Abstract
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.
Highlights
The apprehension over the ecological issues generated by the wide scale appropriation of anthropogenic energy has propelled researches into the development of alternative energy resources that is cleaner, renewable and environmentally acceptable
The present study proposes the use of microporous beads as solution to mass transfer limitations in immobilized cells
Ethanol production by the free and immobilized cells was conducted by separate batch simultaneous saccharification and fermentation (SSF) ahead of the repeated batch experiments
Summary
The apprehension over the ecological issues generated by the wide scale appropriation of anthropogenic energy has propelled researches into the development of alternative energy resources that is cleaner, renewable and environmentally acceptable. Ethanol is an attractive option to fossil fuel because it is a renewable bio-based resource that is oxygenated and reduces emissions from compression ignition engines (Hansen et al, 2005). Ethanol production using immobilized yeast cells offer several advantages over the free or suspended cell systems It facilitates cell separation and reuse after fermentation, brings about reduction in adaptation phase and decreases inhibition caused by high concentrations of substrates and products (Lee et al, 2011; Devi and Nagamani, 2018). These attributes promote ethanol production and reduce operating costs (Duarte et al, 2013). Gel degradation and low mechanical strength are some of the common issues associated with their use, mass transfer limitations remain by far, the most significant challenge encountered with the use of alginate beads as immobilization carriers (Bangrak et al, 2011)
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