Kinetics of extraction and degradation of pectin derived compounds from onion skin wastes in subcritical water

Abstract

In this work the valorization of onion skin wastes (OSW) as a source of the pectic oligosaccharides (POS), prebiotic hydrocolloids, using subcritical water (SubW) was thoroughly studied and modeled (using the Panchev's model) in a batch extractor at temperatures up to 165 °C and extraction times up to 180 min. The extraction of pectin derived compounds was notably sensitive to both variables, with the most favorable extraction rates occurring within a severity factor (logR0) range of 2.8–3.1. The highest recovery, 32 g of OGalA per 100 g of raw material, was achieved at 135 °C for 100 min. This process yielded various families of compounds categorized by their weight average molecular weight (Mw), with 85 kDa being the most prevalent, followed by 27, 17, and 7 kDa. The changes in Mw as a function of time and temperature was modeled using the Emsley equation, which enabled the prediction of POS molecular weight based on extraction conditions, as well as the calculation of the energy of activation for pectin derived compounds degradation (116 ± 2 kJ/mol). Additionally, along with the loss in molecular weight, the presence of degradation products such as formic acid and furfural increased with the intensity of the extraction conditions. In summary, the hydrothermal extraction conducted at temperatures between 125 and 135 °C facilitated the retrieval of POS with MW ranging from 7 to 85 kDa. This stands in contrast to the high MW (>300 kDa) and linear pectin obtained through conventional acid-water extraction processes.