From raw sugar to raw materials
|Researchers in Louisiana look at sugar, blackstrap molasses, bagasse, and filter mud and see raw materials for a wide range of products.
Drive down a back road west of the Mississippi River in southern Louisiana on a summer afternoon. Giant grass plants8 to 10 ft highline both sides of the road. The plants are sugarcane, Saccharum officinarum, a major crop in the Gulf South. Come back in the fall and you will see truck after truck loaded with chopped cane. Huge plumes of steam rise from the sugar mills that convert the cane into crystallized sugar.
When we see the cane fields, most of us think of the white crystals we sprinkle on our morning cereal and stir into a cup of coffee. When scientists at the Audubon Sugar Institute at Louisiana State University, Baton Rouge, see the cane, they see a chemical factory and a host of lucrative new businesses. I spoke to Professors Willem Kampen and Donal Day, whose enthusiasm about the possibilities of value-added products from sugar is contagious. I first found out how sugar is made, then some alternative uses for it and its byproducts.
How cane becomes sugar
After the cane is chopped and the leaves are removed, it travels by truck to a sugar mill. The cane is transferred to a conveyor belt that takes it first to rotating knives, then to a shredder, and finally to a crusher and mill tandem, which forces the cane juice out of the cells. The juice is muddy sugar water containing about 12% sucrose. To clarify the juice, milk of lime (calcium hydroxide) is added and the solids (filter mud) are removed by filtration. The filtrate is evaporated under vacuum to leave ~60% sugar solution.
Further evaporation yields the first crop (A-strike) of raw sugar. After the crystals are collected by centrifugation, two more crops (B- and C-strikes) are harvested from the mother liquors. The C-strike is not used commercially; it becomes seed crystals for the next crop. The crystals of the A- and B-strikes are the raw sugarabout 98% sucrose. The raw sugar is shipped to a refinery, where it is recrystallized into table sugar99.9% sucrose.
The final mother liquor is known as blackstrap molasses, and the remaining plant material is bagasse. Each of the four componentssugar, molasses, bagasse, and filter mudhas several uses and can lead to value-added products (Figure 1). Last year, the price for sugar was >21¢/lb, but it dropped to 18 19¢ by the end of the year (1). Two-thirds of the proceeds typically go to the cane farmers. The price is further threatened by imports from Mexico. If sugar and its byproducts could be converted to high-value chemicals and other products, farmers and mill operators would prosper and new jobs would be created.
Sucrose as a starting material
Crop yields in 1999 were so good for Louisiana sugar farmers that the mills produced an excess of raw sugarmore than they could selland the price dropped. Many farmers could not repay their annual government loans. The U.S. Department of Agriculture (USDA) has come to the rescue and plans to buy 150,000 tons of sugar to save the mills and stabilize the price (1). This amount of sugar will cost $60 millon, and the outlay may rise to $500 millon as more sugar is purchased (2). Officials say loan forfeiture would cost even more, so the purchase is justified.
What will the government do with all this sugar? One suggestion is to use it to make ethanol, which is replacing methyl tert-butyl ether (MTBE) as the preferred oxygenated gasoline additive (3). Ethanol can also be converted to ethyl tert-butyl ether, an alternative fuel additive. It makes senseconvert an excess commodity to one that is in demand. However, because of pressure from corn-growing statesmost fuel ethanol in the United States is produced from cornstarchthe cane probably will not be used to make ethanol.
Another use for sucrose is in the production of dextran, a polysaccharide used as a blood plasma extender. This use may seem strange to veteran sugar producers who worry about too much dextran formation in the cut cane awaiting processing. While working on ways to control dextran formation, Kim and Day (4) developed a mutant microorganism that synthesizes dextranase, the enzyme that breaks down dextran. They have patented a process that uses a dextran-producing organism along with a dextranase-producing organism (5). By varying the ratio of the two species, the researchers can synthesize dextrans in specific molecular weight ranges. This process should be especially useful for the plasma-grade dextran that requires a molecular weight of ~75,000.
The researchers also found that some dextrans can be used to replace antibiotics in animal feeds (6). The dextran feeds beneficial intestinal bacteria to the detriment of harmful ones. Trials using dextran in chicken feed are planned. If they are successful, researchers believe, the bacterial risks associated with chicken consumption will be reduced.
Molasses: Feed or feedstock?
The mother liquors of sugar crystallization are not the molasses of cookies and pumpernickel bread. Table molasses is a mixture of raw sugar and the first mother liquor. In baked goods, its moisture-retention properties help extend the natural shelf life. The antioxidant properties of molasses help preserve fat-soluble vitamins and oils. The organic and inorganic salts in molasses create a buffer that controls the pH in yeast-raised baked goods. The acids in molasses contribute to the leavening action in baked goods.
Although it is sometimes touted as a health food, blackstrap molasses, the final mother liquor, is a dark, smelly substance that is not very appetizing. However, its 44% sugar content makes molasses an ideal starting material for a host of products.
One use for blackstrap molasses is alcohol productiontraditionally in the manufacture of rum. A new firm, the Louisiana Rum Co., Baton Rouge, is planning a state-of-the-art facility to produce rum from Louisiana molasses. After clarifying the molasses to improve yield, it is fermented with a special yeast that produces glycerin and alcohol (7). Distillation separates the alcohol portion, which is subjected to a new proprietary aging process. The new process produces a drinkable rum in just one day; the traditional process takes two years. However, the rum will be aged in oak casks for at least six months to develop its color and flavor. Then the rum is diluted with water, filtered, and bottled.
The distillation residue, called stillage, is the source of many of the flavor components of rum. For example, with the use of native and added bacteria, stillage produces acetic and lactic acids. Further fermentation produces caproic and succinic acids, which are esterified to ethyl caproate and ethyl succinate. These esters are important flavor components along with certain aldehydes and fusel oils (chiefly amyl alcohols). Light and heavy rums are produced by controlling the relative amounts of these components (7).
