An attempt was made for the first time to investigate and optimize ultrasound-assisted solid-liquid extraction (USLE) of phenolic compounds from spent coffee grounds (low-cost source of valuable phenolic compounds) using three levels, four factors (ultrasonic power, extraction temperature, extraction time and solid-liquid ratio) Box-Behnken response surface design (BBD). From the results, mathematical models were developed to illustrate the effects of process variables and their interactions on the extraction yield of phenolic compounds. The optimum process conditions were found to be:...