Aqueous Extraction and Pectin–Calcium Stabilisation of the Phenolic Fraction of Orange Processing Waste

Orange processing waste is the largest residue stream of the Mediterranean citrus industry and a concentrated source of pectic polysaccharides and phenolic compounds. Both fractions were recovered from the same residue, and the recovered pectin was evaluated as a wall material for the phenolics. Extraction was carried out by rapid solid–liquid dynamic extraction in a 40 L Naviglio® extractor with water as the only solvent, at 25–27 °C and 10 bar, over fifteen 40 min cycles preceded by a 20 min pre-impregnation, for approximately 10 h in total. Aqueous citrus extracts are rich in ascorbate and reducing sugars, so total phenolic content was determined both directly and after solid-phase clean-up, and only the corrected value is reported as phenolic. Corrected total phenolic content was 312 ± 18 mg GAE/100 g DW against 487 ± 26 mg GAE/100 g DW uncorrected, so non-phenolic reductants accounted for 36.0 % of the raw signal. Radical-scavenging capacity was 21.6 ± 1.4 µmol TE/g DW by DPPH and 48.3 ± 2.9 µmol TE/g DW by ABTS. Hydroxycinnamic acids dominated the chromatographic profile, with flavanone glycosides in the minority, as expected of an aqueous solvent. Pectin recovered from the residue was high-methoxyl at 68.4 ± 1.9 % esterification and was therefore not calcium-reactive as extracted. Enzymatic de-esterification with orange pectin methylesterase gave 38.2 ± 1.6 % with essentially no loss of molecular weight and a blockwise charge distribution (DBabs 19.1 %), whereas an alkali control at matched esterification lost 47 % of its chain length. Continuous-flow gelation of low-methoxyl pectin gave 176 ± 24 nm particles with an encapsulation efficiency of 88.6 ± 2.1 %. The maltodextrin-dried powder retained 71.0 % of its phenolics after 90 days at 25 °C against 22.9 % for the unprotected extract, and intestinal bioaccessibility rose from 31.5 ± 3.6 % to 68.3 ± 4.1 %. The degree of esterification of the recovered pectin determines whether the circular version of the process is chemically available, and is reported here on that basis.