Pectin is mostly extracted from various plant sources and is of great variation in term of quality. Consequently, pectin is purified and restructured in order to achieve constant and reproducible gel strength, for example HMP is improved its quality by dilution with sucrose. MPP is typical of high methoxyl content which is unable to form gel by interaction with calcium ions due to an insufficient number of carboxylic groups
[36][37]. Thus, to improve its functionality for a specific purpose, de-esterification using either acidic or basic chemicals is necessary. The characteristic compositions of the extracted MPP are illustrated in
Table 2.
The residues of galacturonic acid (GA) (
Figure 1a) are generally recognised as the backbone of the pectin structure. Its chemical structure composes of an aldehyde group at C1 and a carboxylic acid group at C6
[41]. The GA can be partially methyl-esterified at C6 with methanol and acetylated at the O2 or O3 positions with acetic acid (
Figure 1b,c)
[42]. The GA content can be determined by either the colorimetry
[34] or high performance liquid chromatography
[43]. The ratio of methyl-esterified galacturonic acid groups to the total galacturonic acid groups is defined as the degree of esterification (DE)
[44][45][46]. The degree of esterification and acetylation of pectin affects the gelling properties of the pectin; a higher DE increases the capacity to form gels, whereas a higher degree of acetylation inhibits gelling
[47]. The analytical quantification of DE include the titrimetric technique
[34][48], gas liquid chromatography and colorimetric uronic acid analyses
[49]. Furthermore, the content of GA in foods is very important because their presence can affect the chemical and sensorial characteristics of the matrix such as pH, total acidity, microbial stability, sweetness, consumer acceptability and therefore, provide precious information on the wholesome quality of the food or on the optimisation needed to impart select technical features
[50]. Meanwhile, the molecular weight of pectin depends on the raw materials and the extraction techniques. Bagherian et al.
[4] reported that continued heating of pectin extraction may lead to pectin networks disaggregation, thus decreasing the molecular weight.
In case of pectin recovered from mango peel, the GalA contents varied depending on the extraction techniques. Process optimization of extraction methods to obtain the minimal GalA level of 65% in MPP has been highlighted in many research studies
[39][51][52]. Geerkens et al.
[53] claimed that the preparation processes of the peel (blanching, particle size reduction) and fruit ripening stage reduced the GalA content, however the highest content obtained was 48%. Regarding the DE content, the values were in a range between 56% and 93%, which categorized it as high methoxyl pectin
[54]. Both GalA and DE of pectic polysaccharides are involved in the commercial uses of pectin as gelling and thickening agents
[55][56].