Abstract
The feasibility of ethanol production using an enzymatic hydrolysate of pretreated cladodes of Opuntia ficus-indica (prickly pear cactus) as carbohydrate feedstock was investigated, including a comprehensive chemical analysis of the cladode biomass and the effects of limited aeration on the fermentation profiles and sugar utilization. The low xylose and negligible mannose content of the cladode biomass used in this study suggested that the hemicellulose structure of the O. ficus-indica cladode was atypical of hardwood or softwood hemicelluloses. Separate hydrolysis and fermentation and simultaneous saccharification and fermentation procedures using Kluyveromyces marxianus and Saccharomyces cerevisiae at 40 and 35 °C, respectively, gave similar ethanol yields under non-aerated conditions. In oxygen-limited cultures K. marxianus exhibited almost double the ethanol productivity compared to non-aerated cultures, although after sugar depletion utilization of the produced ethanol was evident. Ethanol concentrations of up to 19.5 and 20.6 g l−1 were obtained with K. marxianus and S. cerevisiae, respectively, representing 66 and 70 % of the theoretical yield on total sugars in the hydrolysate. Because of the low xylan content of the cladode biomass, a yeast capable of xylose fermentation might not be a prerequisite for ethanol production. K. marxianus, therefore, has potential as an alternative to S. cerevisiae for bioethanol production. However, the relatively low concentration of fermentable sugars in the O. ficus-indica cladode hydrolysate presents a technical constraint for commercial exploitation.
Highlights
Opuntia ficus-indica, commonly known as the prickly pear cactus, is a drought-resistant plant commonly found worldwide in arid and semi-arid regions
The low xylose and negligible mannose content of the cladode biomass used in this study suggested that the hemicellulose structure of the O. ficus-indica cladode was atypical of hardwood or softwood hemicelluloses
This study showed that the O. ficus-indica cladode biomass had a high content of galactan and fructan that compensated to some extent for the low glucan content
Summary
Opuntia ficus-indica, commonly known as the prickly pear cactus, is a drought-resistant plant commonly found worldwide in arid and semi-arid regions. O. ficus-indica possesses up to a fivefold greater efficiency of water utilization than C4 plants such as corn and sugar cane, giving it the possibility to produce up to 50 tonne dry mass ha-1 year-1 and its cultivation requires a low agronomic input (Inglese et al 2002; Nobel 2002). With increasing focus on the utilization of inexpensive lignocellulosic biomass for the production of bioethanol, the cladodes (the ‘‘leaves’’, which are the stems) of O. ficus-indica might serve as a second generation feedstock for the production of bioethanol, without competing for prime agricultural land or significantly replacing natural vegetation.
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