Author
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Lingle, Sarah |
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Submitted to: Meeting Proceedings
Publication Type: Proceedings Publication Acceptance Date: 2/1/2011 Publication Date: N/A Citation: N/A Interpretive Summary: Sweet sorghum is a crop like corn that stores sugar in its stalk like sugarcane. It is well adapted to grow in many parts of the country. It could be processed in existing sugarcane factories before the sugarcane harvest season begins, and the sugars in the juice can be used to make ethanol, a biofuel. To be useful for making ethanol, the sugars in the juice need to be stored in some way so that ethanol can be made year-round. One way to store the sugars is to make syrup from the juice. Some research was done in the middle of the 20th century to make sugar from the juice, similarly to sugarcane juice. There were two major problems: starch and aconitic acid. Starch is a molecule made of glucose molecules linked together. It tended to thicken as syrup was made, and sometimes gelled when the syrup was cooled. Aconitic acid is a compound that interfered with making sugar, so that it had to be removed. Methods were worked out to manage starch and aconitic acid so that sugar could be made from sweet sorghum syrup, but these methods were uneconomical, and the idea of making sugar from sweet sorghum was dropped in the U.S. To make syrup for future ethanol production, the steps utilized to make sugar will need to be modified, because technology to make sugar has improved since the earlier research was done. On-going research will find new ways to make sorghum syrup for ethanol production. Technical Abstract: Sweet sorghum is an attractive biofuel crop for many reasons. It is widely adaptable, grows fast, and stores sugar in its stalk like sugarcane. Its short maturity time, 90 to 140 days, and ability to re-grow for a second crop in some areas increase the sugar yield from sweet sorghum on an annual basis. In sugarcane growing areas, one or two crops of sweet sorghum could be grown and harvested to be processed by existing sugarcane factories before the sugarcane harvest season starts. Because the crop is seasonal, storing the product will be desirable. One way to store the sugar from sweet sorghum juice is to make higher brix syrup. Most of what we know about processing sweet sorghum juice to syrup comes from research in the 1940s to the early 80s to make raw sugar from the crop. Sweet sorghum juice differs from sugarcane juice in that it is lower in purity, with more glucose and fructose, and higher in starch and aconitic acid. It also has a seed head. The starch and aconitic acid are especially problematic for crystallizing sucrose. Starch makes the juice difficult to filter and increase viscosity during boiling. If not removed, aconitic acid crystallizes at the later stages of the process. Much of the starch and aconitic acid can be degraded or removed during processing so that sugar can be made, but these additional steps add to the cost of making sugar, and decrease the quality of the product. Therefore, the idea of using sweet sorghum juice as a feedstock for sugar was dropped by the sugar industry in the U.S. However, the possible use of sweet sorghum as a feedstock for biofuel has increased interest once again. Sweet sorghum should be harvested in the soft-to hard-dough stage of grain development, when the sugars in the stalk reach a maximum. If harvested earlier, the pigments from the leaves cause problems during clarification. To make sugar, clarification of sweet sorghum juice was done at a higher pH (pH 7.7 to 7.9) and a lower temperature (50 to 58°C) than is normal for sugarcane juice. Since sugar will not be the product from sorghum processing now, these conditions may not be warranted. Some unanswered questions: can some of the pigments in immature sorghum be removed during clarification by adding biochar or activated carbon? How much of the starch needs to be removed during processing to reduce processing problems? The enzyme alpha-amylase can break starch into smaller fragments less likely to thicken. Does the increase in glucose justify the cost of a-amylase? What other glucan polysaccharides exist in sweet sorghum? Can those be easily broken down to glucose? Is aconitic acid enough of a problem to have breeders select for low-aconitic acid lines? How do harvesting conditions affect processing? How does post-harvest deterioration affect processing? These questions will be addressed by on-going research. |
