# Extraction, Characterization, and Applications of Pectins from Plant By-Products
## 5 Techno-Functional Properties and Application of Pectin  
### Pectin Gel  
A pectin gel is a three-dimensional network of macromolecules including a solvent. It is formed by physical or chemical changes that tend to decrease the solubility of pectin, promoting the formation of local crystallizations [10]. Physico-chemical and functional properties of pectins, such as gelling properties, are highly related to their structures, including their Mw, DE, GalA content, and monosaccharide composition, which depend on plant sources and extraction methods [26; 40]. Regardless of their DM, pectin can form gels by different mechanisms. In the case of HM pectin, the higher the DM is, the faster the gel is formed. LM pectin is capable of forming gel by strongly binding divalent ions [72]. Commercial HM pectin (generally at 8-11% methoxyl content) are capable of forming a gel with a high sugar content (>65% sugar) and acidic pH (2.20-2.80) [112; 113]. The low pH reduces the electrostatic repulsion and the sugar helps to bring the chains closer together and thus the establishment of hydrogen bonds due to the reduction in the water activity [114]. The pectin-water interaction is therefore favored by these two elements. The formation of the three-dimensional network is allowed in part because of the hydrophobic interactions existing between the methyl groups [115]. The formation of gel depends on the time, temperature, and the kind of sugar used [116]. LM pectin gel occurs when coordination bonds are formed between carboxylate groups (at the level of homogalacturonic unesterified zones) and divalent cations (calcium) [117]. The formation of pectin-calcium cation gels is characterized by the \"egg box\" model (Figure 5) [118]. LM pectins with less than 7% methoxyl content can form gels at a lower sugar content than HM pectins [119]. Therefore, the number of applications of LM pectins is larger and different to those of HM pectins. Gels formed by high molecular weight pectins (typically Mw \(\geq\) 300 kDa) always have compact network structures with high mechanical strength, rupture strength, and viscosity [120]. Pectin with a low molecular weight (\(\leq\)10 kDa) was not able to form gels [14]. The methylation degree decreased gradually when increasing the acidity of extraction media and extraction time [40; 93]. Galacturonic acid (GalA) residues of pectins dominate the cross-link between pectin and Ca\({}^{2+}\) ions. Branched chains can negatively affect the gelation of pectin, inhibiting binding between pectin and Ca\({}^{2+}\)[121]. The high percentage of GalA is essential for the formation of Ca-dependent gels with increased stability, mechanical strength, and water-holding capacity [122]. The Ca-dependent gelation of pectin and gel properties can also be affected by the inter- and intra-molecular distribution of non-methoxylated GalA blocks [123]. Compared to a random distribution, pectin with a block-wise distribution of non-methoxylated GalA blocks showed a better gelation ability and a stronger mechanical strength of the final gels [123; 124]. The degree of methoxylation strongly affects Ca-pectin gel properties. Pectin with a low DM forms gels with high viscosity and hardness, because  
Figure 5: Gelation of LM pectins in the presence of Ca\({}^{2+}\) cations. the methoxylated carboxyl groups in GalA units cannot bind Ca\({}^{2+}\) as they are not negatively charged and, accordingly, affect the formation of the egg box structure and thus the gel mechanical properties [124; 125]. The acetylation of GalA in pectin resulted in a reduced number of Ca-binding sites, which is unfavorable to the gel formation [123]. The acetyl groups were reported to strongly prevent the formation of dimers (Ca-pectinate) [126]. The concentration of pectin can significantly influence their gelation process and gel properties (e.g., viscosity, elasticity, hardness, strength, or rupture strength) [127]. As compared with low Mw, LM pectin with higher Mw, accordingly, has a longer molecular chain and thus more Ca\({}^{2+}\)-binding sites.