Elsevier

Applied Energy

Volume 102, February 2013, Pages 170-178
Applied Energy

Using an efficient fermenting yeast enhances ethanol production from unfiltered wheat bran hydrolysates

https://doi.org/10.1016/j.apenergy.2012.05.059Get rights and content

Abstract

Wheat bran, an abundant residue of the flour milling industry, is a promising feedstock for bioethanol production. The first objective of the present study was to define the optimised hydrolysis conditions for wheat bran that gave both high sugars concentration and little or no inhibitors formation. This low-cost material was subjected to acid treatment and then hydrolysed using increasing dosages of commercial enzymes.

With the aim of selecting a robust yeast suitable for industrial ethanol production, new Saccharomyces cerevisiae yeasts were isolated. Among the strains having remarkable ethanol yields from glucose in a minimal broth with high sugars concentrations, S. cerevisiae MEL2 was selected to be used for wheat bran fermentation. The strain produced ethanol levels higher than the benchmark industrial yeast S. cerevisiae DSM70449, exhibiting an outstanding ethanol yield of 96% of the theoretical and also showing promising fermentative activity on the whole unfiltered hydrolysates.

This study demonstrated that dilute acid pre-treatment and the use of a properly selected yeast had a great impact on the overall ethanol yield from wheat bran.

Highlights

► Wheat bran was efficiently hydrolysed after a mild physical–chemical pre-treatment. ► Newly isolated yeasts with outstanding ethanol yields were reported. ► Unfiltered bran hydrolysates were fermented with a yield of 96% of the theoretical.

Introduction

Liquid biofuels, such as ethanol, produced from low-cost biomass are regarded as an attractive alternative to fossil fuels to reduce dependence on oil and decrease carbon dioxide emissions, the main cause of greenhouse effect [1], [2], [3]. In principle, ethanol could be obtained from any material containing simple or complex sugars. Currently, the main feedstock for bioethanol is starch-rich biomass (corn, wheat, and potato), as it is rapidly hydrolysed by amylases, giving high yields of glucose.

One of the main costs in ethanol production is the substrate and the International Energy Agency (IEA) indicates that producing ethanol from cheap substrates such as energy-crops, food processing residues, agricultural and forest waste, will be required [4]. Lignocellulosic biomass is the most promising raw material for bioethanol considering its great availability and limited price. Despite these advantages, lignocellulose is much more expensive to process than starch-rich material because of the need for costly pre-treatments and large dosages of commercial enzymes. Therefore, more efficient and cost-effective methods for the conversion of lignocellulosic biomass to ethanol are needed.

To meet the increasing ethanol demand it is crucial not only to select a suitable raw material, but also to exploit it more efficiently by converting all the components, including lignocellulosic residues [5]. Currently, wheat is used as feedstock and wheat starch is converted into ethanol in several European plants [6]. Wheat bran is produced in large quantities as residue of the milling process accounting for 14–25% of the grain [7], [8]. On the basis of the data from the Food and Agriculture Organization [9], 6.87 billion tons of wheat were globally produced in 2009, corresponding to about 1.7 billion tons wheat bran available for the production of ethanol and other valuable compounds.

A promising approach to gradually introduce second generation biofuels may take advantage, in many European countries, of the existing capacities and logistics of the present wheat-to-ethanol first generation processes. Indeed, wheat bran contains a significant amount of sugars, such as hemicellulose, residual starch and cellulose [7], which could be converted to ethanol enhancing the overall alcohol efficiency of the plants. In this perspective, wheat bran has the great potential to be a low-cost lignocellulosic feedstock for bioethanol and may be considered as a model of other cheap and abundant agricultural waste.

