Issue Date: May 12, 2008
THE SUGAR INDUSTRY may be one of the oldest in the Western hemisphere, but it continues to innovate. Those innovations come in areas that include the agricultural practices used to grow sugarcane, the methods to process sucrose, and the technology to convert sugar or leftover portions of the sugarcane plant into high-value goods, including polymers or biofuels.
At the American Chemical Society spring national meeting in New Orleans, researchers reported on recent advances in these areas at a symposium sponsored by the Division of Carbohydrate Chemistry and organized by division chair Gillian Eggleston. The symposium was held in memory of Margaret A. Clarke, former managing director of the Sugar Processing Research Institute in New Orleans, who died 10 years ago.
Benjamin Legendre, a sugarcane specialist at the Louisiana State University Agricultural Center, described agricultural practices that affect sugar yield and quality. Research over the last several years shows that the amount of nitrogen fertilizer used can be significantly decreased without adverse effects. "We can reduce the amount of nitrogen fertilizer by as much as 30% and maintain maximum sugar yields per acre," Legendre said. "Tons of cane per acre may go down slightly, but the recoverable sugar per ton far exceeds the decrease in tons of cane per acre."
Another agricultural practice that can speed the formation of sugar is the application of chemical ripeners such as glyphosate (perhaps better known as the main ingredient in Roundup herbicide), which stops the plant's vegetative growth and diverts all of its photosynthetic products to sucrose storage. Some growers, Legendre said, are still skeptical of the use of ripeners. However, "ripeners can increase recoverable sugar by anywhere from five to 30lbs per ton," he said.
Sugarcane that has partially deteriorated due to bacterial growth can cause problems in the processing plants that extract the sugar, according to Eggleston, a chemist at the U.S. Department of Agriculture's Southern Regional Research Center in New Orleans. But it can still be worthwhile to extract sugar from it. "It's not economical to throw the load away, but the factory needs to know if the sugarcane is partially deteriorated because they're going to have to change some of their processing parameters," she said.
One marker of this deterioriation is mannitol, a sugar alcohol formed primarily by Leuconostoc mesenteroides bacteria. The mannitol itself causes problems such as slower boiling rates and reduced sucrose recovery. In addition, it is a marker for dextran, a polysaccharide produced by the same bacterium when sugarcane deteriorates. This is because the disaccharide sucrose is split into its substituents, fructose and glucose; the former is fermented to form mannitol, and the latter is polymerized to form dextran.
The bacteria that produce mannitol and dextran hinder the sugar production process by "gumming up the works," said Gregory Coté, a researcher at USDA's National Center for Agricultural Utilization Research, in Peoria, Ill. The bacteria grow as a thick biofilm that is held together by dextran and related polysaccharides, collectively known as glucans.
Eggleston described an enzymatic method for detecting mannitol in deteriorated sugarcane and sugar beets. The method replaces an ion-chromatography method that was too sophisticated to use in sugar factories, Eggleston said. In the enzymatic method, mannitol is converted to fructose by the enzyme mannitol dehydrogenase using the coenzyme nicotinamide adenine dinucleotide (NAD). In the reaction, the coenzyme is reduced to NADH, which can be measured with a spectrophotometer.
The method is simple and cheap enough to be used in factories, Eggleston said, and is now being used in several other countries. The U.S. sugar industry, however, has been slow to adopt the method. "They're very conservative here," she said.
Mannitol and dextran may be hated by the sugar industry, but other groups are finding uses for the polysaccharide and its glucan cousins. For example, glucans can be used as thickening agents, Coté said. In another application, Coté's team collaborated with Cargill to develop a low-glycemic-index sweetener called Sucromalt, which is made from glucans other than dextran.
RESEARCHERS ARE also figuring out how to use the parts of sugarcane and sugar beets, another source of sucrose, that are left over after the sugar has been extracted. Such applications include using sugar beet pulp in biopolymers and sugarcane bagasse, the juiceless remains of sugarcane, as a cellulosic feedstock for ethanol production.
Arland T. Hotchkiss Jr. described work being done by his coworker LinShu Liu at USDA's Eastern Regional Research Center, in Wyndmoor, Pa., in which Liu blends sugar beet pulp with polylactic acid (PLA). The resulting thermoplastic composite materials have properties similar to petroleum-derived thermoplastics, but the sugar beet pulp reduces the cost and density compared with pure PLA.
No more than 30% sugar beet pulp generally be used without losing some of the plastic's mechanical properties, but by using cross-linking agents, however, 50% sugar beet pulp can be incorporated into the composite, Hotchkiss said. Such composites could be used as hard plastics in construction, appliance, microelectronic, and automotive applications, he said. In addition, pectin from sugar beet pulp can be combined with PLA for biomedical applications such as tissue engineering scaffolds.
Sugarcane bagasse and sugar beet pulp can also be used as feedstock for ethanol production. The U.S. produces approximately 1.5 million tons of sugar beet pulp each year, which could be used to produce approximately 150 million gal of ethanol, Hotchkiss said. Sugar beets are a more important crop and biofuel feedstock in Europe, which has 10 times higher sugar beet production than the U.S.
"We envision that this would be done on a regional basis," Hotchkiss said. Sugar beets are grown in the upper midwestern and western portions of the U.S. "You probably wouldn't have a cost-effective way of getting all the sugar beet pulp together and sending it to one refinery," he said. "These ethanol plants would probably have to be put on the back end of sugar production plants."
In other countries such as Brazil, ethanol is being fermented from sugarcane juice as well as from the bagasse. But in the U.S., "it's just not economical to make alcohol from juice because we can get a lot more for sugar," Eggleston said. Import quotas elevate the U.S. price of sugar. Making alcohol from molasses could be cost-effective in the U.S., she said.
Using bagasse rather than sugarcane juice in the U.S. averts the criticism often leveled at ethanol production from corn that ethanol production diverts corn from the food supply and adversely affects food prices. Bagasse, which would otherwise be a waste product, represents a way to get an extra product out of sugarcane.
Bagasse has some advantages over other potential cellulosic feedstocks that could be used for ethanol fuel production, said Gregory Luli, vice president of research at Verenium BioFuels in San Diego. For example, "the sugar mills have already run the bagasse through a series of grinding or crushing steps that reduce the material in size," he said. "We can feed the sugarcane bagasse as we receive it from the mill directly into our process." Other crops dedicated to fuel production need to be ground or crushed before they can be further processed.
Producing ethanol from bagasse and other cellulosic materials is more complicated than producing ethanol directly from sugar. The sugarcane bagasse still contains about 70% carbohydrates, but they are in the form of cellulose and hemicellulose, which need to be broken down into simple sugars before they can be fermented.
Describing Verenium's process for producing cellulosic ethanol, Luli said that it starts with a thermochemical step that uses heat and acid to hydrolyze the hemicellulose portion of bagasse and then uses an enzymatic process to break down the cellulose part of bagasse. Cellulose is a polymer of glucose, whereas hemicellulose contains a variety of sugars. Then further fermentation to ethanol involves two different bacteria, one for hemicellulose sugars and one for cellulose sugars. "Our processing conditions are slightly different" for the two different portions, "so we take advantage of organisms that are best suited to handle the processing conditions that optimize the conversion of sugar into ethanol," he said.
Verenium has pilot- and demonstration-scale facilities in Jennings, La., for producing ethanol from bagasse. On the second day of the symposium, participants toured the new demonstration facility. The demonstration plant is still in start-up phase, and Verenium will continue to evaluate systems startup and testing for the next three months. The plant will be used to validate the economics of the company's technology.
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