Detoxification-Assisted Bioprocessing for Sustainable Conversion of Defatted Spent Coffee Grounds to Bioplastics and Biofunctional Metallic Nanoparticles

Spent coffee grounds (SCGs) are waste products obtained after brewing roasted coffee beans. The detoxification-based process used in the preparation of defatted spent coffee grounds (SCGO) involves: (i) direct acid pretreatment followed by enzymatic-hydrolysis (AE), (ii) solvent extraction followed by acid pretreatment and enzyme-hydrolysis (SAE), and (iii) solvent extraction followed by acid pretreatment and enzymatic-hydrolysis coupled with activated carbon detoxification (SAEAc) for polyhydroxyalkanoates (PHA) production. SCGO was subjected to optimized acid pretreatment (100 °C, 2.0% H2SO4, 6 h), followed by enzyme saccharification (20 FPU/g of SCGO). The effect of detoxification on the obtained hydrolysates was comparatively evaluated based on their inhibitor concentrations and suitability as substrates for fermentation by Lysinibacillus sp. RG.S. and PHA biosynthesis. The maximum biomass productivity (5.6 ± 0.44 g/dm3), PHA accumulation (53.0 ± 1.22%), and PHA yield (2.97 ± 0.32 g/dm3) were produced by the hydrolysates that were generated by the SAEAc-detoxification process with 1% corn steep liquor (CSL) and 1% acetic acid. The physicochemical and thermo-stable properties of the produced PHA were found to be comparable to those of standard PHB, making it appealing for a variety of applications. Extracted bioactive compounds from SCGO were employed as reducing and stabilizing agents for the synthesis of silver nanoparticles (AgNPs). Analytical results suggest that the SCGO-AgNPs were well phyto-fabricated and evenly dispersed, with particle diameters of about 10–40 nm. Furthermore, SCGO-AgNPs demonstrated potential antioxidant and antidiabetic activities, indicating their prospective for biomedical and nutraceutical applications. The foregoing results illustrate that detoxification strategy enhances the fermentability of SCGO hydrolysate and extracted bioactive compounds are used for biofunctional synthesis of AgNPs. In summary, the integrated cascading biorefinery concept supports sustainable and economically viable valorization of SCGO.