Spent coffee grounds (SCG) and spent tea leaves (STL) are leftovers of the two most diffused beverages all over the world, coffee and tea. Functional components commonly found within raw materials, i.e. caffeine and phenolic compounds such as chlorogenic acid, gallic acid, and catechin derivatives, are not fully extracted from coffee grounds and tea leaves during beverage preparation. Therefore, SCG and STL can be regarded as a source of bioactives to be valorised in the formulation of various functional foods and beverages.
In the present study, ultrasound-assisted extraction (UAE) and conventional solvent extraction aimed at the recovery of natural antioxidants and caffeine from SCG and STL were compared. In particular, ethanol 60% (v/v) and boiling water were used as solvent media and the effects of two different ultrasound (US) waves amplitudes (80 and 152 µm) and treatment time (5 and 10 min) were investigated. This study closes a literature gap on UAE from SCG and STL using GRAS (Generally Recognized as Safe) solvents, in particular with water.
US had an evident positive effect on the recovery of natural antioxidants when the solvent media was aqueous ethanol. In particular, applying the US treatment at 152 µm for 10 min, the total phenolic compounds content of SCG (24.01 ±0.01 mgGAE/gTS) and STL (243.67 ±12.36 mgGAE/gTS) extracts doubled and quadrupled, respectively. Likewise, caffeine recovery significantly increased after sonication up to 2.19 ±0.02 mg/gTS from SCG and up to 12.74 ±0.36 mg/gTS from STL.