# Use of Anionic Polysaccharides in the Development of 3D Bioprinting Technology
## 2 Overview of Natural Anionic Polysaccharides from Plant, Bacteria and Algae
### Polygalacturonic Acid from Pectin  
Pectin is a glucidic polymer of high molecular weight, present practically in all plants where it contributes to the cell structure [16; 17]. The term pectin covers a number of polymers, which vary according to their molecular weight, chemical configuration and neutral sugars content [17]. It is an heteropolysaccharide found in and between plant cell walls. The amount and composition of the polymer vary with the plant species but all pectins are based on a backbone of anhydroogalacturonic acid [17; 18]. The dominant feature of pectins consists of a linear chain of \(\alpha\)-(1\(A\))-linked p-galacturonicacid (GaA) units called polygalacturonic acid (PGA) (Figure 1) [17; 19; 20], together with various neutral sugars, mainly rhamnose (Rha), arabin (Ara) and galactose (Gal). In addition, non-sugar components such as methanol, acetic acid and ferulic acid (FerA) are often associated with pectic polysaccharides (Table 1). The galacturonic acid chain is partly esterified as methyl esters. Although the esters are the most significant components on the galacturonic acid backbone, other chemicals, such as acetyl, can be important in specific pectin types [17]. The percentage of galacturonic acid in the whole molecule is defined as the galacturonic acid content (% GA), which is set at a minimum of 65% in the definition of pectin as a food additive. The percentage of esterified galacturonic acid units relative to the total number of galacturonic acid units in the pectin structure is defined, respectively, by the degree of methylation (DM) and the degree of acetylation (DA) [17; 18]. Colorimetric procedures are the most widely used methods for galacturonic acid content determination. In concentrated acid medium, furfuric derivatives are generated and conjugated with aromatic compounds (m-hydroxybiphenyl, 3,5-dimethylphenol) [28; 29] to give colored products that are quantified by colorimetry. [PERSON] (1979) [30] automated the m-hydroxybiphenyl method. Ttirimetry is also widely used and is the main reference method for commercial pectin quantification. In their acidic form, pectins are titrated by a dilute alkali, saponified and titrated again. This \"double titration\" gives access to both DM (degree of methylation) and galacturonic acid content, but it suffers from a lack of specificity and cannot be used on acetylated pectins. The method also requires large quantities of pectins (from 100 mg to 1 g) for its operations. Regarding rheological behavior, solutions with more than 1% (w/v) pectin exhibit pseudoplastic behavior and are strongly affected by calcium. They gel in the presence of calcium ions or other divalent cations. Then, there are \"eggbox\" junction zones between the regular regions of acidic homogalacturonans, which can stack on top of one another. It is clear that the dominant factor in the gelling of pectin strongly depends on its degree of esterification, but the structure of the complex network is most often the result of a combination of several mechanisms. The assertion that high ester pectins will gel with sugars and acids, while gelling \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Origin** & **GalA** & **Rha** & **Ara** & **Xyl** & **Gal** & **FerA** & **Reference** \\ \hline
**Citrus** & 792 & 14 & 11 & 2 & 24 & ND\({}^{*}\) & [21] \\ \hline
**Apple** & 731 & 23 & 44 & 17 & 42 & ND & [21] \\ \hline
**Quinoa** & 625 & 38 & 33 & 18 & 51 & 3 & [22] \\ \hline
**Carrot** & 613 & 43 & 35 & 3 & 79 & ND & [23] \\ \hline
**Beet** & 558 & 28 & 124 & 3 & 47 & 7 & [24] \\ \hline
**Grape** & 533 & 58 & 77 & 6 & 44 & ND & [25] \\ \hline
**Pea** & 449 & 102 & 144 & 155 & 75 & ND & [26] \\ \hline
**Pear** & 380 & ND & ND & ND & ND & ND & [20] \\ \hline
**Potato** & 350 & ND & ND & ND & ND & ND & [20] \\ \hline
**Glasswort** & 289 & 22 & 462 & 3 & 43 & 17 & [27] \\ \hline \hline \end{tabular} \({}^{*}\)ND: not determined.  
\end{table} 
Table 1: Galacturonic acid and neutral sugars contents (mg/g) of some acid extracted pectins.