**2. Pectin structure**

Pectin is a negatively charged branched heteropolysaccharide, composed of up to 17 different monosaccharides with more than 20 types of linkages [5, 6]. This polysaccharide was first reported in 1825 by Braconnot and defined as a biopolymer rich in galacturonic acid (GalA; up to 65%) [7]. Although the precise structure of pectin has not yet been fully elucidated due to its complexity, three major polysaccharide domains are recognized; as shown in **Figure 1**, the most abundant is based on a linear homopolymer of α-(1–4)-linked-D-galacturonic acid (GalpA, GalA) residues that can be methyl esterified at the C-6 position and to a lesser extent O-acetylated in C-2 and C-3; this domain is defined as homogalacturonan (HG) [5, 7]. In the rhamnogalacturonan I (RG-I) domain, the rhamnose (Rhap, Rha) residues disrupt the HG structure to form a preferably ramified structure of pectin (20–35%) due to the presence of the repeating disaccharide [→4)-α-D-GalpA-(1→2)- α-L-Rhap-(1-]. Here, the GalA residues are not methyl esterified, and attachment of neutral sugar side chains [α-L-arabinose (Araf, Ara) and β-D-galactose (Galp, Gal)] to the C-4 positions of Rha residues can be suitable, leading to linear side chains (LC-A) when α(1→5)-L-Araf or linear type I (β(1→4)-L- Galp) or branched side chains (LC-B) when α(1→2,3)-L-Araf or branched

#### **Figure 1.**

*The schematic representation of the pectin structure contains the HG, RG-I, and RG-II domains. L-AcA: L-Aceric acid. Adapted from [8].*

*New Sources of Pectin: Extraction, Processing, and Industrial Applications DOI: http://dx.doi.org/10.5772/intechopen.109579*

type II β(1→3,6)-D-Galp and arabinogalactans. The branching design of the structure in RG-I depends on the pectin source, the extraction conditions, and the presence of other sugars such as xylose (D-Xyl), fucose (L-Fuc), and glucuronic acid (D-GlucA), among others [9]. The RG-II domain (1–8%) is constituted of around nine α(1→4) linked GalpA units partially methyl esterified with four heteropolymer side chains attached, mainly composed of 11 monosaccharide residues, including apiose (D-Api), 2-O-methyl-L-fucose, 2-O-methyl-D-xylose. 3-C-carxy-5deoxy-L-xylose, 3-deoxy-Dmanno-octulosonic acid (Kdo), and 3-deoxy-D-lyxoheptulosaric acid (D-Dha), which are linked with up to 22 glycoside bonds [10, 11].

Some investigations about the basic structure of pectin establish that although the pectin source may influence the structure diversity by partially modifying the chain conformation of the macromolecule, the RG-II region seemed to be well preserved among the different sources [12]. Moreover, pectins contain functional groups besides carbohydrate type, such as phenolic acids, methanol, acetic acid, and some amide groups. Methanol and acetic acid are relevant in the esterification of galacturonic acid residues for developing the inherent structure functionalities of pectin. The degree of methylation (DM) is a helpful tool for describing the structure of pectin and potential applications; high methoxy pectins (HM) contain more than 50% of carboxyl groups in methylated form, while those with lower content are defined as low methoxy pectins (LM). Most common native pectins are characterized by being methyl esterified. Likewise, acetylation in pectins rarely occurs in native pectins. The degree of acetylation (DA) in pectins is defined as the percentage of galacturonosyl residues that can be acetylated per unit of monosaccharide. DA can be larger than 100% and is usually found in the branched RG regions. In pectins from citrus and apple, the acetyl groups in the HG region are present in low content, rather than in pectins from sugar beet and potato, where higher amounts have been found [13, 14]. Amidation of pectins does not occur naturally; instead, it is induced chemically or enzymatically to improve the functional properties such as solubility in water, gelling, and rheological properties through modifying some non-esterified carboxyl groups into amide groups by using various amino compounds [15–17].
