Ultrasound-assisted extraction (UAE) of bioactive compounds from coffee silverskin: Impact on phenolic content, antioxidant activity, and morphological characteristics

Coffee silverskin (CSS) has various nutritional and functional properties, enabling it for valuable applications. Ultrasound-assisted extraction (UAE) is an efficient and economic extraction technique. In this research, UAE was employed in the extraction of bioactive compounds from CSS with water and methanol-water mixture as solvent. Extraction efficacy between the UAE-treated extracts and the UAE-untreated (control) was assessed in terms of the total phenolic content (TPC), caffeine content, chlorogenic acids (CGAs), and antioxidant capacities. The yields of TPC were in the range from 5.80 +/- 0.06 to 8.94 +/- 0.01 mg GAE/g sample. Phenolic content and antioxidant activity were highly enhanced by applying UAE compared to the controls. Noticeable caffeine content in the range from 3.27 +/- 0.10 to 4.29 +/- 0.01 g/100 g sample was found in the CSS samples. CGAs (3-CGA, 4-CGA, and 5-CGA) were all quantified. Methanol-water extracts showed significantly (p < .05) higher TPC, caffeine content, and antioxidant capacity than water extracts. The micro-structure investigation of CSS indicated UAE can cause cell disruption and micro cracks so that it can be an effective extraction technique. Peleg's model showed a good fitting (R-2 > 0.94) to the experimental results on the kinetics of UAE of phenolics compounds. Practical applications Coffee is a most consumed beverage internationally. Due to its great production, large quantities of residues including coffee pulp and husk, coffee silver skin, and spent coffee grounds are generated in the coffee industry. Among them, coffee silverskin (CSS) is a by-product during the coffee roasting process, which contains nutritional and functional properties. As an efficient and economic novel extraction technique, ultrasound-assisted extraction (UAE) was employed in the current study to extract bioactive compounds from coffee silverskin with water and methanol-water mixture as solvent in order to overcome various disadvantages encountered by the conventional extraction methods. Results indicated that application of UAE pretreatment showed significantly higher recovery of phenolic compounds compared to the controls and high ultrasound intensity (38 W/cm(2)) was more effective in recovering polyphenols compared to 5 W/cm(2) ultrasonic treatments. Therefore, results from the current study provide useful and practical information for the coffee industry.