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 âleavesâ, 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-08152012-120748 |
| Date | 15 August 2012 |
| Creators | Kuloyo, Olukayode Olakunle |
| Contributors | Prof SG Kilian, Prof JC du Preez |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-08152012-120748/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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