Upcycling of carrot waste into pectin-arabinogalactan and lignin-cellulose films via hydrothermal treatment, ultrafiltration/diafiltration, and casting

Abstract

Discarded carrots were valorized to obtain different films. The pulp was subjected to hydrothermal treatment (140 and 180 °C) at pilot scale, with one and with several flow-through reactors in series, allowing the extraction of pectin-containing arabinogalactan (P-AG) of very high molecular weight. Ultrafiltration and diafiltration allowed obtaining purified fractions with molecular weights be-tween 3.48-102.75 kDa. The solid residue from the hydrothermal process was subjected to mechanical treatments to obtain lignin-containing cellulose nanofibers (L-CNFs). The P-AG film had a better oxygen barrier (67.73 vs. 239.83 cm3·µm/m2/kPa/day) and the L-CNFs film had a higher tensile strength (7.74 vs. 3.14 MPa). The combination of both fractions showed that L-CNFs should be added in percentages higher than 15 % (w/w) so that their presence does not harm the P-AG based film. There was a synergistic effect on hydrophobicity when mixing both fractions, reaching a 57.5 % higher water contact angle (125.8 °). As for the P-AG sample, a higher molecular weight decreased the oxygen permeability up to 14.6 % (41.14 cm3·µm/m2/kPa/day) but increased the water vapor permeability up to 11.4 % (24.01 g·mm/m2/kPa/day). The tensile strength was up to 150.9 % higher at lower molecular weight and higher pectin content (2.84 MPa) due to the high degree of branching of the arabinogalactan. Elongation was up to 157.9 % higher at higher arabinogalactan content (15.28 %). For the first time, purified fractions of P-AG and L-CNFs from carrot waste were obtained and used for film formation. The films had acceptable properties for food-packaging.