Hydrothermal production of high-molecular weight hemicellulose-pectin, free sugars and residual cellulose pulp from discarded carrots

Abstract

Discarded carrots account for 30% of the total production ending up in landfills, land, or a small part as cattle food. Valorization of discarded carrot pulp was studied by hydrothermal treatment, fractionating free sugars, hemicellulose and pectin in the liquid phase and residual pulp in the solid phase. Extraction took place in flow-through mode at 140, 160 and 180 C, achieving recoveries up to 211.0 g/kg dry pulp of free sugars, 29.13 g/kg dry pulp of homogalacturonan pectin, and 70.45 g/kg dry pulp of arabinogalactan hemicellulose. The residual pulp reached a cellulose content of 57.5 wt% while before the treatment it was 10.7 wt%. Most of the free sugars were extracted in the preheating stage in batch, so they could be obtained separately from the biopolymers. The flow-through extraction allowed to obtain hemicellulose and pectin of molecular weights >30 kDa. The evolution of different ranges of molecular weight was studied in detail for a better understanding of the phenomenon of autohydrolysis and the link between hemicellulose and pectin. The ample molecular weight distribution in the hydrolyzate allows for a subsequent fractionation via ultrafiltration membranes, to obtain a high molecular weight fraction for applications such as film-forming (in combination with the residual pulp).