Bioethanol production from sweet potato by co-immobilization of saccharolytic molds and Saccharomyces cerevisiae
Highlights
► We develop and evaluate a single-step system for bioethanol fermentation. ► This system was using a co-immobilization method for the fungi and yeast. ► The mixed blends of molds and S. cerevisiae gave the highest ethanol yields. ► The maximum ethanol production was 4.08% (v v−1) when the Aspergillus:Monascus (A:M) ratio was 1:2.
Introduction
In most developing countries, energy depends on imports and more than 90% of total energy comes from non-renewable fuel sources. This relationship causes pressure on oil supply, emission of CO2 to the atmosphere, inducing climate change environmental pollution [1]. Therefore, bioenergy (eg. bioethanol) is considered as one of the key renewable energy resources in the future, with economic and environmental benefits [2], [3], [4], [5]. Worldwide bioethanol production is dominated by Brazil and the USA. In recent years, the development and application of bioethanol from sweet potato is the main goal of Taiwan Renewable Energy Policy, as the advantages of sweet potato are its easy growth, adaptation to many farming conditions and prices are more stable than other agricultural major energy crops [6], [7], [8].
The ethanol fermentation processes from starchy materials commonly involves two stages [9]: (i) liquefaction of starch by α-amylase and enzymatic saccharification of the low molecular weight liquefaction products such as dextrin to produce glucose; (ii) fermentation of glucose to ethanol. It has been estimated that the energy input of the first stage is about 30–40% of the total energy during bioethanol production from starch for high temperature (around 90 °C) to precook and dissolve the particles [10]. The development a process for simultaneous liquefaction, saccharification and fermentation of starch would reduce the energy input and increase the efficiency of substrate utilization [11]. Many researchers have been attempted to combine the two-stage fermentation process in a single-step [12], [13]. Co-culturing methods have also been used, but not on an industrial scale. Because the two strains used in co-cultures do not always have similar culture requirements such as pH, temperature, nutrient, oxygen demand, etc [14], it is very difficult to optimize the conditions for one strain without affecting the other strains. Therefore, co-immobilization different kinds of microorganisms within the same porous matrix by co-immobilization and combination two-stage fermentation process in a single-step can reduce the energy input and resolve the above mentioned problem. The purpose of this study was to develop and evaluate a simultaneous single-step system for bioethanol fermentation from sweet potato starch using a co-immobilization method for the aerobic fungi (Aspergillus oryzae or Monascus purpureus) and the facultative anaerobic yeast (Saccharomyces cerevisiae) under limited aerobic culture conditions without imposing special artificial conditions.
Section snippets
Tested organisms and culture media
A. oryzae BCRC 30289, M. purpureus BCRC 31615, and S. cerevisiae BCRC 21494 were obtained from Bioresource Collection and Research Center, Taiwan. The amylase-producing molds were cultured on media containing (%, w v−1) sweet potato starch, 5.0; NaNO3, 0.3; KH2PO4, 0.3; CaCl2 ·2H2O, 0.01; MgSO4 ·7H2O, 0.1; Fe2(SO4)3 ·7H2O, 0.001; agar, 1.5 at 30 °C and pH 5.0. S. cerevisiae BCRC 21494 was cultivated on YPD agar containing (%, w v−1) yeast extract, 1.0; peptone, 2.0; dextrose, 2.0; agar, 1.5 at
Effect of initial ethanol concentration on ethanol tolerance of free and immobilized yeast cells
The effects of initial ethanol concentration on cell viability, ethanol production, and fermentation efficiency of free and immobilized yeast cells are shown in Fig. 1. The viability of free cells decreased sharply from 86.67 to 64.36% when the initial ethanol concentration increased from 6 to 8%. In contrast, the viability of immobilized cells declined only from 95.10 to 92.50%. Free cells were more sensitive to ethanol than immobilized cells. Ethanol production of free and immobilized cells
Acknowledgment
The authors thank the National Science Council of Taiwan (96-ET-7-002-005-ET) for their support and Dr. Hui-Na Chou for her technical assistances.
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