The levels employed on the fractional factorial design (defined as low and high) were 70 and 85 °C for temperature, 1:8 and 1:20 for the S:S, and 0 and 30 min for indirect ultrasound assistance (42 Khz and 100 W, 8891-26 Cole-Parmer Ultrasonic bath, Cole Parmer Instruments, USA). For the indirect ultrasound pre-treatment the sample in a plastic tube with a diameter of 30 mm, was immersed in an ultrasound bath, the tube was shaken periodically with an orbital shaker and the liquid level inside the tube was about 1 cm below the liquid surface in the bath.
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