Reactor configuration for subcritical water extraction of pectin-derived compounds from onion peel wastes: a comparative study

Abstract

The increasing interest of pectin derived compounds as functional additives in the pharmaceutical and food industries demands new sources and eco-friendly extraction approaches. In this regard, onion peel wastes (OPW) hold great potential, with over 450,000 tons of OPW being discarded annually [1], despite their remarkable pectin content. Pectins are commonly extracted acid hydrolysis (pH 2, >85 °C), that yield mainly the HG domain, due to complete hydrolysis of the branched domains or enzymatic hydrolysis, which is time consuming, but yields branched pectin rich in RG-I and RG-II domains. As an alternative to the conventional process for producing pectin derived compounds (PDC), hydrothermal hydrolysis based on subcritical water (SubW) is emerging as an eco-friendly technique. SubW refers to water at temperatures ranging from 100 °C (boiling temperature) to 374 °C (critical temperature), which remains in a liquid state due to the application of pressure. SubW can facilitate the extraction of pectin from natural matrices, and by controlling the operational parameters, pectin can be tailored into smaller units with varying degrees of polymerization to produce PDC. However, in practice, accurate control of experimental conditions has emerged as the primary challenge of the SubW extraction process, which can lead to the formation of undesired degradation products, and uncontrolled molecular weight reduction of the pectin.