Ultrasound as a Sustainable Technology for the Isolation of Polyphenols from Coffee Grounds

Coffee is one of the most commonly consumed beverages, but after brewing, a large amount of residue is produced, that remains unused and is discarded as waste. One of the priorities of the European Green New Deal is to convert and reuse materials into new value-added products thus significantly reducing waste disposal and its negative impact on the environment and human health. Since the phytochemicals are not completely extracted during the brewing process, spent coffee grounds is very abundant in bioactive compounds (carbohydrates, proteins, lipids, oils, alkaloids, minerals, phenols, etc) some of which may have antioxidant properties. Therefore, it shows a high potential for recovery and can be used as multipurpose material in mushroom growing, soil enrichment, pellet production, and different industries (pharmaceutical, agricultural, food, cosmetic). Using new sustainable technologies, such as ultrasound-assisted extraction (UAE), which enables low energy consumption, faster extraction, and higher yields of antioxidants also helps minimize negative ecological impacts and therefore is classified as a green technology. For this study, the coffee residue was collected after brewing with a catering coffee machine. Samples were prepared from 5 g of spent coffee grounds and as a green solvent 50% and 80% ethanol (v/v) was used. The aim was to optimize the extraction method by determining the influence of different variations of the probe and bath UAE on the content of polyphenolic compounds and antioxidant capacity of spent coffee grounds extracts. The type of equipment, ethanol concentration, time, and amplitude were tested. The highest total phenolics content (TPC) and total non-flavonoid content (TNFC) (835.68 and 477.62 mg GAE/100 g, respectively) were determined during 10-minute extraction with 50% EtOH and a probe system with 60% of amplitude. For the determination of antioxidant capacity, the FRAP method proved to be more suitable than the ABTS method for this type of raw material, and the highest value was observed when using the probe system (20% amplitude) with 50% EtOH (v/v) and 5-minute extraction. In general, the UAE with probe system proved to be a better solution for the extraction of polyphenols from spent coffee grounds than the bath system. This research has shown that different treatments of UAE can have an influence on polyphenol extraction, and instead of being treated as waste, spent coffee grounds can be utilized as a source of bioactive compounds. It is important to conclude that coffee grounds recovery can have many applications, can greatly reduce waste accumulation and be a significant by-product for further reuse.