Optimization of Agitation and Aeration for Very High Gravity Ethanol Fermentation from Sweet Sorghum Juice by <em>Saccharomyces cerevisiae </em>Using an Orthogonal Array Design

Optimization of three parameters: agitation rate (A; 100, 200 and 300 rpm), aeration rate (B; 0.5, 1.5 and 2.5 vvm) and aeration timing (C; 2, 4 and 6 h), for ethanol production from sweet sorghum juice under very high gravity (VHG, 290 g L⁻¹ of total sugar) conditions by <em>Saccharomyces cerevisiae</em> NP 01 was attempted using an L₉ (3⁴) orthogonal array design. The fermentation was carried out at 30 °C in a 2-L bioreactor and the initial yeast cell concentration was approximately 2 × 10⁷ cells mL⁻¹. The results showed that the optimum condition for ethanol fermentation should be <em>A</em>₂<em>B</em>₃<em>C</em>₂ corresponding to agitation rate, 200 rpm; aeration rate, 2.5 vvm and aeration timing, 4 h. The verification experiments under the optimum condition clearly indicated that the aeration and agitation strategies improved ethanol production. The ethanol concentration (<em>P</em>), productivity (<em>Q_p</em>) and ethanol yield (<em>Y_p/s</em>) were 132.82 ± 1.06 g L⁻¹, 2.55 ± 0.00 g L⁻¹h⁻¹ and 0.50 ± 0.00, respectively. Under the same condition without aeration (agitation rate at 200 rpm), <em>P</em> and <em>Q_p</em> were only 118.02 ± 1.19 g L⁻¹ and 2.19 ± 0.04 g L⁻¹h⁻¹, respectively while <em>Y_p/s</em> was not different from that under the optimum condition.