The proposed rum plant has a strong chance of commercial success because it is capable of producing flavorful vinegars and liqueurs, in addition to rum. The new plant will also be able to produce animal feed by mixing the stillage with two other byproducts available in Louisiana: fish waste and sugarcane bagasse. Finding a commercial use for the stillage generated at the plant will also ease environmental concerns, because there will be very little remaining waste.
Ethanol and glycerin
The proposed rum plant will not be large enough to produce many chemical products, but a molasses-based fuel ethanol plant would be. Such a plant could be a significant new source of glycerin. Kampen, Njapau, and Munene have studied the fermentation that produces both ethanol and glycerin (7), and Kampen holds patents for increased glycerin production (8, 9). The glycerin can be recovered by using membrane filtration and liquid chromatography. If the osmotic pressure in the fermentation vessel is increased, the cells undergo plasmolysis and survive by raising their osmotic pressure. The cells raise their pressure by making polyolschiefly glycerin.
Glycerin is in demand by manufacturers of chemical, tobacco, personal care, and pharmaceutical products, and its price is rising. Most glycerin comes from animal fats, and European concern about mad cow disease has increased interest in plant-derived glycerin, especially in the pharmaceutical industry.
Currently, most fuel ethanol plants depend on government subsidies. If they could make additional high-value products, these plants would make a profit. One corn-based ethanol plant, operated by High Plains Corp. (Wichita, KS), has licensed Kampens technology and is starting glycerin production (10). High Plains expects to produce 99.7% pure glycerin for 15¢/lb and sell it for 60¢ to $1/lb.
More from molasses
Many other chemicals are potentially available from fermentation and purification of molasses or stillage.
Bagasse: Trash or treasure?
Bagasse, the sugarcane cellulose left over after milling, is burned as fuel to supply heat for concentrating cane juice. However, the manufacturing plants produce excess bagasse and pay for its disposal. Therefore, it makes sense to use the bagasse as a starting material for another process. Bagasse will be the feedstock when BCI (Dedham, MA) rebuilds a failed grain-to-ethanol plant in Jennings, LA, into a bagasse-to-ethanol plant (13). The technology for the plant is based on a genetically engineered strain of Escherichia coli developed by Lonnie Ingram at the University of Florida, Gainesville, which converts five- and six-carbon sugars to ethanol. The celluloses in the bagasse will be hydrolyzed with sulfuric acid and then subjected to fermentation with the patented Ingram bacteria.
An article in Chemical & Engineering News (Dec 7, 1998) quoted Harry Parker, a professor of chemical engineering at Texas Tech University: Mark your calendar for 18 months from now. Im betting there will be no cellulose-to-ethanol plant in sustained commercial operation in Jennings.
As he predicted, the plant is yet to be built, although a groundbreaking ceremony was held on November 20, 1998. However, Louisianas bond commission has approved the sale of up to $120 millon in tax-free bonds to finance the plant (14), and BCI negotiated an $82 millon contract with a construction firm (15). Although the economic success of the plant is in question, the essentially free starting material and the 54¢/gal federal subsidy for ethanol are in its favor. Negative factors are the cost of shipping the bagasse from Louisianas 18 sugar mills to Jennings and the plants dependence on government aid. Making value-added products could eventually offset the high costs.
Another chemical product from bagasse is furfural, which has applications ranging from foundry resins to rocket fuel (16). In a recent Chemical Innovation article, Karl Zeitsch explained that bagasse is submitted to acid digestion, followed by hydrolysis to pentoses, which are then dehydrated to furfural. If the process is continuous, significant quantities of the value-added products diacetyl and 2,3-pentanedione are produced. These products are used as buttery flavorings for margarine, cookies, ice cream, and other foods.
In addition to its use for compost and animal feed, bagasse may be used in paper and particleboard. Acadia Board Co. (New Iberia, LA) hopes to manufacture a composite board material for home building from a combination of epoxy and bagasse (17). Chemists at the USDAs Southern Regional Research Center in New Orleans have produced epoxy polymers from sugar. The combination of epoxy and bagasse forms a building material that is resistant to termites.
Waste not, want not?
Even the filter mud, removed from the raw cane juice before it is concentrated, has value. In addition to soil and plant material, the mud contains a wax that had coated the sugarcane stalks. The mud is usually returned to the cane fields, but Michael Saska of the Audubon Sugar Institute is working on ways to extract the wax (6). The mud yields up to 1% of a high-value food-grade wax. Cane wax is a potential replacement for a rain forest product, carnauba wax, that is widely used in cosmetics, foods, and pharmaceuticals. In addition, with supercritical fluid extraction, the wax is a source of long-chain aliphatic alcohols. Octacosanol (C28) is reported to increase physical stamina and remedy damaged nerve cells and even to stimulate sex hormones (18). Sugarcane is also a source of sterols, particularly β-sitosterol, the active ingredient in saw palmetto berries, a nutritional supplement for prostate problems.
Although the chemical possibilities for sugarcane seem limitless, few of these products are currently in production. The tradition-bound sugar industry is slow to adopt the new technologies required for implementation, but the scientists of the Audubon Sugar Institute hope to change that mindset.
Anne Kuhlmann Taylor is a consultant and freelance writer specializing in scientific topics (5420 S. Woodchase Ct., Baton Rouge, LA 70808; 225-767-0818; firstname.lastname@example.org). After 14 years in pharmaceutical analysis at Schering-Plough and Warner-Lambert, she became a consultant in GMP compliance and writing for regulatory submissions. She has a B.A. degree in chemistry from Gettysburg College and a Ph.D. in chemistry from Cornell University.