Pectin structure in citrus fruit

Overview & Synopsis

Pectin, a key component of plant cell walls, is a widely used, after extraction, as gelling agent in food industry. The structure of pectin is based on a backbone of polygalacturonic acid, which is called homogalacturonan (HG). Another major building block of pectin is rhamnogalacturonan I (RGI), which consists in alternating sequences of L-rhamnosyl and D-galacturonosyl residues ramified with side chains of arabinans, arabinogalactans and galactans. The length of both HG and RGI domains, the fine structure of RGI, and the HG/RGI ratio are likely to influence pectin functionality both in vitro and in planta. Dry citrus peel (orange, grapefruit, lime and lemon) was subjected to nitric or oxalic acid extractions. We showed that both the type of acid and pH value are important parameters influencing pectin features. Pectin samples with different sub-domain structure and diverse macromolecular characteristics were obtained depending on extraction conditions and citrus sources. Notably, oxalic acid allowed efficient extraction of very well-preserved pectin macromolecules exhibiting particularly high molecular weight and intrinsic viscosity values. Variation in (i) pectin overall composition, (ii) macromolecular characteristics, (iii) HG/RGI ratio, (iv) HG length, and (v) RGI fine structure depending on extraction conditions and citrus sources will be presented and discussed.