Comparison of saccharification and fermentation of steam exploded rice straw and rice husk

Biotechnology for Biofuels, Sep 2016

Background Rice cultivation produces two waste streams, straw and husk, which could be exploited more effectively. Chemical pretreatment studies using rice residues have largely focussed on straw exploitation alone, and often at low substrate concentrations. Moreover, it is currently not known how rice husk, the more recalcitrant residue, responds to steam explosion without the addition of chemicals. Results The aim of this study has been to systematically compare the effects of steam explosion severity on the enzymatic saccharification and simultaneous saccharification and fermentation of rice straw and husk produced from a variety widely grown in Vietnam (Oryza sativa, cv. KhangDan18). Rice straw and husk were steam exploded (180–230 °C for 10 min) into hot water and washed to remove fermentation inhibitors. In both cases, pretreatment at 210 °C and above removed most of the noncellulosic sugars. Prolonged saccharification at high cellulase doses showed that rice straw could be saccharified most effectively after steam explosion at 210 °C for 10 min. In contrast, rice husk required more severe pretreatment conditions (220 °C for 10 min), and achieved a much lower yield (75 %), even at optimal conditions. Rice husk also required a higher cellulase dose for optimal saccharification (10 instead of 6 FPU/g DM). Hemicellulase addition failed to improve saccharification. Small pilot scale saccharification at 20 % (w/v) substrate loading in a 10 L high torque bioreactor resulted in similarly high glucose yields for straw (reaching 9 % w/v), but much less for husk. Simultaneous saccharification and fermentation under optimal pretreatment and saccharification conditions showed similar trends, but the ethanol yield from the rice husk was less than 40 % of the theoretical yield. Conclusions Despite having similar carbohydrate compositions, pretreated rice husk is much less amenable to saccharification than pretreated rice straw. This is likely to attenuate its use as a biorefinery feedstock unless improvements can be made either in the feedstock through breeding and/or modern biotechnology, or in the pretreatment through the employment of improved or alternative technologies. Physiological differences in the overall chemistry or structure may provide clues to the nature of lignocellulosic recalcitrance.

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Comparison of saccharification and fermentation of steam exploded rice straw and rice husk

Wood et al. Biotechnol Biofuels Comparison of saccharification and fermentation of steam exploded rice straw and rice husk Ian P. Wood 0 Huong‑Giang Cao 2 Long Tran 2 Nicola Cook 1 Peter Ryden 0 David R. Wilson 0 Graham K. Moates 0 Samuel R. A. Collins 0 Adam Elliston 0 Keith W. Waldron 0 0 The Biorefinery Centre, Institute of Food Research , Norwich Research Park, Colney, Norwich NR4 7UA , UK 1 The Earlham Institute , Norwich Research Park, Norwich NR4 7UG , UK 2 Vietnam Academy of Agricultural Science , Hanoi , Vietnam Background: Rice cultivation produces two waste streams, straw and husk, which could be exploited more effectively. Chemical pretreatment studies using rice residues have largely focussed on straw exploitation alone, and often at low substrate concentrations. Moreover, it is currently not known how rice husk, the more recalcitrant residue, responds to steam explosion without the addition of chemicals. Results: The aim of this study has been to systematically compare the effects of steam explosion severity on the enzymatic saccharification and simultaneous saccharification and fermentation of rice straw and husk produced from a variety widely grown in Vietnam (Oryza sativa, cv. KhangDan18). Rice straw and husk were steam exploded (180-230 °C for 10 min) into hot water and washed to remove fermentation inhibitors. In both cases, pretreatment at 210 °C and above removed most of the noncellulosic sugars. Prolonged saccharification at high cellulase doses showed that rice straw could be saccharified most effectively after steam explosion at 210 °C for 10 min. In contrast, rice husk required more severe pretreatment conditions (220 °C for 10 min), and achieved a much lower yield (75 %), even at optimal conditions. Rice husk also required a higher cellulase dose for optimal saccharification (10 instead of 6 FPU/g DM). Hemicellulase addition failed to improve saccharification. Small pilot scale saccharification at 20 % (w/v) substrate loading in a 10 L high torque bioreactor resulted in similarly high glucose yields for straw (reaching 9 % w/v), but much less for husk. Simultaneous saccharification and fermentation under optimal pretreatment and saccharification conditions showed similar trends, but the ethanol yield from the rice husk was less than 40 % of the theoretical yield. Conclusions: Despite having similar carbohydrate compositions, pretreated rice husk is much less amenable to saccharification than pretreated rice straw. This is likely to attenuate its use as a biorefinery feedstock unless improvements can be made either in the feedstock through breeding and/or modern biotechnology, or in the pretreatment through the employment of improved or alternative technologies. Physiological differences in the overall chemistry or structure may provide clues to the nature of lignocellulosic recalcitrance. Bioethanol; Biomass; Saccharification; Fermentation; Rice straw; Rice husk; Steam explosion; Pretreatment Background Rice is the third most widely grown cereal crop in the world after maize and wheat. It is the staple food and a considerable source of income for many tropical nations. Very large quantities of agricultural lignocellulosic residues are generated from rice cultivation. Annually, paddy rice cultivation produces over 660 million tonnes of rice, along with over 800 million dry tonnes of agricultural residues (mostly straw) including over 113 million tonnes of rice husks (hulls) [ 1, 2 ]. Vietnam is a major rice grower and produces over 60 million tonnes of rice straw and husks every year. The bulk of this biomass is disposed of by burning, resulting in substantial emissions of black carbon, methane and the generation of tropospheric ozone leading to high levels of air pollution [ 3 ]. This has negative impacts on air quality and human health, reduces crop productivity and contributes to global warming [ 4, 5 ]. Decreasing emissions from burning agricultural waste, amongst other measures, should be adopted as a priority measure by the international community if we are to meet the proposed 2  °C target for limiting anthropogenic global temperature increases. Other potential disposal methods, such as incorporation into wet soil, are also responsible for increased methane emissions [6]. Hence, there is great interest in developing approaches to exploit the energy potential of such biomass, for example, through conversion to energy or to biofuels. There have been a number of studies on the pretreatment, enzymatic saccharification and in some cases simultaneous saccharification and fermentation (SSF) of rice straw for the production of ethanol fuel. A range of pretreatments have been assessed, including steam explosion [ 7, 8 ], steam explosion and biological pretreatment [9], alkaline pulping [ 10 ], microwave alkali heating [ 3 ] organosolvent pretreatments [ 11 ] and fine milling [ 12 ]. There have been comparatively fewer studies on rice husk which have included in (...truncated)


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Ian Wood, Huong-Giang Cao, Long Tran, Nicola Cook, Peter Ryden, David Wilson, Graham Moates, Samuel Collins, Adam Elliston, Keith Waldron. Comparison of saccharification and fermentation of steam exploded rice straw and rice husk, Biotechnology for Biofuels, 2016, pp. 193, 9, DOI: 10.1186/s13068-016-0599-6