Production of antioxidants and other value-added compounds from coffee silverskin via pyrolysis under a biorefinery approach

Abstract

The coffee roasting industry produces about 0.4 Mt of coffee silverskin (CSS) per year, the only residue generated from the roasting process that is mostly disposed as industrial waste. The aim of this study is to convert CSS into value-added products by intermediate pyrolysis, transforming the waste into a resource within an integrated biorefinery perspective. To this end, bio-oils and biochars from the intermediate pyrolysis of CSS at 280 °C, 400 °C and 500 °C have been studied. GC–MS analysis showed that bio-oils were composed of value-added products such as caffeine, acetic acid, pyridine and phenolics, the latter being the most interesting due to their antioxidant properties. Total phenolic content and antioxidant capacity of the samples were determined through Folin-Ciocalteu (FC) and DPPH methods, revealing an increase in phenolics in bio-oils compared to CSS extract directly from the feedstock. The bio-oil with the highest phenolic content and antioxidant properties was produced at 280 °C and contained 6.09 and 3.02 mg of gallic acid equivalents /g of bio-oil determined by FC and DPPH methods, respectively. This represents a global potential of up to 487 and 242 tones of gallic acid equivalents per year, considering the FC results and DPPH respectively. The resulting 280 °C biochar presented significant calorific values (22 MJ/kg), indicating its potential use as an energy source. Hence, CSS pyrolysis converts a waste into a by-product and a resource, increasing the environmental benefits and contributing to the circular economy and bioeconomy.