Corn as a chemical feedstock

Corn as Chemical Feedstock

The use of plant materials to produce industrial products is not a new concept. With the rise of the petroleum industry, biomass-based industrial production of fuels and chemicals decreased substantially in the 1950s and 1960s. The advent of "biotechnology" in the 1970s, recognition of the limitations of fossil fuels and emphasis on renewable sources of energy has rekindled interest in "bioprocessing". Bioprocessing is the adaptation of biological methods of production to large-scale industrial use. It thus involves the integration of the biological, chemical and engineering sciences for the development of economically viable products and processes. Competitive advantages in biotechnology may well depend as much on developments in bioprocess engineering as on innovations in genetics and molecular biology.

The key step in this technology is the conversion of a carbohydrate source (e.g., corn starch) to desirable products, using biological catalysts such as enzymes and microorganisms. Dextrose (glucose), which is a sugar that can be produced from corn starch, is the most common feedstock for these fermentation processes. The corn refining industry is a good example of the use of renewable resources for producing a variety of products, from food, feed and fiber to industrial fuels and chemicals. The most visible example of this fermentation technology is the use of yeast to produce ethanol, mostly as an additive to gasoline. US capacity today exceeds 1.5 billion gallons (5.7 million cubic meters) annually. However, ethanol is only one of several chemicals that can be manufactured from corn. Others include:

  • Acetone and butanol, which are high-value industrial solvents.
  • Butanediol, an industrial solvent and a precursor to synthetic rubber.
  • Lactic acid, valuable by itself as a commodity chemical, but also in the future as a raw material for polylactates and acrylates, which are biodegradable polymers useful in packaging and medicine.
  • Acetic acid, the main component of vinegar, but which has a large market as an industrial chemical, primarily for the production of vinyl acetate, monochloroacetic acid, acetic anhydride, cellulose acetate, acetate solvents, terephthalic acid, various dyes and pigments and as a solvent in the chemical process industry. The textile industry uses acetic acid as a buffering agent in dye baths and for neutralization. The pharmaceutical industry uses acetic acid to manufacture vitamins, antibiotics and hormones. The food industry uses it as an acidulant and for the preservation of fresh meat products.World production is about 3.5 million tonnes per year, of which about 2 million tonnes is produced in USA.

    It also has potential as an environment-friendly, noncorrosive highway deicer in the form of calcium-magnesium acetate (CMA), to replace the chloride salts now used to clear roads in the winter. If even half the salt used on the nation's highways were replaced with CMA, the demand for corn would increase by 100-300 million bushels annually and create a $1-3 billion market for a new corn-based product.

  • Citric acid, a major acidulant in food products, which is used in industrial detergents as metal finishing and cleaning solutions. Much of the citric acid available in the United States today is produced by corn processors.

The limitations

There are technical constraints in the use of fermentation to produce these products. A variety of microorganisms are used in these processes, e.g., yeast (Saccharomyces cerevisiae) for ethanol; bacteria such as Lactobacillus delbreuckii for lactic acid; Clostridium thermoaceticum or Acetobacter aceti for acetic acid and fungi such as Aspergillus niger for citric acid. Frequently, the "wild" strain of the microorganism found in nature is inefficient in that it produces the chemical too slowly (for example, ethanol fermentation typically takes 24 to 48 hours, acetic acid fermentation 36 to 200 hours) and in too low a concentration (for example, ethanol is 10 percent of the fermentation mixture, acetic acid only 2 to 5 percent, acetone-butanol 1.5 percent). These inefficiencies result in high fermentation and downstream processing costs, the latter for removing large quantities of water and for purifying the chemical. (In contrast, petroleum refiners essentially start with 100 percent product in the crude oil: they simply have to distill the oil by heat and blend the fractions to obtain the desired products).

The solutions

Industrial microbiologists and biochemical engineers are addressing these inefficiencies in producing fuels and chemicals from corn. Genetic engineering can result in microorganisms with improved biosynthetic capabilities. Researchers at the University of Illinois have developed strains of Clostridium acetobutylicum that now produce 40% higher concentration of butanol of parent strains. Clostridium thermoaceticum has been mutated and improved to produce 400% higher concentrations of acetate than the parent wild strain.

Similarly, UI biochemical engineers have designed continuous bioreactors with productivities that are 10 to 30 times higher than the technology used today. In addition, modern separation techniques based on synthetic membranes are expected to dramatically change the way in which 21st century corn refineries operate.

But .... more limitations

Although these technologies will improve the manufacturing process, the economics will depend to a large extent on factors beyond the control of microbiologists and engineers. The corn itself accounts for 50-70 percent of the cost of ethanol and 20-40 percent of the cost of organic acids. Energy, much of it produced by coal and natural gas, probably accounts for another 15-25 percent of the total corn-refining cost. The prevailing price of oil on the international market has much to do with the economics of corn-based versus petroleum-based chemicals. However, petroleum is a finite resource that is largely imported, whereas corn is annually renewable and available in abundance within our own borders. One must also consider the military and political consequences of keeping imported oil available at low cost.

Considering these factors, and with continued technological advances, the cornfields of the Midwest could be as important a source for fuels and chemicals in the future as the oil fields of the Middle East are today.

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Updated January 2011 by