Sequential Recovery of Non-Polar and Polar Compounds from Spent Coffee Grounds Using scCO₂ Followed by SWE and UAE

Spent coffee grounds (SCG), a solid by-product generated during coffee preparation, are produced in substantial quantities yet remain largely underexploited. The valorization of SCG offers significant environmental, economic, and social advantages, contributing to the advancement of a circular bioeconomy. In this study, SCG valorization consisted of two steps. In the first step, the SCG was depleted by supercritical CO₂ at 250 bar for 4 h, and by using two different temperatures: 40 °C and 60 °C. In the second step, the defatted SCG was subjected to subcritical water extraction at different temperatures, and ultrasound-assisted extraction at both different amplitudes and temperatures. The objective of this study was to evaluate a sequential extraction approach for the recovery of fatty acid methyl esters (FAMEs) and the subsequent maximization of caffeine extraction from SCG. The FAME profile was determined in ten supercritical fluid extracts and two Soxhlet extracts, while caffeine content was evaluated in five subcritical water extracts and six ultrasound-assisted extracts obtained from SCG, previously exhausted by supercritical CO₂ extraction. The highest caffeine content was present in a subcritical extract obtained at 180 °C, while the most abundant fatty acids found in all the tested extracts were linoleic acid (43.79-54.15%) and palmitic acid (32.73-45.85%). The predominance of linoleic and palmitic acids in the obtained extracts suggests their potential application as feedstocks for biodiesel production and oleochemical industries. Furthermore, the high content of linoleic acid, an essential polyunsaturated fatty acid, may enhance the value of these extracts for cosmetic and nutraceutical applications.