Sequential Extraction Transformation of Brown Onion Skin into Cellulose-Based Enzyme Immobilization Carrier

The present study aimed to design a process of brown onion skin transformation by sequential extraction to a cellulose-based immobilization carrier, along with detailed analysis of obtained extracts, pointing to approaching a “zero-waste” model of circular economy. The process of brown onion skin transformation started with semi-continuous sequential subcritical extraction via consecutive use of five solvents of increasing polarity (96, 75, 50, and 25% ethanol and water), followed by alkaline liquefaction of solid residue by 10% aqueous solution of sodium hydroxide. The designed BOS transformation process resulted in 16.62 g of cellulose-based immobilization carrier derived from 100 g of brown onion skin. Extracts obtained by semi-continuous sequential subcritical extraction contained 37 mg/g of proteins, 40 mg/g of sugars, 17.5 mg/g of uronic acids, 28 mg/g of polyphenols, and 36 mg/g of flavonoids, while those obtained by alkaline liquefaction 19 mg/g of proteins, 58 mg/g of sugars, 10 mg/g of uronic acids, 6.6 mg/g of polyphenols, and 0.5 mg/g of flavonoids. The suitability of the cellulose-based enzyme immobilization carrier was evaluated by B. cepacia lipase immobilization by adsorption, where a maximal 31 U of lipase activity per 1 g of wet carrier was achieved. Based on the results obtained, it seems that the proposed process of brown onion skin transformation shows the possibility of being used for the production of a cellulose-based immobilization carrier, approaching the “zero-waste” model of a circular economy.