Pectin Structure
Pectin is a highly complex polysaccharide made of three main domains that are covalently linked one to another, homogalacturonan, rhamnogalacturonan I and rhamnogalacturonan II. The dominant feature of pectin consists of a linear chain of α-(1,4)-linked d
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Pectin Structure David Ropartz and Marie-Christine Ralet
2.1 Introduction Pectin is a natural constituent of all terrestrial plants that is particularly abundant—together with hemicelluloses, cellulose, and low amounts of structural proteins—in the primary cell walls of eudicotyledons and non-graminaceous monocotyledons (Carpita and Gibeaut 1993). Once extracted from citrus peel or apple pomace, commercial pectin is widely used as gelling, thickening, stabilising and emulsifying agent in various food products such as jams, acidic milk drinks, ice creams, or salad dressings, and it is well agreed that the fine structure of pectin deeply affects its functionality and applicability (Willats et al. 2006). The existence of a “jelly” in tamarind extract was discovered more than two centuries ago by the French pharmacist and chemist Louis-Nicolas Vauquelin (1790). The word “pectin” from the Greek πηκτός (“pêktós”, which means “thick”) was first used in 1825 by Braconnot, who had resumed Vauquelin’s work (Braconnot 1825a, b). Smolenski in 1923 was the first scientist to describe pectin as a polymer of galacturonic acid (GalA) and Kertesz (1951) defined pectin as a hetero-polysaccharide containing mainly partly methylesterified GalA together with some neutral sugars. In the 80s, the work of de Vries and co-workers was instrumental in showing that neutral sugars were present as side-chains arranged in blocks in so called “hairy regions” while >90% of the GalA residues could be isolated as chains comprising solely GalA (de Vries et al. 1981, 1982, 1983). Nowadays, it is widely accepted that pectin is a heterogonous macromolecule composed of interlinked distinct domains, the relative amount and structure of which vary according to the botanical origin, the organs and cell types considered, the stages of cellular development, and the precise location within the cell wall (Voragen et al. 2009). On top of this biological diversity, pectin structure varies broadly depending on the extraction D. Ropartz · M.-C. Ralet (*) INRAE, Biopolymères Interactions Assemblages, Nantes, France e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2020 V. Kontogiorgos (ed.), Pectin: Technological and Physiological Properties, https://doi.org/10.1007/978-3-030-53421-9_2
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method used. Different pectin populations that are more or less strongly anchored in the cell wall co-exist and a high number of extraction steps is needed to extract all pectin (Broxterman 2018). Two main families of pectin structural elements are usually considered: galacturonans and rhamnogalacturonan I (RG-I). Galacturonans are made of a backbone of α-(1,4)-linked d-galacturonic acid (GalA) residues. This galacturonan backbone may be unbranched (homogalacturonan) or decorated with more or less complex side-chains. The backbone of RG-I is made of the diglycosyl repeating unit [2-α-l- Rha-(1,4)-α-d-GalA-(1] (Lau et al. 1985). Rhamnose (Rha) residues are ramified at O-4 (mainly) and O-3 (scarcely) positio
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