Wheat bran starch can be hydrolysed by commercial amylases. Although the practice of converting starch to ethanol by an enzymatic process is a fairly mature technology, energy cost is high and the need to develop a more energy-efficient process is evident [10], [11]. A raw-starch hydrolysing and fermenting yeast could yield substantial cost reductions and improve the energy balance for the resulting one-step conversion of starch into ethanol [12], [13]. However, utilising also the hemicellulose/cellulose fraction of wheat bran could increase ethanol production but enzymatic hydrolysis is not enough to degrade these complex polysaccharides to simple sugars. In addition, pre-treatment with chemical and physical methods are also required. The development of effective pre-treatment procedures is thus essential for converting all wheat bran components into bioethanol.

Agricultural residues respond well to dilute acid pre-treatment and enzymatic hydrolysis. However, despite a plethora of accounts reporting the use of dilute acid on many lignocellulosic substrates, there has been only one on wheat bran, describing good sugar yields but high inhibitors levels release [6]. In the present study, the mild dilute acid treatment of wheat bran has been performed. In this respect, the objective of this investigation was to define the optimised hydrolysis conditions for wheat bran that gave both high sugars concentration and little or no inhibitors formation.

Once developed a cheap and efficient method to process bran into sugars, the hydrolysates should be efficiently fermented [14]. To date, many valuable works have been published on the pre-treatment of lignocellulosic materials aiming to reach the maximum sugar releases from the feedstock [reviewed in [1], [15]]. However, very few reported the use, in the following fermentation phase, of yeasts selected on the basis of their fermentative traits and industrial scale adaptability [16]. In this work, several newly isolated Saccharomyces cerevisiae strains were evaluated for their fermentative ability in minimal media supplemented with high glucose and xylose concentrations and the most proficient yeast has been adopted for the wheat bran fermenting phase.

To the best of our knowledge, this is the first account reporting i) the efficient mild dilute acid pre-treatment and enzymatic hydrolysis of wheat bran ii) the selection of a S. cerevisiae yeast capable of fermenting unfiltered lignocellulosic hydrolysates with high yield (96% of the theoretical).

Section snippets

Feedstock and commercial enzymes

Wheat bran (WB), obtained from Grandi Molini Italiani (Rovigo, Italy), was stored in plastic bags at 4 °C. The material had a dry matter content of 89% and the main components, determined according to international standard methods [17], were (%): ash 5.0 ± 0.2, cellulose 14.8 ± 1.0, hemicellulose 33.5 ± 0.9, lignin 2.0 ± 0.1, protein 12.1 ± 1.0, starch 20.1 ± 1.3.

The enzymes contained in the Novozymes Biomass Kit intended for conversion of lignocellulosic materials were kindly supplied by Novozymes

Fermentative abilities of wild type S. cerevisiae strains in minimal medium supplemented with high sugars concentrations

A set of S. cerevisiae strains was selected mainly among those involved in processes related to ethanol production (isolated from wines, grape marcs or beer), since they are expected to produce and tolerate high ethanol concentrations. S. cerevisiae MH1000, robust industrial ethanol yeast, and the type strain S. cerevisiae DSM70449 were also included in this study for comparison of relatively diverse phenotypic backgrounds (Table 2). Both yeasts have been used in many works reporting the

Conclusions

Currently, the main feedstock for bioethanol is starch-rich biomass, as it is efficiently hydrolysed by amylases, giving high yields of glucose. However, since ethanol could be produced from any material containing simple or complex sugars, the need of high amounts of cheaper substrates, possibly not conflicting with the food production chain, directed the attention to waste such as the 1.7 billion tons of wheat bran, potentially suitable for ethanol production. Unfortunately, only few studies

Acknowledgements

This work was partially financed by the BiotechIIbis and BiotechIII projects (Regione Veneto). Dr. Favaro is recipient of “Assegno di ricerca Junior” grant from the University of Padova (Padova, Italy). The authors wish to acknowledge Novozymes for kindly providing enzyme preparations. Annatjie Hugo (Stellenbosch University, Stellenbosch, South Africa) and Stefania Zannoni (University of Padova) are acknowledged for HPLC analysis. Prof. Antonio Berti (University of Padova) is gratefully thanked

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