Ethanol production by yeast fermentation of an Opuntia ficus-indica biomass hydrolysate
Kuloyo, Olukayode Olakunle
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Opuntia ficus-indica, the prickly pear cactus, is well adapted for cultivation in arid and semi-arid regions, with a yield of 10 to 40 tonnes (dry wt) cladode biomass per ha. The cladodes (the “leaves”, which in fact are the stems) might serve as lignocellulosic biomass feedstock for second generation bioethanol production, without competing for agricultural land or replacing significant natural vegetation. The main objective of this study was to investigate the feasibility of bioethanol production from an enzymatic hydrolysate of O. ficus-indica cladodes. The potential of a Kluyveromyces marxianus isolate UOFS Y-2791, a yeast capable of utilising a wider range of carbon substrates and of ethanol production at higher temperatures than Saccharomyces cerevisiae, was investigated for bioethanol production using an O. ficus-indica cladode enzymatic hydrolysate as feedstock. S. cerevisiae UOFS Y-0528, a wine yeast strain, was used as benchmark. Compositional analysis of the cladode biomass indicated that it had a low lignin content of 8% (dry wt). The content of readily fermentable carbohydrates in the cladode, which was 34.3 g per 100 g dry biomass of which 23 g was glucose, was comparable to other conventional biomass feedstocks such as sugar cane bagasse and corn stover, whereas it had a low xylose content. By applying a statistical design experimental approach where acid concentration and contact time were varied, optimum conditions for dilute acid pretreatment of the dried and milled cladode were determined to be 1.5% (w/w) sulphuric acid for 50 min at a temperature of 120oC and a dry biomass loading of 30% (w/v). Enzymatic hydrolysis experiments were performed with varied enzyme loadings of cellulase and β-glucosidase with or without the addition of pectinase, and the enzyme loadings chosen were 15 FPU cellulase, 15 IU β-glucosidase and 100 IU pectinase per gram of dry biomass. These parameters yielded an O. ficus-indica hydrolysate containing (per litre) 45.5 g glucose, 6.3 g xylose, 9.1 g galactose, 10.8 g arabinose and 9.6 g fructose. Using a chemically-defined medium with a sugar composition similar to the hydrolysate as benchmark, K. marxianus and S. cerevisiae were grown in the O. ficus-indica hydrolysate at 40oC and 35oC, respectively, under non-aerated conditions, whereas the performance of K. marxianus was also investigated under oxygen-limited conditions where the DOT was controlled at less than 1% saturation. The fermentation profiles of both yeasts were compared using separate hydrolysis and fermentation (SHF) and simultaneous hydrolysis and fermentation (SSF) process configurations, at a water-insoluble solids (WIS) content of 14%. Both yeasts achieved comparable ethanol yields in SHF and SSF under nonaerated conditions, although K. marxianus exhibited a lower volumetric ethanol productivity than S. cerevisiae. K. marxianus, cultivated under oxygen-limited conditions, achieved a lower ethanol yield than both yeasts cultivated without aeration. However, K. marxianus exhibited the highest volumetric ethanol productivity of 2.3 g l-1 h-1 and 1.57 g l-1 h-1 in SHF and SSF, respectively, although the ethanol produced was assimilated upon hexose depletion. K. marxianus utilised galactose poorly in the absence of aeration, but completely consumed the sugar under oxygen-limited conditions. The overall ethanol productivity of SSF was double that of SHF. An ethanol concentration of 20.6 g l-1; the highest concentration achieved in this study, was an improvement on the 14 g l-1 previously reported elsewhere. This study provided more information on the chemical composition of the O. ficus-indica cladode, particularly regarding its constituent carbohydrates, and also highlighted the feasibility of ethanol production from the cladodes, albeit at low concentrations from an industrial point of view. K. marxianus demonstrated its potential as an alternative to S. cerevisiae for bioethanol production from lignocellulosic biomass.