Mass transfers pose significant challenge in fermentation due to wide diffusion gradient existing between the culture broth and the immobilized cells. In the present study, Saccharomyces cerevisiae LC 269108 was immobilized in a recently described microporous alginate beads. The new method was compared with the conventional calcium alginate gel beads in a repeated batch process for the production of ethanol. The fermentation conditions implemented were 110 rpm, pH 5.5 and temperature of 30°C for 60 h. The bead diameters were 3 mm while the calcium chloride concentration was 2.0%. In separate batch experiments conducted by the simultaneous saccharification and fermentation (SSF) method, the free cells maintained plateau at peak ethanol concentration of 7.50±0.33% after 36 h. In the conventional alginate (6.51±0.05%) and microporous beads (7.06±0.10%), ethanol dropped in concentration until reaching final volumes of 5.65±0.33 and 6.56±0.64%, respectively. In the repeated batch experiments, five fermentation batches or runs were conducted over a 12 h period each. The concentrations of ethanol produced in batches with the cells immobilized in calcium alginate were 2.91±0.34, 5.80±0.22, 5.01±0.39, 4.41±0.14 and 3.77±0.21%, respectively. Cells immobilized in microporous beads had higher ethanol output with concentrations 2.33±0.07, 6.62±0.04, 6.16±0.32, 5.90±0.2 and 4.70±0.26% obtained after five respective batches. Glucose metabolism was found to be lower with cells immobilized in alginate beads. From initial glucose concentration of 14.30±0.2%, residual glucose was detected after the first (3.61±2.11%), fourth (3.18±0.98%) and fifth (5.30±0.86%) batches of fermentation. In the batches containing microporous beads, residual glucose (5.36±0.29%) was confirmed after the first batch only. The present study demonstrates the feasibility of using microporous beads in the production of ethanol.
Key words: Mass transfer; microporous beads, immobilized cells, ethanol, calcium alginate.
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