Eco-friendly valorization of espresso coffee grounds as a sustainable source of natural antioxidants in protein bar formulation

Abstract Spent coffee grounds (SCG) are agro-industrial waste that is generated in very large quantities and is an excellent potential source of bioactive compounds. This paper explored and compared conventional solvent extraction (CSE), ultrasound-assisted extraction (UAE), and microwave-assisted extraction (MAE) as eco-friendly approaches to recover phenolic compounds from SCG. Extraction efficiency, total phenolic content, antioxidant activity, and chlorogenic acid concentration were systematically measured. At first, MAE results surpassed the other methods and showed the highest extraction yield (25.31%) and total phenolic content (74.8 mg GAE), with chlorogenic acid being quantified at 42.1 mg g⁻ 1 extract. The MAE extract showed a powerful antioxidant capacity with an IC₅₀ value in the DPPH test of 44.8 mg L⁻ 1 . In order to assess the real usage of the product, the optimized MAE extract was used in protein bar formulations at different concentrations (0.5,1.0, and 2.0% w/w). Progressive increases in extract levels improved the antioxidant activity and final product phenolic content; nevertheless, sensory evaluation disclosed a drop in the consumer acceptability at concentrations exceeding 0.5% mainly due to a bitterness increase. Through 60 days of storage testing, antioxidant stability showed concentration-dependent variations. In conclusion, this work provides evidence that extracts from SCG through green extraction techniques can be effectively utilized in functional foods systems, only if the level of the formula is adjusted to provide a good balance between the functional and sensory properties. Graphical Abstract