Evaluating pretreatment techniques for converting hazelnut husks to bioethanol Yalçın Çöpür a,⇑ , Ayhan Tozluoglu a , Melek Özkan b a Düzce Üniversitesi Orman Fakültesi, Orman Endüstri Mühendislig ˘i Bölümü, Konuralp Yerles ßkesi, 81000 Düzce, Turkey b Gebze Yüksek Teknoloji Enstitüsü, Çevre Mühendislig ˘i Bölümü, Muallimköy Kampüsü, 41400 Gebze, Turkey highlights " NaBH 4 delignified the highest amount of lignin (49.1%) from the husk structure. " NaOH treated husk resulted in the highest xylan solubility (77.9%). " NaOH treated husk had the highest glucan to glucose conversion (74.4%). " NaOH treated husk the highest ethanol yield (52.6 g/kg husks). article info Article history: Received 10 September 2012 Received in revised form 9 November 2012 Accepted 10 November 2012 Available online 24 November 2012 Keywords: Bioethanol Enzyme Hazelnut husk Hydrolysis Pretreatment abstract This study examined the suitability of husk waste for bioethanol production and compared pretreatment techniques with regard to their efficiencies. Results showed that 4% NaBH 4 (90 min) delignified the high- est amount of lignin (49.1%) from the structure. The highest xylan solubility (77.9%) was observed when samples were treated with 4% NaOH for 90 min. Pretreatment with NaOH and NaBH 4 , compared to H 2 O 2 and H 2 SO 4 , resulted in selective delignification. The highest glucan to glucose conversion (74.4%) and the highest ethanol yield (52.6 g/kg husks) were observed for samples treated with 2% NaOH for 90 min. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Almost 70% of the world’s hazelnuts are grown in Turkey, which makes it a significant hazelnut producer. Based on this production, the amount of husk waste is estimated to be 200,000 ton/year (Mid- illi et al., 2000). This abundant agricultural waste has had no eco- nomic value to date and is usually burned in the fields, causing air pollution and soil erosion. In addition, the burning decreases the biological activity of the soil (Arslan and Saracoglu, 2010). Any pos- sible industrial usage of hazelnut husks can be expected to yield economic as well as environmental dividends. The literature on using husk waste in industrial applications has been very limited. In earlier studies, the possible usage of husk waste in particleboard (Copur et al., 2007) and medium-density fiberboard (Copur et al., 2008) productions was examined. The usage of several agricultural residues in bioethanol production has been studied (Balat et al., 2008). On the other hand, no known effort has been made to utilize hazelnut husks as a resource for bioethanol production. Like woody biomass, husk structure consists mainly of cellulose, hemicelluloses and lignin. Separation of individual lignocellulosic biomass components, such as cellulose or lignin, can increase their value dramatically. Cellulose, if not combined with lignin, can be converted into the biofuel ethanol through hydrolysis and subse- quent fermentation (Lynam et al., 2012). Therefore, several pretreatment techniques have been used in the process for efficient conversion of the structural carbohydrates to fermentable sugars. However, these applications remove some carbohydrates from the structure. Physical, physico-chemical, chemical and biological pretreatment methods have been utilized (Olsson and Hahn-Hägerdal, 1996), and all these techniques are ex- pected to improve the efficiency of cellulose accessibility of hydro- lytic enzymes. In addition, the ideal technique has to minimize the formation of degradation products because of their inhibitory ef- fects on subsequent hydrolysis and fermentation processes. Ongo- ing studies seek to improve the efficiency of pretreatments by increasing the conversion rate for more economical ethanol production. Several chemicals, including acids, alkalis, organic solvents, etc., are utilized in the pretreatment step of bioethanol production 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.11.058 ⇑ Corresponding author. Tel./fax: +90 380 5421137. E-mail address: yalcincopur@duzce.edu.tr (Y. Çöpür). Bioresource Technology 129 (2013) 182–190 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech