Pectins are a component of the complex heteropolysaccharide mixture present in the cell wall of higher plants. Structurally, the pectin backbone includes galacturonic acid to which neutral sugars are attached, resulting in functional regions in which the esterification of residues is crucial. Pectins influence many physiological processes in plants and are used industrially for both food and non-food applications. Pectin-based compounds are also a promising natural source of health-beneficial bioactive molecules. Olives, the fruit of the olive tree, are consumed as part of the healthy Mediterranean diet or processed into olive oil. Pectins from olives have recently emerged as promising compounds with health-beneficial effects.
Pectins are complex heteropolysaccharides, which include at least 17 kinds of monosaccharides and over 20 types of linkages, with a backbone of α-1,4-D-galacturonic acid (70%) in which homogalacturonan (HG), rhamnogalacturonan (RG-I and RG-II), and xylogalacturonan (XG) domains, linked by covalent or ionic interactions, can be distinguished [14,19]. Homogalacturonan linear domain monosaccharides are partially C-6 methyl-esterified and may be C-2/3 O-acetylated in some plant sources, and the degree of esterification is a parameter that affects pectin functionality . This “smooth region” of HG is the most abundant pectin domain (comprising 60–65%) in plant cell pectins  and has been recently related to epidermal morphogenesis in plants . The “hairy” regions of pectin molecules include both RG-I and RG-II, to which nonionic side chains containing many neutral sugars are attached . RG-I domains include rhamnose residues in the galacturonic acid backbone with many side chains containing other neutral sugars, such as galactose or arabinose . It is well established that the monosaccharide composition and architecture of both HG and RG-I domains vary significatively during plant development . Only little structure variations in pectin RG-I domains have been reported in different plants . RG-II is a much more complex domain, in which up to 12 types of sugar may be present, including the rarely observed apiose, xylose, or fucose .
From the raw biomass, the industrial process of extraction requires pre-extraction protocols, followed by hydrolysis and isolation of pectins and post-extraction solubilization.
Both single digestions and combined methods have been used extensively for pectin extraction [1,14,24,42]. Single extraction methods use acid or alkali solutions in addition to enzyme treatments to release pectins from the cell wall, where it forms complex networks with cellulose and hemicellulose.
Pulsed electric field extraction or the use of hot water or chelating agents, such as oxalate or sodium hexametaphosphate, are also single extraction methods [1,45]. A pulsed electric field applies a high voltage during a short time to a food product, increasing cell membrane permeability and facilitating bioactive molecules release . Nevertheless, these protocols are time- and energy-consuming, with low extraction yields and inadequate pectin quality or functionality, as well as environmental disadvantages due to contaminants generated [14,46]. However, the structure and properties of pectins are influenced by the extraction method; thus, there is a need to find novel extraction techniques that achieve the optimal yield and quality of the by-products generated and the isolated pectic polysaccharide products . Accordingly, combined techniques using subcritical water-, ultrasound-, microwave-, or ultrasonic/microwave-assisted protocols are promising approaches for pectin extraction .
At present, two-phase extraction is preferred in the olive oil industry as it reduces the consumption of water and the generation of liquid pollution. The resulting solid phase includes water and vegetable mass and is commonly known as “wet olive pomace” [15,27]. Pectins are minor compounds in the olive fruit but comprise up to 35% of the olive pomace during processing [25,27], depending on the ripening stage and other factors related to cultivar conditions and olive variety [33,47,48].
Pectins can be extracted from olive pomace as an “alcohol-insoluble residue” (AIR), which also includes additional cell wall materials such as cellulose, hemicellulose or proteins . Conventional methods already described, such as high temperature or acid solvents, have been used extensively in extraction protocols [47,49,50]. Some data point to low molecular weight pectins as bioactive compounds and, accordingly, hydrothermal treatment has appeared as a promising technology for the production and solubilization of pectins from olive pomace, as temperature is a critical parameter for maintaining the bioactivity of pectin [51,52]. Regarding the million tons of olive pomace produced every year by the olive oil industry, this by-product appears to be a noteworthy source of bioactive molecules, including pectic polysaccharides.
New ventures to find natural sources of pectins in plants have the potential to expand what is known about vegetal polysaccharides as bioactive compounds that are available in large quantities but are still considered as waste. Many biomaterials are based on the pectin molecule, and many studies have assessed the efficiency of pectins as wound-healing agents  or in tissue engineering [22,78,79]. Pectins are a common dietary source of oligosaccharides from fruits and vegetables that are fermented in the colon by the gut microbiota. Promising activities include bactericidal, immunomodulatory, anti-inflammatory [80,81], antioxidant, cardioprotective, probiotic , cholesterol , serum glucose-reducing , and intestinal and obesity regulator [25,77,85] functions for pectin oligosaccharides. Moreover, low molecular weight fragments from pectins exhibit antitumoral activities [25,86,87,88,89,90]. Recent studies have also pointed to the importance of fruit and vegetables as an important source of pectin molecules containing the RG-I domain [23,91].
This entry is adapted from the peer-reviewed paper 10.3390/biology10090860