Stability of the process of simultaneous saccharification and fermentation of corn flour. The effect of structural changes of starch by stillage recycling and scaling up of the process
Introduction
The economical competitiveness of different technologies for fuel ethanol production depends on the cost of the first stage of this process – obtainment of a solution of fermentable sugars. Three types of raw material requiring different processing can be used for ethanol manufacturing. Sugar crops (sugarcane or sugar beet) require only the extraction process before fermentation. From starch-containing plants (cereal grains, potato tubers, cassava roots) the polysaccharide first has to be extracted and then hydrolyzed. Lignocellulosic biomass (wood, straw) has to undergo the most complicated pretreatment prior to fermentation that includes removal of lignin (non-saccharide fraction) followed by hydrolysis of cellulose. The latter process is far more difficult than the saccharification of starch. Therefore, despite the fact that most of agricultural by-products are lignocellulosic materials, production of second generation bioethanol is currently economically not competitive [1]. For this reason, almost all bio-ethanol is produced from grain and sugarcane. Moreover, much effort is made to decrease the cost of the production of this biofuel using starchy raw materials, i.e. to improve the hydrolysis stage.
The conventional process of enzyme hydrolysis of starch to produce fermentable sugars involves following steps: gelatinisation, liquefaction with thermostable α-amylase, and saccharification [2]. The energy consumption of theses processes usually amounts to about 30–40% of all energy required for ethanol production [3]. However, recently a new enzyme mix – STARGEN 001 – has been developed by Genencor International that hydrolyzes granular starch. Employing this enzyme makes it possible to effectively perform the process of simultaneous hydrolysis of native starch and ethanol fermentation [4]. During this process sugars liberated by hydrolysis are instantly consumed by yeast. The process of simultaneous saccharification and fermentation (SSF) is energy- and water-saving and results in higher ethanol productivity by avoiding the loss of fermenting sugars, which may occur during heating of fermentation broth (i.e. in the Maillard reaction).
There are a few important factors determining economic effectiveness of bioethanol production, one of the most important being full utilization of sugars and the related efficient conversion of granular starch into ethanol. The key role in the hydrolysis of granular starch is played by their supramolecular structure, crystallinity and the presence of complexing agents [5], [6]. These factors are determined by starch origin. The comparison of the four cheapest commercial starches in their native form in terms of their susceptibility to amylolysis sets them in the following diminishing order: corn ⩾ wheat > cassava > potato [7], [8].
The second important factor is water management. Firstly, the growing global water shortage leads to an increase in its costs. Secondly, distillery stillage is ranked among especially burdensome industrial effluents, particularly in large biorefineries. One of the methods of water cost reduction applied by distilleries is the reutilization of stillage in the production process which enables not only to reasonably utilize this by-product, but also to significantly reduce the demand for production water. In order to improve the economic outcome of the SSF process, the zero-discharge fermentation system is introduced. This means that the solid-containing whole stillage is first separated using a decanter centrifuge. Afterwards, the solid phase is dried in a drum dryer to produce DDGS (distillers dry grain solids), a valuable co-product used for animal feed, whereas the liquid phase is evaporated in a double-effect evaporator [9]. Another way of improving the zero-discharge fermentation system is the application of the repeated SSF process with complete recycling of stillage liquid fraction. This approach assumes liquid phase recirculation into the simultaneous saccharification and fermentation process [9]. It should be emphasized that coupling hydrolysis to fermentation in time and space through SSF reduces the costs and duration of the process and eliminates the necessity of using two separate vessels [10].
The number of reports on the subject reflects the awareness of the key role played by the efficient hydrolysis of granular starch [11], [12], [13], [14], [15], [16]. In spite of much effort of different research groups within the field, a lot of questions remain answered. Still it is not clear which regions of starch granules, crystalline or amorphous, are more susceptible to amylolysis [12], [11], [17], [18]. Moreover, most of the reports utilize experimental systems significantly different from those occurring in industrial practice. The aim of the present study was to examine the factors influencing the effectiveness of simultaneous saccharification and fermentation of corn flour with full stillage recycling. The effects of structural changes of starch granules during the long-term repeated SSF process as well as scaling up of the process were investigated.
Section snippets
Materials
Commercially available corn flour (BIO CORN, Ziębice, Poland) was used as raw material for lab scale and pilot plant fermentation experiments. In the industrial scale corn grain ground using 1 mm mesh size was used. The corn grain contained 69% starch in dry mater and was significantly contaminated with soil and sand. Freeze-dried distiller’s yeast – Red Star Ethanol Red (Saccharomyces cerevisiae) obtained from Lesaffre Company (France) was used for the production of ethanol from corn mashes. A
Results and discussion
Our previously reported study on the process of simultaneous saccharification and fermentation of raw corn starch using GSH (granular starch hydrolyzing) enzymes proved that although glycerol, lactic and acetic acid as well as inorganic ions content slightly increases with the increase in the number of recycling cycles, the yield of ethanol production does not decrease [9]. Moreover, the resulting increase in osmotic pressure did not influence yeast condition and enzyme activity. A slight
Conclusions
The described repeated batch SSF process with stillage recycling conducted with corn flour and the use of STARGEN 001 enzyme preparation runs efficiently in spite of the accumulation of glycerol, organic acids and inorganic ions in the fermentation broth. The most important factor influencing effectiveness of this processes is the quality of the raw material. Bacterial contamination of the raw material caused lower ethanol productivity despite similar utilization of starch. Instead of ethanol,
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