Effect of sodium hydroxide pretreatment on physical, chemical characteristics and methane production of five varieties of sorghum C. Sambusiti a, b, * , E. Ficara a , F. Malpei a , J.P. Steyer b , H. Carrère b a Politecnico di Milano, DICA, Environmental Section, Piazza L. da Vinci, 32, 20133 Milano, Italy b INRA, UR0050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, 11100 Narbonne, France article info Article history: Received 9 October 2012 Received in revised form 6 March 2013 Accepted 13 April 2013 Available online 18 May 2013 Keywords: Anaerobic digestion FTIR Sodium hydroxide pretreatment Sorghum abstract Sorghum bicolor [L.] Moench, represents an interesting substrate for methane production. The aim of this study was to evaluate the effect of alkaline pretreatment on chemical composition, physical structure and methane production of five varieties of sorghum (S1, S2, S3, S4, and S5). The pretreatment was conducted in closed bottles, at 55  C for 12 h. Samples were soaked in a NaOH solution at 4 and 10 gNaOH/100 gTS, with a solid concentration of 35 gTS/L. Sodium hydroxide pretreatment led to a reduction of lignin (50 e70%), hemicelluloses (18e35%), cellulose (16e45%) and galacturonic acids (up to 100%), for all varieties of sorghum, as also confirmed by Fourier transform infrared spectroscopy analyses. The reduction of lignin content and thereafter the solubilization of cellulose and hemicelluloses, previously observed, can accelerate the disintegration and the hydrolysis steps during anaerobic digestion. Indeed, an increase in the first order kinetic constant was observed by increasing the alkaline dosage (by 40%, 61%, 64%, 54%, and 40% for sorghum S1, S2, S3, S4, and S5, respectively). Nevertheless, the sodium hydroxide pre- treatment had no positive effect in enhancing the methane yields (270  13, 335  11, 294  1, 327  9 and 303  24 mL CH 4 /gVS for S1, S2, S3, S4 and S5, respectively). Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Current energy policies address the use of renewable energy sources, instead of fossil fuels. In this context, biomasses offer a huge potential for the production of biofuels (i.e. biodiesel, bio- ethanol, methane and biohydrogen) and their use could be bene- ficial to reduce both the world’s dependency on oils and the global emissions of greenhouse gases [1]. Biofuels, which are predominantly produced from biomasses, can be categorized into three generations, according to the origin of the biomasses used [2]. First generation biofuels are produced from the edible part of the plant (i.e. sugars, grains and seeds), second generation ones come from the non-edible part of the plants (i.e. lignocellulosic substrates) and the third generation ones come from microalgae [3]. The production of first generation biofuels is in an advanced state with mature technologies and relatively well- understood processing and production pathways. However, the production of first generation biofuels is controversial due to both considerable economic and environmental limitations. The major limitations include the impact that they may have on biodiversity and competition with agriculture arable land used for food pro- duction [4]. Therefore, lignocellulosic biomasses (i.e. agricultural residues and energy crops) can offer the potential to provide novel “second generation” biofuels (i.e. bioethanol, biogas and bio- hydrogen), due to the fact that they do not create competition for lands used for food production [5,6]. Among energy crops, sorghum (Sorghum bicolor [L.] Moench), with a world cultivated land of 40 million ha in 2009 [7] and with a hectare yield up to 25 t (dry weight) per year, represents an interesting substrate for methane production, in alternative to corn. Sorghum is a warm-season, short-day annual grass and it grows best under relatively high temperatures and under sunny condi- tions. It requires less water than corn, so it is likely to be grown as a replacement to corn and it produces better yields than corn in hotter and drier areas. Moreover, as a quick-growing energy crop, sorghum can be rotated as a part of an annual cropping system [8]. Sorghum bicolor [L.] Moench is a genus with many species and subspecies. In general, sorghum can be classified into forage and grain types [8]. Recently, new cultivars of sorghum have been commercialized, so called biomass sorghum for great biomass production to energy purpose. Forage sorghums are grouped into * Corresponding author. Politecnico di Milano, DICA, Environmental Section, Piazza L. da Vinci, 32, 20133 Milano, Italy. Tel.: þ39 (0)223996433; fax: þ39 (0) 223996499. E-mail addresses: cecilia.sambusiti@mail.polimi.it (C. Sambusiti), elena.ficara@ polimi.it (E. Ficara), francesca.malpei@polimi.it (F. Malpei), steyer@supagro.inra.fr (J.P. Steyer), carrere@supagro.inra.fr (H. Carrère). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.04.025 Energy 55 (2013) 449e456