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Production of 16% ethanol from 35% sucrose
Gaber Z. Breisha*
Department of Agricultural Microbiology, Faculty of Agriculture, Minia University, Minia, Egypt

article info
Article history: Received 13 December 2008 Received inrevised form 17 March 2010 Accepted 22 March 2010 Available online xxx Keywords: Sucrose Ethanol Saccharomyces cerevisiae Nitrogen Thiamine Oxygen

abstract
A strain of Saccharomyces cerevisiae, which showed marked fermentation activity, ethanol and temperature tolerance and good flocculation ability, was selected for ethanol production. A stuck fermentation occurred at sucrose concentration of 25%.Increasing the yeast inoculum volume from 3% to 6% showed positive effects on fermentation from 25% sucrose. The ratio of added nitrogen to sucrose, which gave the best results (for the selected yeast strain), was determined. It was concluded that this ratio (nitrogen as ammonium sulphate at a rate of 5 mg gÀ1 of consumed sucrose) is constant at various sugar concentrations. Addition of nitrogenat this ratio produced 11.55% ethanol with complete consumption of 25% sucrose after 48 h of fermentation. However fermentation of 30% sucrose at the above optimum conditions was not complete. Addition of yeast extract at a level of 6 g lÀ1 together with thiamine at a level of 0.2 g lÀ1 led to complete utilization of 30% sucrose with resultant 14% ethanol production. However the selected yeaststrain was not able to ferment 35% sucrose at the same optimum conditions. Addition of air at a rate of 150 dm3 minÀ1 m3 of reactor volume during the first 12 h of fermentation led to complete consumption of 35% sucrose and 16% ethanol was produced. This was approximately the theoretical maximum for ethanol production. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

To increase theefficiency of an existing fuel bio-alcohol plant, one potential improvement would be the use of high sugar concentration for fermentation to produce high levels of ethanol. Loss of yeast viability however is a serious problem in fermentation using high sugar concentrations. Limtong et al. [1] reported a decrease in ethanol production at sugar concentrations higher than 22%. They attributed thedecrease in ethanol production to various factors including high osmotic pressure and high temperature. Furthermore high ethanol concentrations are produced in the bioreactor systems, the high concentration of ethanol is toxic to yeast growth [2]. In fact, the most expected success in enhancing the yeast’s performance and tolerance for ethanol may be

achieved by supplementing the media with sourcesof nitrogen and vitamins. Despite numerous studies are carried out on this topic, the results are not always in agreement and no clear conclusions can be made on the process feasibility. Some authors have reported that nitrogen deficiency reduces biomass concentration and can lead to stuck or sluggish fermentation [3e5]. According to other authors, an excessive nitrogen addition increases the riskfor production of acetic acid [5], higher alcohols [6], ethyl carbamate [7], or in some conditions hydrogen sulphide [8]. According to Berthels et al. [9] ammonium addition could counteract turnover of high sugar affinity transporters and also activate phosphofructokinase. As early as 1992, McCaig et al. [10] reported that addition of free amino nitrogen (FAN) leads to higher final ethanolconcentrations in the fermented media and higher

* Tel.: þ20 2 03 5776848; fax: þ20 2 086 2362182. E-mail address: gaberbresha@yahoo.com 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.03.017

Please cite this article in press as: Gaber, Production of 16% ethanol from 35% sucrose, Biomass and Bioenergy (2010), doi:10.1016/j.biombioe.2010.03.017...