wash (dry, if necessary) and mill the seaweed. Alginate isolation is essentially an ion exchange process, and alginate is brought into solution as sodium alginate by treating it with a strong alkali, after a pre-treatment with hydrochloric acid before the extraction with sodium carbonate [13, 14]. There are several methods to separate the alginate

from other soluble substances from the crude alginate extract solution.

For example, addition of alcohol [2] would precipitate out sodium alginate. Adding a solution of calcium chloride with good stirring would precipitate out calcium alginate, whereas adding hydrochloric acid would precipitate out alginic acid.

*(a) Laminaria ochroleuca from Ínsua beach, afife, north of Portugal; (b) Fucus ceranoides from mondego river estuary (coordinates: 40° 7'31.39"N, 8°46'15.76"W), center-north of Portugal; (c) Macrocystis pyrifera; (d) Ascophyllum nodosum from Praia do norte, Viana Do Castelo, north of Portugal; (e) Alaria esculenta from Eskifjördur, Iceland; (f) Chlorella vulgaris immobilized in calcium alginate gel beads. Original images: (a, d, and e) University of Coimbra (MACOI), (b and f) João cotas images in public domain: (c) Shane* 

*Introductory Chapter: Alginates - A General Overview DOI: http://dx.doi.org/10.5772/intechopen.88381*

*Alginates - Recent Uses of This Natural Polymer*

the algae with water or diluted acid, then extracting with sodium carbonate, and then precipitating the alginate present in the solution by addition of acid [4]. In the second decade of the twentieth century, some scientific groups working separately with alginate found that uronic acid was one of the constituents of alginic acid. Moreover, this discovery led to further study in the years to come. These investigations led to the discovery of D-mannuronic acid in hydrolyzed alginate samples. The nature of the bonds in the uronic acid residues in the alginate

It was only in the 1950s that with the work of Fischer and Dörfel [5], through a chromatographic study of uronic acids, the presence of a different uronic acid from what had been identified was discovered, identifying this new acid with L-guluronic acid. And that acid had a considerable quantity in the sample analyzed, and as such, a quantitative method was developed to determine the two acids pres-

Thereafter, alginate was identified as a binary copolymer composed of residues of guluronic and mannuronic acids, but in general, it was reported that alginic acid was chemically homogeneous and of equal chemical structure, independent of the

This principle had to be scientifically proven, and the alginate had to be fractionated chemically from different sources to prove the theory. The development of fractionation techniques was done mainly by Haug [6], who helped the characterization alginate as a block copolymer and in the correlation between the block structure and the physical properties of an alginate with that composition.

The alginate is an unbranched biopolymer family. The alginates consist of 1,4-β-Dmannuronic acid (M) and 1,4 α-L-guluronic acid (G) monomers, with a homogeneous (poly-G, poly-M) or heterogeneous (MG) block composition, which was proven by partial acid hydrolysis. That is, each alginate-producing species may exhibit different alginate compositions and as such differences in the ratio of mannuronic and guluronic acid blocks, varying in composition and sequence [7]. The proportions of the two acids

vary from species to species and from different parts of the same seaweed [8]. It was proven that the alginates do not have regular repeating units and that the distribution of the monomers along the polymer chain could not be described; hence the knowledge of the monomeric composition was not sufficient to deter-

Alginate is found in seaweeds as salts of different metals, primarily sodium and calcium, in the intercellular regions and cell walls. Its biological functions in seaweeds are of structural and ion exchange type. Alginate enriched in polymannuronic acid is found in young cell wall tissue and/or intercellular regions, whereas polyguluronic-rich alginate is found in the cell wall having a high affinity for Ca2+, which is mainly responsible for gel strength [9]. Alginate polymer is synthesized in

The principal differentiation between algal and bacterial alginates at the molecular level is the presence of Ο-acetyl groups at C2 and/or C3 in the bacterial alginates [11].

