**3.1 Degree of esterification**

The DE of pectin is the ratio of esterified D-galacturonic acid (GalA) groups to total GalA groups [34, 61]. Depending on the species, tissue, and maturity of the plant, the DE can have a wide range. In general, the structure of pectin is mostly composed of homogalacturonan (HG), regions (partially 6-methylated and 2- and/or 3-acetylated poly-α(1–4)-D-galacturonic acid residues), alternating with rhamnogalacturonan I (RG-I), regions (branched α(1–2)-L-rhamnosyl-α(1–4)-Dgalacturonosyl chains substituted with side chains of mainly α-L-arabinofuranose and α-D-galactopyranose) [18, 62]. The interconnection of HG "smooth" (responsible for the gelling capability) and RG-I "hairy" (play a gel-stabilizing role) regions, in relative proportions determine the flexibility and rheological properties of the polymer in solution [63, 64]. The gelling mechanism of pectin is dictated by its degree of esterification (total methoxyl content) [65]. Pectin based on the DE can be classified as high methoxyl (HM) pectin with DE > 50% or low methoxyl (LM) pectin DE < 50%, which are either the conventionally demethylated or the amidated molecule [66–68]. The two groups of pectin gel by different mechanisms. To form gels, high methoxyl pectin requires a minimum amount of soluble solids and a pH of around 3.0.

HM pectins are generally hot water-soluble, thermally reversible, and often contain dextrose (a dispersion agent) to prevent lumping. Conversely, LM pectins produce gels independent of sugar content, are less sensitive to pH compared to the HM pectins, and require the presence of a controlled amount of calcium or other divalent cations for gelation [41].

The specific application to which pectin will be put is a function of its gelling behavior, which is dependent on its DE, the monosaccharide content (HG), and the spatial disposition of the cross-linking blocks (RG) [69]. While HM pectins have been used in tablet formulations as a binder, controlled-release matrix and taste masker through complexation with bitter molecules, the LM pectins have been used as sustainedrelease matrices in microspheres produced by ionotropic gelation [19, 53, 69].

#### **3.2 Degree of blockiness**

Pectin, an anionic cell wall polysaccharide through its non-methyl esterified galacturonic acid units, interacts with divalent cations [40, 47]. At pH values above the pKa of pectin (2.8 to 4.1), non-methyl esterified GalA residues can be negatively charged, giving pectin the ability to interact with cations [34, 70]. Thus, the lower the DE of pectin, the higher the number of non-methyl esterified GalA residues present, the higher the cation-binding capacity. Due to LM pectin's higher number of negatively chargeable carboxyl groups (non-methyl esterified carboxyl groups) compared to HM pectin, it exhibits a higher charge density, further showing that the cation-binding capacity of pectin increases with decreasing DE [40, 70, 71]. Studies have shown that regardless of the method used, a stronger and higher bound interaction occurs between pectin with decreasing DE and cations (Fe2+, Zn2+, or Ca2+) [47]. Furthermore, the DE and the intramolecular distribution of the non-methyl esterified carboxyl groups within the pectin determine pectin's anionic nature and associated functionality [72, 73]. Interestingly, less described in the literature is the influence of the distribution pattern of non-methyl esterified GalA units on the cation-binding capacity of pectin compared to DE [47].

Daas et al. first quantified the relative occurrence of blocks of non-methyl esterified GalA units within a pectin chain as the degree of blockiness, DB [47, 74]. Apart from the DB, the absolute number of non-methyl esterified GalA units present in blocks can be expressed as the absolute degree of blockiness (DBabs). Both parameters (DB and DBabs) were established by exhaustive enzymatic degradation of pectin using endo-polygalacturonase (endo-PG) of *Kluyveromyces fragilis*, which required at least four consecutive non-methyl esterified GalA units to hydrolyze the linkage between two non-methyl esterified GalA units [70]. The DB is the proportion of galacturonic acid units (mono-, di-, and tri-) released by the enzyme to the total amount of non-methyl esterified GalA units, while DBabs is the number of

*Pharmaceutical Applications of Pectin DOI: http://dx.doi.org/10.5772/intechopen.100152*

GalA oligomers released in the endo-PG digest to the total number of GalA units in the pectin polymer, without adjustment of the DE [47, 74, 75]. Thus, to characterize the presence of blocks of non-methyl esterified GalA units, these parameters (DB and DBabs) are used [70]. For most of the cations (divalent cations), the binding between them and pectin is known to follow the egg-box model [47]. The egg-box model of ´binding was mainly described for pectin-Ca2+ binding but assumed to be applicable for interaction between pectin and other divalent cations [76]. However, Assifaoui et al. [77] reported that the egg-box model was more appropriate for Zn2+ binding than Ca2+ as they found that Zn2+ interacts with both carboxyl and hydroxyl groups, comparable to the egg-box model, whereas Ca2+ binds only via carboxyl groups [40]. This egg-box model yields stronger gels [78, 79].

Applications to which a high DB is required would thus mean high cationbinding capacity and hence the use of LM pectins and the converse is true.

#### **3.3 Ash value**

The ash content of pectin is a valuable tool in determining the purity as well as the gel-forming capability of the polymer. The ash content of pectin has been found to increase as the yield of pectin decreases [80]. High levels of ash in pectin may be caused by elevated concentrations of negatively charged carboxylic groups of pectin and the counterions in solution during pectin precipitation [41]. However, for gel formation, low ash content (≤ 10%) is a more favorable criterion as this will aid in determining the applicability of the polymer [47, 81, 80]. Ash content along with the anhydrouronic acid value of pectin has also been used to determine its purity [82, 83].

#### **3.4 Solubility**

Pectins are soluble in pure water. The solubility appears to depend on the valency of the cation salt; monovalent cation salts of pectin and pectic acids are usually soluble in water, while the di- and trivalent cation salts are weakly soluble or insoluble in water. Dry powdered pectin hydrates very rapidly when added to water, but tends to form clumps. These clumps are semidry packets of pectin within a highly hydrated outer coating. Dry mixing the powder with water-soluble carrier material can prevent the formation of clumps or by the use of specially treated pectin that has improved dispersibility [20, 83]. Studies have shown that pectin extracted with distilled water showed a high yield and low ash content when compared to other solvents [79]. High ash content and the drying process of the extracted pectin, however, may reduce the solubility of pectin [47]. It has been shown too that a decrease in the esterified carboxylic group reduced the solubility of extracted pectin; this insolubility of the extracted pectin is probably due to the presence of electrolytes in de-methylated pectic acid [47]. Thus, pectins with lower DE are less hydrophilic [69].

Dilute pectin solutions are Newtonian in behavior but at a moderate concentration, they exhibit the non-Newtonian, pseudo plastic behavior characteristics. Solubility, viscosity, and gelation are generally related. Whatever factors increase gel strength will increase the gelling tendency, viscosity, decrease solubility, and vice versa [84].

#### **3.5 Antioxidant activity**

Another property of pectin that could affect its application is its antioxidant activity. However, there are limited studies to show how this property may be applied to either the food industry or the pharmaceutical sector [85].
