Influence of pretreatment and modifiers on subcritical water liquefaction of spent coffee grounds: A green waste valorization approach

Spent coffee ground (SCG) is the main residue of the coffee industry with a worldwide annual generation of 6 million ton. SCG contains large amounts of organic compounds (i.e., polyphenols, lignin, cellulose, hemicellulose, and other polysaccharides) that can be exploited as a source of value-added products. Effective valorization of SCG depends on the extraction methods. Therefore, the aim of this study was to investigate the influence of pretreatments such as microwave and ultrasonic and modifiers such as nitrogen and carbon dioxide on subcritical water (SCW) liquefaction of SCG at different temperatures (180–240 °C) and pressures (20–60 bar). The hydrolysate was studied for their total phenolic content (TPC), total flavonoids content (TFC), reducing sugars (RSs), protein content antioxidants, and antimicrobial activities. The extracts showed that the TPC value ranged from 33.1 ± 1.89 to 51.2 ± 1.87 and the TFC value ranged from 15.13 ± 1.73 to 25.51 ± 1.55 mg/g of GA and CE respectively. The RSs and protein content ranged from 8.88 ± 1.19 to 37.9 ± 4.21 g/100 g and 20.05 ± 2.33 to 47.38 ± 4.41 respectively. DPPH and ABTS+ radical scavenging antioxidant activities ranged from 0.36 ± 0.03 to 0.51 ± 0.08 and from 0.39 ± 0.04 to 0.76 ± 0.01 mmol TE/g, respectively. The extracts also showed very good antimicrobial activity against common foodborne pathogens with Staphylococcus aureus being the most sensitive bacteria having an MIC value of 5 mg/ml lyophilized extracts. This study showed that pretreatment and the use of modifiers could increase the liquefaction of SCG and composition of the extracts by reducing the severity index.