The extraction methodology of alginate is recurring on the transforming of the insoluble mixture of the alginic acid salts prevenient of the cell wall in a

mine the sequential structure of alginates from diverse species.

the cytoplasm and then transported to the cell surface [10].

was identical to that in the cellulose, through the β 1, 4 bond.

ent in alginate, mannuronic acid and guluronic acid.

raw material from which it was extracted.

**3. Chemical structure**

**4. Alginate extraction**

**4**

soluble alginate salt, which is naturally recommended for aqueous high-affinity polysaccharide obtained from the main species of brown algae (e.g., Fucales and Laminariales). The industry uses brown seaweeds mainly from the genus *Macrocystis*, *Laminaria* (**Figure 1a**), *Lessonia*, *Ascophyllum*, *Alaria*, *Ecklonia*, *Eisenia*, *Nereocystis*, *Sargassum*, *Cystoseira*, and *Fucus* (**Figure 1b**), with *Macrocystis pyrifera* (**Figure 1c**) and *Ascophyllum nodosum* (**Figure 1d**) being the principal resources utilized.

Various species are harvested, and some are even cultivated offshore (e.g., *Laminaria* and *Alaria* (**Figure 1e**)) for alginate production, between southern and northern hemispheres. The species used are generally harvested from natural resources. China and some northern hemisphere countries, however, cultivate these species, which is an expensive way of alginate production. In this case, *Laminaria* is mostly used as food, and if surplus amounts are cultivated, then it can be used for alginate production.

Brown seaweeds that grow in cold water and those growing at a temperature up to 20°C are more used because they have more alginic acid content than the brown seaweeds found in more warm waters. And due to more turbulent waters, the seaweeds produce more content than the same species in calmer waters [12].

The alginic acid appears in the seaweeds as an insoluble mixed salt. To extract it, it is necessary to convert the alginic acid into its soluble salt forms such as sodium or potassium [12]. The alginate is made alternately insoluble and soluble in solvent by ion exchange reactions to separate out from the other constituents of algae. As large molecules must diffuse out from the plant tissues, the seaweed is preferably reduced to small particles as a preliminary step. Therefore, the first step is to wash (dry, if necessary) and mill the seaweed. Alginate isolation is essentially an ion exchange process, and alginate is brought into solution as sodium alginate by treating it with a strong alkali, after a pre-treatment with hydrochloric acid before the extraction with sodium carbonate [13, 14]. There are several methods to separate the alginate from other soluble substances from the crude alginate extract solution.

For example, addition of alcohol [2] would precipitate out sodium alginate. Adding a solution of calcium chloride with good stirring would precipitate out calcium alginate, whereas adding hydrochloric acid would precipitate out alginic acid.

#### **Figure 1.**

*(a) Laminaria ochroleuca from Ínsua beach, afife, north of Portugal; (b) Fucus ceranoides from mondego river estuary (coordinates: 40° 7'31.39"N, 8°46'15.76"W), center-north of Portugal; (c) Macrocystis pyrifera; (d) Ascophyllum nodosum from Praia do norte, Viana Do Castelo, north of Portugal; (e) Alaria esculenta from Eskifjördur, Iceland; (f) Chlorella vulgaris immobilized in calcium alginate gel beads. Original images: (a, d, and e) University of Coimbra (MACOI), (b and f) João cotas images in public domain: (c) Shane Anderson.*

The extraction of alginate is done by mild acid treatments that remove undesirable compounds (normally, hydrochloric acid) and modify the cell wall alginate into alginic acid to obtain the best extract efficacy because the intercellular mucilage has been regarded as the principal site of alginic acid [15]. The alginic acid is recovered as a soluble sodium form by neutralizing with sodium carbonate or sodium hydroxide. The insoluble residue is removed by filtration, flotation, or centrifugation, and the soluble alginate is precipitated by conversion into alginic acid or calcium/sodium alginate. The alginic acid is then converted into the required counter ion by neutralization with appropriate hydroxides or chlorites. The difference in the alginate recovery process depends on the source and structure of constituents of alginate [13].
