2. Methods
2.1. Spent coffee grounds
Spent coffee grounds (SCG) were obtained from a local coffee shop (Fortaleza, Ceará, Brazil) that uses Arabica coffee grounds. The SCG were dried in an oven at 105 °C for 24 h to reduce its water content to the mass fraction of 3%. Drying aimed to avoid spoilage of the SCG and to avoid the growth of microorganisms.
2.2. Ultrasound-assisted oil extraction
The spent coffee grounds were placed inside a paper sachet and immersed in hexane inside a 500 mL hermetically sealed beaker. Sonication was carried out for 45 min at 40 kHz and temperatures ranging from 20 °C to 60 °C. A 2.7 L ultrasonic bath (UltraCleaner 1450A, Unique, Brazil), delivering 160 W to the bath, was used in the experiments. After the extraction, the sachet containing the SCG was removed and the mixture containing the oil was transferred to a rotary evaporator to remove hexane under moderate vacuum. The oil was transferred to an oven drier set at 104 °C and kept there for 6 h to remove any hexane that might have remained in the oil.
A 22 factorial experimental design was carried out to evaluate how the solvent to SCG ratio (mL g−1) and the temperature influences the amount of oil extracted from the SCG. In the study, the solvent to SCG ratio ranged from 2 mL g−1 to 4 mL g−1 and the temperature ranged from 20 °C to 60 °C (Table 1). These ranges were based on common ranges used in the literature for oil extraction. The data were analyzed using the software Statistic v7.0 (Statsoft Inc., USA) via multiple regression analysis using the least squares method, taking into account the isolated terms and interaction of the studied variables.