Valorization of coffee industry wastes: Comprehensive physicochemical characterization of coffee silverskin and multipurpose recycling applications

Every year, the coffee supply chain produces tons of waste materials from the processing of the coffee bean. Coffee silverskin (CS) is the only waste product from the coffee bean roasting phases but as roasting is a key step in obtaining the coffee beverage, there are tons of CS collected and thrown away by coffee roasting industries. This study is based on the characterization of this integument, with a view to its use in nutritional, nutraceutical and industrial fields. From the results obtained, silverskin can be used in the food sector for its nutritional profile with a good amount of protein (18.9%) and total dietary fiber (34.7%) andits low-fat content (3.0%). Food safety was also determined by evaluating the low levels of contaminants such as ochratoxin A (OTA) and acrylamide (AA), with values < 0.1 mg/kg and <0.02 mg/kg respectively. Low concentrations of heavy metals and polycyclic aromatic hydrocarbons (PAHs) completed the safety-related overview in terms of CS contamination. For the nutraceutical aspect, an in-silico analysis of the peptide fraction was proper to evaluate the presence of angiotensin-converting enzyme (ACE) inhibitory peptides, antioxidants and dipeptidyl peptidase (DPP) IV inhibitors with promising uses for the treatment of diabetes and its side effects. Finally, through morphological-structural characterization of CS, the main constituents such as lignin, cellulose and hemicellulose were identified spectrally by Fourier transform infrared spectroscopy (FT-IR) and solid-state CP/MAS 13C NMR spectroscopy. The CS thermostability was monitored by thermogravimetric (TG) analysis, and the tegument surface was evaluated by Scanning Electron Microscopy (SEM) and volumetric nitrogen adsorption analysis. This information provides a broader view of the possible uses of CS as a filler for biocomposite materials. Therefore, from the perspective of recovering waste materials from food supply chains, coffee silverskin could be a good product to focus on.