Cascade disassembling of spent coffee grounds into phenols, lignin and fermentable sugars en route to a green active packaging

Coffee is one of the most popular beverages worldwide, whose consumption generates spent coffee grounds (SCGs) as one of the major by-products. A new biorefinery flowchart aimed at the integral valorization of SCGs in view of the possible implementation of a sustainable active packaging is proposed in this work. The cascade approach started with a delipidization step to get coffee oil (13.0 g/100 g SCGs). Then, a hydroalcoholic extraction followed by a treatment with a deep eutectic solvent (composed of choline chloride and lactic acid) allowed to recover low molecular weight phenolic compounds (9.6 g/100 g SCGs) and lignin (6.2 g/100 g SCGs), respectively. Both fractions were found to display promising antioxidant properties in chemical assays. The residual solid material was then submitted to saccharification through a statistical approach, enabling the optimization of the enzymatic cocktail needed to address biomass structural and compositional complexity. The process resulted in 57 % and 73 % saccharification yields for cellulose and hemicellulose respectively, corresponding to the recovery of 11.6 g of fermentable sugars per 100 g of SCGs, and led to a homogeneous residual solid (4.3 g/100 g SCGs) still endowed with antioxidant properties. Finally, the possible use of the inhibitory-free saccharification extract for the microbial production of polyhydroxybutyrate was demonstrated using a proper engineered Escherichia coli strain, achieving about 0.7 g/L of biopolymer (1.2 g/100 g SCGs). Overall, the combination of the sequential processes applied in this work allowed to recover functional compounds accounting for ca. 45 % w/w of the starting biomass, paving the way to the development of a coffee-biorefinery aimed at the designing of fully biobased materials composed of polyhydroxyalkanoates and active additives, i.e. phenols and lignin.