Energy & Fuels 2009, 23, 519–525
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Research on Solid-State Ethanol Fermentation Using Dry Sweet Sorghum Stalk Particles with Active Dry Yeast Fei Shen and Ronghou Liu* Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong UniVersity, 800 Dongchuan Road, Minhang District, Shanghai 200240, People’s Republic of China ReceiVed July 2, 2008. ReVised Manuscript ReceiVed NoVember 11, 2008
An effective method of sweet sorghum stalk storage in dry particles for 8 months was reported in this study. Meanwhile, the experiments of solid-state fermentation (SSF) for ethanol using dry sweet sorghum stalk particles (DSSSPs) with active dry yeast were carried out to clarify the effects of different variables, including temperature, particle size, yeast inoculation rate, and water content on yeast growth, CO2 and ethanol formation, and sugar use. The results indicated that the effects of temperature and particle size on yeast growth and ethanol yield were significant. In addition, the yeast inoculation rate and water content were greatly related to yeast growth, even though they had less significant effects on ethanol yield. The suitable ranges of temperature and particle size for yeast growth were 25-30 °C and 1.6-2.5 mm, respectively, in which higher values of Ycell/sugar of 0.2681 and 0.3538 mg mg-1 were obtained. The suitable ranges for ethanol production were 35-40 °C and 0.9-1.6 mm, in which higher values of Yethanol/sugar of 0.2404 and 0.2702 mg mg-1 could be obtained. The adequate yeast inoculation rate should be 0.250, and higher values of Yethanol/sugar and Ycell/sugar could be up to 0.2486 and 0.3017 mg mg-1, respectively. In addition, the suitable water content for ethanol production and yeast growth was 76.47%. With this water content, the Yethanol/sugar and Ycell/sugar could reach 0.2593 and 0.4211 mg mg-1, respectively.
Introduction In recent years, a new round of enthusiasm in biomass and bioenergy has been initiated with the recognition that the global crude oil reserve is finite and its depletion is occurring much more seriously than previously predicted. In addition, the environmental deterioration resulting from the overconsumption of petroleum-derived products, especially the transportation fuels, is threatening the sustainability of human society.1 Ethanol, as an alternative fuel energy resource, has been a subject of great interest since the oil crisis of the 1970s.2 Therefore, a strong need exists for efficient ethanol production with low cost in raw material and the production process.3 As for materials, one of the prime sources being investigated for ethanol is sweet sorghum. Sweet sorghum [Sorghum bicolor (L.) Moench] is a high-yielding sugar crop that has many characteristics, such as wide adoptability and tolerance to abiltic stresses, such as drought, water logging, salinity, and alkalinity.4-6 Meanwhile, sweet sorghum stalk contains quite a few quantities of soluble (glucose, fructose, and sucrose) and insoluble (cellulose and hemicellulose) carbohydrates.7 Besides, the stalk or bagasse of sweet sorghum can be synthetically used as raw materials for * To whom correspondence should be addressed. Telephone: 0086-2134205744. Fax: 008-21-34205877. E-mail:
[email protected]. (1) Bai, F. W.; Anderson, W. A.; Moo-Young, M. Biotechnol. AdV. 2008, 26, 89–105. (2) Tao, F. J.; Miao, J. Y.; Shi, G. Y.; Zhang, K. C. Process Biochem. 2005, 40, 183–187. (3) Liu, R. H.; Shen, F. Bioresour. Technol. 2008, 99, 847–854. (4) Bryan, W. L. Enzyme Microb. Technol. 1990, 12, 437–442. (5) Prasad, S.; Singh, A.; Jain, N.; Joshi, H. C. Energy Fuels 2007, 21, 2415–2420. (6) Christakopoulos, P.; Li, L. W.; Kekos, D.; Macris, B. J. Bioresour. Technol. 1993, 45, 89–92. (7) Jasberg, B. K.; Montgomery, R. R.; Anderson, R. A. Biotechnol. Bioeng. Symp. 1983, 13, 113–120.
ethanol and some byproduct with high additional values, such as forage, paper making, and laminate floor, which can cut the cost of ethanol production from sweet sorghum. Therefore, of the many crops currently being investigated for energy and industry, sweet sorghum is one of the most promising energy crops, particularly for ethanol production.8 However, two issues should be solved if the whole sweet sorghum stalk, including the soluble and insoluble carbohydrates, is industrially used for fuel ethanol production. One is long-time and effective storage of fresh stalk or juice. The sugars in fresh stalk or juice will be easily deteriorated in ordinary conditions, and the harvest season for sweet sorghum is annually limited from September to December, resulting in shortage of raw materials for the whole year.9 Thus, the storage of fresh stalk or juice is very important to its application. The other is the use of lignocelluloses in sweet sorghum stalk for ethanol. As a kind of sugar crop, considerable attentions have been paid to soluble carbohydrates in sweet sorghum stalk for ethanol. While the insoluble part in stalk for ethanol has not been given enough attention because of higher cost and more complex processes. The pretreatment of lignocelluloses in the stalk of sweet sorghum is different from that of other nonsugar crop stalks for refining ethanol. The sugars in the sweet sorghum stalk were necessary to be removed before hydrolysis. Otherwise, the sugar inhibition to cellulase would have a serious effect on hydrolysis.10 Therefore, on the basis of the research on sweet sorghum stalk storage in dry ways and the necessity of sugar removal from stalk before hydrolysis of lignocelluloses, the solid-state fermentation (SSF) using the dry sweet sorghum stalk (8) Gnansounou, E.; Dauriat, A.; Wyman, C. E. Bioresour. Technol. 2005, 96, 885–1002. (9) Mei, X. Y.; Liu, R. H.; Shen, F. Trans. China Soc. Agric. Eng. 2008, 24, 218–223. (10) Henk, L. L. Appl. Biochem. Biotechnol. 1996, 57/58, 489–501.
10.1021/ef800531e CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
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particles (DSSSPs) has been very important and necessary. Recently, some related research on SSF for ethanol using carob pod, grape pomace, apple pomace, sugar beet, and fresh sweet sorghum stalk was carried out, and the effects of some important variables, such as temperature, particle size, pH, and addition of inorganic salt, on fermentation were also reported.11-15 However, the related research on SSF for ethanol from DSSSPs using active dry yeast has not been investigated. The aims of this investigation were to examine the potential of DSSSPs, stored for 6 months, as a source for ethanol production by active dry yeast via SSF, as well as to study the effects of various fermentation variables, such as temperature, particle size, yeast inoculation rate, and water content, on the fermentation processes, especially, on the yeast growth, ethanol and CO2 production, and sugar use. The research can offer an effective method to store sweet sorghum stalk and to use the whole stalk of sweet sorghum for ethanol production.
Shen and Liu Table 1. Experimental Design temperature particle size water content yeast inoculation part run (°C) (mm) (%) rate (φ, wt/wt)a
A
B
C
D
Materials and Methods Sweet Sorghum and Its Preparation. Sweet sorghum cultivar Liaotian No. 1 grown on the farm of Shanghai Jiao Tong University was used for this work. The fresh crops were harvested, and the leaves were stripped from the fresh stalks by hand. The fresh stalks were chopped and ground into wet particles with a small-scale industry rubbing machine for less than 4 mm theoretical length before it was used for storage preparation. The wet sweet sorghum stalk particles were sun-dried, in which the water content was less than 15% for the long-time storage. The DSSSPs with 6 months of storage were used for the fermentation. The five particle fractions (
