A Sequential Fermentation Biorefinery to Produce Ethanol from Corn Processing Co-products

Grant# 04-04
Principal Investigator: J (Hans) van Leeuwen
Organization: Department of Civil, Construction and Environmental Engineering, Iowa State University
Technical Area: Efficiency & Renewable Energy

Dependence on imported oil is a difficult economic and strategic challenge for the U.S. Energy self-sufficiency is an important motivation for the U.S. government’s strategy towards more bioenergy and other biobased products. Microbial processes could play an important role in this because these are much faster than plant or animal production of biomass and because microbes can use a much larger variety of feedstocks. Key to success is the use of a cheap substrate to produce valuable byproducts and useful microbial biomass.

Corn milling and ethanol fermentation produces large quantities of fibrous, lignocellulosic co-products of low value. Yeasts, required to make ethanol, will not grow on such complex organic substrates and pretreatment would be needed to use these feedstocks for more ethanol production. The proposed research aims to develop an innovative “biorefinery” to sequentially ferment these co-products using white rot or brown rot (micro)fungi followed by yeast fermentation to produce ethanol. Both of the brown and white rot fungi are known to be able to degrade very complex organic substrates such as lignin and cellulose, producing a simple substrate for yeasts to ferment to ethanol. Residual fungal and yeast biomass have additional value as additives to animal feeds.

Shake flask studies will determine the most suitable fungal species to ferment a mixture of the co-products from wet and dry milling of corn. Continuous bench-scale fermentation to optimize growth conditions for these fungi will then be tested. The remaining fermentation broth, following fungal harvesting, will be analyzed for simple carbohydrates suitable for yeast fermentation. The fungal treatment will be optimized to maximize such products as required for yeast fermentation. Shake flask studies followed by larger batch studies, will be used to optimize the yeast fermentation and ethanol production. An economic analysis based on conversion rates to ethanol and other useful products and the operational requirements will be done to determine the feasibility of the proposed biorefinery.

The same team at Iowa State University successfully developed fungi production on corn processing wastewater with U.S. Department of Agriculture funding and now further supported directly by industry. These fungi will be used as a source of valuable biochemicals.

It is expected that this research will improve the understanding of fungal and yeast fermentation processes in such a way as to provide an opportunity for industrial scale value-adding to low-value corn milling co-products. Ultimately, other low value agricultural co-products could be investigated as feedstock to such biorefineries. The industrial scale biorefineries could help to solve the U.S. energy dependence problem while creating jobs in industry and adding value to U.S. farm products. This will be of particular value in Iowa and the Midwest as major sources of corn and other agricultural products and also the seat of many existing corn milling operations.

The main aim will be to make more ethanol from corn by also making some ethanol from lower value corn milling co-products that are presently not used for ethanol fermentation. These co-products are complex carbohydrates and are resistant to biodegradation by most organisms. In the proposed research, the co-products will first be degraded by fungal fermentation to liberate simpler carbohydrates. These breakdown products will be used as feedstock in subsequent yeast fermentation to produce additional ethanol.

  1. Select the most suitable white rot or brown rot fungi to be cultivated on low-value co-products from wet and dry corn milling.
  2. Establish the best operational protocols and conditions to continuously cultivate fungi on the corn milling co-products.
  3. Determine the suitability and efficacy of the remaining degraded substrate as a feedstock in yeast fermentation to produce ethanol.
  4. Quantify microbial biomass production in a continuous bioreactor system.
  5. Find other applications of fungal and yeast biomass.
  6. Determine the microbial by-products formed during continuous fungal bioreactor operation.
  7. Disseminate the research findings using proven technology transfer vehicles (peer reviewed technical paper publication, conference presentation, pilot-scale demonstration, and workshop).

Work To Date : (Technical Report – March 2007)

1. The fungal enzyme secretion was enhanced by addition of lignocellulosic biomass (corn fiber) during enzyme induction experiments.  These fungi produced enzymes in situ to hydrolyze the cellulosic, and the hydrolysates were fermented to ethanol in a simultaneous-saccharification and fermentation process.  The net ethanol yield increased to over 10 % (w/w).

2. Enzyme induction test needs to be repeated to improve the enzyme activity and also confirm activities of enzymes especially alpha-amylase and glucoamylase.

3. The methodology under development represents a unique step for lignocellulosic ethanol production.  No polluting chemicals are used and the process operates only slightly above room temperature.

4. Thin stillage can be potential substitution to replace expensive fungal culture media.