Evaluation of Ultrasound Waves on S. cerevisiae Stimulation in the Bioethanol Production from Rice Bran

The ultrasound exposure can affect the Saccharomyces cerevisiae growth and metabolic product generation. This study evaluated the effect of ultrasound on S. cerevisiae stimulation in lag and exponential phase cultured in defatted rice bran (DRB) employing a full-factorial design. Firstly, the DRB was hydrolyzed by protease, α-amylase, and amyloglucosidase; the hydrolyzed DRB (HDRB; 30 g L−1) was used as substrate for S. cerevisiae. The variables ultrasound power (132 to 330 W) and duty cycle (25 to 50%) were evaluated considering the temperature (30 °C), ultrasound frequency (37 kHz), and ultrasound exposure time (15 min) as fixed variables. The responses variables were the bioethanol productivity, cell productivity, cell number, cell viability, and efficiency (conversion yield of sugars into ethanol). A control treatment was performed in parallel. The application of ultrasound at a duty cycle of 50% stimulated cellular metabolism of S. cerevisiae both in lag and exponential phase, evidenced by the faster consumption of reducing sugars, bioethanol production, and biomass improvement. However, at the exponential phase, the ultrasound exposure was more promising due to the increased bioethanol productivity (up to 1.55 after ultrasound and 2.42 g L−1 h−1 at final fermentation process), cellular productivity (up to 0.93 g L−1 h−1), and efficiency (up to 103.17%). Around 60% of the total bioethanol was produced after ultrasound exposure in the exponential phase. The HDRB is a rice byproduct with a great potential for bioethanol production by S. cerevisiae ultrasound assisted, and the optimized process condition was achieved under ultrasound power of 132 W at duty cycle of 50%, applied in the exponential phase of yeast growth curve.