Bioconversion of Processing Byproducts and Wastes

Walker Terry  |  5/6/2005 6:52:00 PM

Terry H. Walker, Caye M. Drapcho and Donal F. Day

The usable carbon and nutrients contained in rice hulls and bran, sugarcane bagasse and sweet potato skins, which are Louisiana agricultural byproducts, may be converted by microorganisms to high-value products. LSU AgCenter researchers are developing bioconversion processes that can be used to produce specialty or nutra-ceutical compounds from these byproducts. The value of these beneficial compounds may exceed $50 per pound as compared to a value of less than 50 cents per pound for white rice, sugar and sweet potatoes.

In 2000, the Biomass Research and Development Board co-chaired by the U.S. Department of Agriculture and Department of Energy established the goal of tripling use of biobased products and bioenergy by 2010. To achieve this, a number of bioconversion and process technologies are needed to increase production while maintaining profitability. One concept that integrates most approaches is the development of a “biorefinery.” This is a process that first extracts valuable components from a feedstock with the goal of complete use at optimum profitability and minimum waste generation.

LSU AgCenter researchers are coordinating efforts with other states to develop a biorefinery. The goal is bioconversion in three categories of biobased products: specialty chemicals such as pharmaceuticals, bulk chemicals such as ethanol and biomaterials. This collaborative effort will result in faster development of processes that will benefit Louisiana agriculture.

Fatty Acid Bioconversion

Beneficial health effects from consumption of certain fish oils have been attributed to essential polyunsaturated fatty acids. These fatty acids have been linked to a reduced risk for coronary heart disease, arthritis, inflammation, hypertension, psoriasis, other autoimmune disorders and cancer. Essential fatty acids are marketed as dietary supplements for adults and children at health food stores in the form of concentrated fish oils and as prescribed medications for humans and pets. The U.S. market is estimated to be at least $100 million per year with a value of about $50 per pound.

Declining marine resources and an increasing demand have prompted the search for alternative sources of polyunsaturated fatty acids. These sources include certain types of algae and filamentous fungi. LSU AgCenter researchers are examining a bioconversion process using the fungi Pythium irregulare. This fungi is effective at converting the carbohydrate in rice bran and rice husks into stored oils that contain high levels of the omega-3 fatty acids, such as eicosapentanoic and arachodonic acids.

This research has shown that 0.31 grams of oil per gram of fungal biomass can be obtained when rice bran and husks are used as the growth medium and that enhanced growth rates are achieved at a temperature of 25 degrees C. Adding enzymes did not enhance oil production, which is an important cost consideration.

This research used rice byproducts obtained from the Sataki pilot-scale rice mill in the Department of Biological and Agricultural Engineering, thus integrating the bioconversion process to traditional grain processing, which exemplifies the biorefinery concept. The oils are extracted by supercritical fluid extraction using carbon dioxide rather than toxic, flammable organic solvents. Supercritical carbon dioxide has advantages over conventional solvents in that high extraction rates are achieved at lower temperatures, which preserves many of the nutrients extracted.

Production of Bioethanol and Xylitol

Research conducted in the Department of Biological and Agricultural Engineering has led to the investigation of a novel bioreactor system to maximize product formation rates for anaerobic or low-oxygen fermentation products. This system is being investigated for both ethanol production from sweet potato wastes and xylitol production from sugarcane bagasse.

Bioethanol Production. Ethanol is used as a precursor to other organic chemical production and as an additive to fuels to significantly reduce noxious emissions from fuel combustion. MTBE, the current fuel additive, is being phased out because of environmental concerns. LSU AgCenter researchers are focusing on the use of Kluyveromyces marxianus, a heat-tolerant yeast, to convert agricultural waste from the processing of sweet potatoes, rice, corn and sugarcane to ethanol. These byproducts contain large amounts of starch, cellulose and hemi-cellulose that must be broken down to sugars at high temperatures by enzymes in a process called saccharification so that the sugars can be used by the yeast for growth. Use of the high temperature-tolerant yeast allows for the saccharification step and the yeast growth to occur in the same reactor, which results in faster hydrolysis and growth rates, and therefore will lower operating and capital costs. AgCenter research has shown that K. marxianus grows well in hydrolyzed sweet potato wastes and that effective conversion of the organic compounds in the sweet potato waste to yeast biomass and ethanol was achieved.

Xylitol Production. Xylitol is an important sugar alcohol that has found wide application in foods. Xylitol is converted to glucose by human metabolism, but at relatively slower rates that do not significantly increase human insulin production. Food-grade xylitol has an economic value of about $12 per pound. AgCenter researchers are conducting studies into the production of xylitol by the yeast Candida tropicalis using sugarcane bagasse and leaf trash. This process involves the hydrolysis of the celluloses and hemicelluloses to the sugars glucose and xylose, followed by the bioconversion of the sugars to yeast biomass and xylitol. Conversion rates of 0.5 – 0.8 grams xylitol per gram of xylose have been found by other researchers. The goal of the research into the two-stage continuous reactor process is to increase production rates while minimizing costs compared to traditional batch fermentation systems.

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