Ultrasound-assisted production of bioethanol by simultaneous saccharification and fermentation of corn meal Svetlana Nikolic ´ a, * , Ljiljana Mojovic ´ a , Marica Rakin a , Dušanka Pejin b , Jelena Pejin b a Faculty of Technology and Metallurgy, Department of Biochemical Engineering and Biotechnology, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia b Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia article info Article history: Received 10 September 2009 Received in revised form 28 December 2009 Accepted 24 February 2010 Keywords: Bioethanol Ultrasound pretreatment Simultaneous saccharification and fermentation Saccharomyces cerevisiae var. ellipsoideus Corn meal abstract An ultrasound-assisted liquefaction as a pretreatment for bioethanol production by simultaneous sac- charification and fermentation (SSF) of corn meal using Saccharomyces cerevisiae var. ellipsoideus yeast in a batch system was studied. Ultrasound pretreatment (at a frequency of 40 kHz) was performed at dif- ferent sonication times and temperatures, before addition of liquefying enzyme. An optimal duration of the treatment of 5 min and sonication temperature of 60 °C were selected, taking into account glucose concentration after the liquefaction step. Under the optimum conditions an increase of glucose concen- tration of 6.82% over untreated control sample was achieved. Furthermore, the SSF process kinetics was assessed and determined, and the effect of ultrasound pretreatment on an increase of ethanol productiv- ity was investigated. The obtained results indicated that the ultrasound pretreatment could increase the ethanol concentration by 11.15% (compared to the control sample) as well as other significant process parameters. In this case, the maximum ethanol concentration of 9.67% w/w (which corresponded to per- centage of the theoretical ethanol yield of 88.96%) was achieved after 32 h of the SSF process. A compar- ison of scanning electron micrographs of the ultrasound-pretreated and untreated samples of corn meal suspensions showed that the ultrasound stimulated degradation of starch granules and release of glu- cose, and thereby accelerated the starch hydrolysis due to the cavitation and acoustic streaming caused by the ultrasonic action. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Bioethanol, one of the most important biofuels, is both renew- able and environmentally friendly (Balat, Balat, & Öz, 2008; Baras, Gac ´eša, & Pejin, 2002). It can be blended with petrol (E5, E10, E85) or used as neat alcohol in dedicated engines, taking advantage of the higher octane number and higher heat of vaporisation, and it is also an excellent fuel for future advanced flexi-fuel hybrid vehi- cles (Chum & Overend, 2001; Kim & Dale, 2005). Fermentation-derived ethanol can be produced from sugar, starch or lignocellulosic biomass. Sugar and starch-based feed- stocks are currently predominant at the industrial level and they are so far economically favourable. In Serbia, one of the most suit- able and available agricultural raw materials for industrial bioeth- anol production is corn. It is reported that in 2009 the average corn yield in Serbia was approximately 6–7 million ton, while the calcu- lated domestic needs for corn are only 4–4.5 million ton (http:// www.b92.net/biz/vesti/srbija.php). This means that there is en- ough corn for non-food uses, such as bioethanol production. Current world bioethanol research is driven by the need to re- duce the costs of production. For example, improvement in feed- stock pretreatment, shortening of fermentation time, lowering the enzyme dosages, improving the overall starch hydrolysis and integration of the simultaneous saccharification and fermentation (SSF) process could be the basis of cutting down production costs. The application of ultrasound pretreatment may significantly in- crease the conversion of starch materials to glucose as well as over- all ethanol yield (Khanal, Montalbo, Hans van Leeuwen, Srinivasan, & Grewell, 2007; Mielenz, 2001). Ultrasonication has been applied widely in various biological and chemical processes. Ultrasound (i.e. mechanical waves at a frequency above the hearing range of humans) can be divided into three frequency ranges: power ultra- sound (16–100 kHz), high frequency ultrasound (100 kHz–1 MHz) and diagnostic ultrasound (1–10 MHz) (Patist & Bates, 2008). When a low frequency ultrasound (that is, power ultrasound rang- ing from 16 – 100 kHz) wave propagates in a medium such as a li- quid or slurry, it produces cavitation and acoustic streaming. It generates large cavitation bubbles resulting in higher tempera- tures and pressures in the cavitation zone. The cavitation generates powerful hydro-mechanical shear forces in the bulk liquid, which disintegrate nearby particles by extreme shear forces. The main 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.02.063 * Corresponding author. Tel.: +381 113370423; fax: +381 113370387. E-mail address: snikolic@tmf.bg.ac.rs (S. Nikolic ´). Food Chemistry 122 (2010) 216–222 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem