Bioethanol production from sugar beet molasses and thick juice using Saccharomyces cerevisiae immobilized on maize stem ground tissue
Highlights
► Sugar beet molasses and thick juice as raw materials for bioethanol production. ► Saccharomyces cerevisiae is immobilized on maize stem ground tissue as new carrier. ► Using this new biocatalyst for fermentation, a superior system was realized. ► Bioethanol fermentation under normal and very high gravity conditions was studied. ► Thick juice was found to be economically favorable, compared to molasses.
Introduction
Bio-fuels are attracting growing interest around the world, with some governments announcing commitments to bio-fuel programs as a way to both reduce greenhouse gas emissions and dependence on petroleum-based fuels. Bioethanol is by far the most widely used bio-fuel for transportation worldwide, because it is a renewable, nontoxic, biodegradable resource and it is oxygenated, thereby provides the potential to reduce particulate emissions in compression–ignition engines [1], [2]. Worldwide, fuel ethanol share could reach 10–20% of the gasoline consumption by 2030, requiring a seven fold increase of ethanol production capacity compared to estimated as of 2005 [3]. In Europe, an annual growth rate is expected for liquid bio-fuels around 6.3% between 2005 and 2030 [4], [5].
In the European countries, beet molasses are the most utilized sucrose-containing feedstock for ethanol production [6]. In the ethanol production processes from sucrose-containing materials, the availability and transport costs of the feedstock continues playing a crucial role, while feedstock costs comprise up to 70% of the final ethanol price [6]. Molasses and other intermediates from sugar beet processing are very good raw materials for ethanol production due to their high content of fermentable sugars, which can be directly used for fermentation without any modification [7]. Molasses is a traditional raw material for distilleries in Serbia, particularly in the Vojvodina province, and about 90% of ethanol production comes from this raw material nowadays. Previous studies confirmed that the production of bioethanol from thick juice as an intermediate of sugar beet processing gives the benefits of reduced water usage, reduced wastewater purification costs, easier mixing with syrup if used warm, lower use of acids for pH buffering, and increased levels of nutrients compared to molasses [8]. Mineral salts and vitamins contained in the feedstock, as micronutrients, have a stimulatory and protective effect either on growth, fermentation, or viability of producing microorganism, which overall stimulates the rate of ethanol production and contributes to an increase in efficiency of ethanol fermentation. The doses of these compounds depend on the raw material used [9].
Saccharomyces cerevisiae strain is the most widely used ethanol producing microorganism. To increase the productivity and cost effectiveness of ethanol production, many process improvements including immobilized cells application and very high gravity fermentation (VHG) technology have been studied [10], [11]. Cell immobilization in the ethanol fermentations has many technical and economical advantages compared to free cell system [12], [13]. It is an effective method for improving the efficiency of substrate utilization and productivities of various fermentation processes and its practical application has been reported in both the food and wastewater-treatment industries [4], [14]. The most widely used immobilization methods are based on cell entrapment in natural or synthetic polymers gels [15], [16], [17]. The main drawback for application of polymer beads as cells carrier is problem of gel degradation, low physical strength, considerable substrate and product mass transfer limitation and gel particles disruption due to intensive CO2 evolution [18]. Another widely used method is based on passive adhesion of cells onto the surfaces of insoluble carrier such as glass beads, stainless steel wire spheres, sorghum bagasse [19], wood chips [20], raisin berries [21], sugar cane bagasse [22], watermelon rind pieces [23], orange peel [24], apple peaces [13] and many others. When yeast cells are immobilized by surface adsorption, their growth is not significantly affected, and some yeast cells can be washed out of the fermentation system and be continuously renewed. Adsorption is moreover influenced by various parameters such as pH, ion concentration, biomass and glucose concentration [25], [26], [27]. In addition, such supporting materials are readily cleaned and microbial contamination can be effectively prevented [28]. However, in order to use this technology in fuel ethanol production, the immobilization carrier must be very cheap and cell immobilization should be achieved with minimal additional cost [29]. There has been a surge in the attempts towards finding out a renewable and biodegradable carrier which essentially is easy to use, not synthetic, cheaper and available naturally in abundance [30]. Maize stem ground tissue is light, chemically and mechanically stable lignocelluloses material, with high porosity and is suitable for yeast cell immobilization [31].
It was reported that immobilization of the yeast cells can eliminate inhibition caused by high concentration of substrate and product, and also enhance ethanol yield and productivity [9]. Relative insensibility of immobilized yeast cells to pH, allows fermentation of different sugar containing raw materials without previous pH adjustments [32]. VHG fermentation technology includes the preparation and fermentation to completion of mashes containing 27 g or more sugar per 100 g mash [33]. Because of high concentration of dissolved carbohydrates and toxic level ethanol concentration in VHG fermentation, yeast cells are exposed to considerable osmotic and ethanol stresses resulting in a loss of cell-viability, growth and fermentation performance [34].
Vojvodina province as agricultural region has significant quantities of maize stems that remain as agricultural residue. Thus, the application of maize stems, as a new biomaterial for yeast cells immobilization in the ethanol production is of great interest. Since many authors reported that, immobilized yeast cells are protected from alterations of the environmental conditions, in the current study we investigated and compared efficiencies of ethanol production from sugar beet molasses and thick juice, by free and yeast cells immobilized onto maize stem ground tissue, under standard and VHG conditions. The purposes of this research were to obtain efficient ethanol production with high ethanol yield and productivity and to lower high operating costs. The effect of initial glucose concentration on the production of ethanol by S. cerevisiae immobilized onto maize stem ground tissue was evaluated and compared with free cells.
Section snippets
Raw materials
Sugar beet thick juice and molasses from a domestic sugar factory were used after dilution with distilled water to give a total sugar concentration of 100, 150 and 300 g/l, respectively. All fermentation substrates were adjusted to 5.5 pH with 10% (v/v) H2SO4 and sterilized by autoclaving at 121 °C for 30 min.
Preparation of support
The stems of Gold Cup maize hybrid were collected from ready-to-harvest corn fields from Budisava site, the Vojvodina region, Serbia. In order to increase specific surface area of the
Results and discussion
Quality parameters of substrates (molasses and thick juice) are presented in Table 1. Based on the results shown in Table 1, substrates can be considered as convenient raw materials for ethanol production.
Fujii et al. [38] estimated that the main factor that influences the efficiency of adsorption, behavior of the immobilized yeast cells and their productivity is thought to be the surface characteristics of the carrier including a pore size, the water content, hydrophilicity and magnetism. The
Conclusion
Taking into consideration significant process parameters, sugar beet thick juice was found to be technologically favorable as raw material for bioethanol production, compared to molasses. The maximum ethanol concentrations of 83.20 g/l for molasses and 132.39 g/l for thick juice were achieved in the VHG fermentation by the immobilized cells on maize stem ground tissue. Using this new biocatalyst for fermentation, a superior system was realized which exhibited lower substrate inhibition and higher
Acknowledgments
Financial support of the Project TR-31002 from the Ministry of Science and Technological Development of the Republic of Serbia is highly acknowledged.
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