Scientists develop more efficient biorefining process to make ethanol from wheat

In a finding that could help put wheat alongside corn on the menu of biofuel sources, researchers in Greece and the United Kingdom report the development of a new method for producing ethanol from wheat. The technology - potentially cheaper and more efficient than conventional methods for producing wheat-based biofuel - has been published as an open-access article in Biotechnology Progress.

As oil prices soar, demand for bioethanol to stretch out supplies of gasoline has increased dramatically, along with frenzied research efforts to find the best raw materials and conversion processes for its economical production.

Cereals and sugar crops are currently the preferred raw materials for bioethanol production due to availability and low processing cost. In the EU, wheat is more widely cultivated and could be regarded as the preferred cereal grain for bioethanol production. The predominant process for wheat conversion into a fermentation feedstock begins with a simple dry milling stage leading to the production of whole wheat flour. The starch content in whole wheat flour is then hydrolyzed into glucose by commercial enzymes. The resulting glucose solution is fermented into ethanol after the addition of nutrient supplements. This process finally leads to the production of only one coproduct (Dried Distiller's Grain), which has a low market value as animal feed.

In the new study, Apostolis Koutinas and colleagues describe a simplified biorefining method that uses fewer steps and less energy and generates fewer waste products but more valuable byproducts (schematic, click to enlarge). The economics and efficiency of bioethanol production from wheat could thus be improved by fractionating the grain into the fermentable fraction and several nonfermentable fractions, including bran, germ and protein, that have a wide spectrum of end-uses.

The main differences between the proposed and the traditional dry milling of wheat are:

• Wheat components that are not fermentable during Saccharomyces cerevisiae cultivations for bioethanol production are separated prior to fermentation. In this way, two coproducts are produced (bran-rich pearling and gluten) with current and potential market outlets that could improve process economics. In addition, the removal of non-fermentable solids from yeast fermentation leads to the production of pure yeast cells that have a much higher market value and diversified market outlets as compared to Dry Distillers Grains (with solubles) produced by traditional wheat dry milling.
• Hydrolysis of starch or any remaining protein and phytic acid is achieved by consortia of enzymes that are produced by Aspergillus awamori fermentation on wheat flour. Simultaneous gelatinization, liquefaction, and saccharification is achieved at temperatures less than 70 C because the crude filtrate used from fungal fermentations contains all of the enzymes required to hydrolyze wheat components. Fungal cells were grown on exactly the same medium to produce enzymes that led to complete hydrolysis of wheat starch and protein during fermentation. Depending on plant capacity, this processing scheme leads to lower energy requirements and capital investment as compared to traditional processing that uses separate liquefaction and saccharification stages.
• Wheat is the sole raw material used throughout this process. A minimum amount of waste is produced by regenerating nutrients consumed during A. awamori fermentation via fungal autolysis. Fungal autolysates are used to supply additional nutrients for yeast fermentation.

In the process presented by the scientists, starch hydrolysis and fungal autolysis are carried out in separate reactions. However, the operating conditions employed for starch hydrolysis (60 C and uncontrolled pH) and fungal autolysis (55 C and uncontrolled pH) are very similar. This creates the opportunity to integrate these two unit operations in a single batch unit operation leading to lower capital investment and processing costs:
energy :: sustainability :: wheat :: biofuels :: ethanol :: efficiency :: by-products :: biorefining :: bioconversion ::

The study then presents experimental results that justify the feasibility of integrating starch hydrolysis and fungal autolysis in the same unit operation for the production of a nutrient-complete medium for bioethanol production.

The feedstock production process including continuous operation for fungal fermentation and combined hydrolytic/autolytic reaction has been cost-optimized by the scientists by nonlinear programming. A continuous scheme for starch hydrolysis is proposed where initial gelatinization and liquefaction is carried out at significantly lower temperature (68 C) and faster reaction rate (up to 10 min residence time) as compared to the conventional process due to the utilization of the enzyme consortium produced from fungal fermentation. Subsequently, starch saccharification is carried out together with fungal autolysis at 55 C in the same unit operation.

Depending on the selected combination of physical and biological treatment, the optimized process yields various fractions enriched in bran, wheat germ and proteins that could be sold or utilized for the extraction or production of value-added products, boosting income of biorefineries, the scientists say.

This process could substitute for the conventional wheat dry milling process that is currently employed in industry. The most important unit operations of the proposed continuous scheme are a fungal fermentation producing enzymes and fungal cells and a combined hydrolytic/autolytic reaction producing a nutrient-complete medium.

References:
Najmul Arifeen, Ruohang Wang, Ioannis Kookos, Colin Webb, and Apostolis A. Koutinas, "Optimization and Cost Estimation of novel Wheat Biorefining for Continuous Production of Fermentation Feedstock", Biotechnol. Prog., 23 (4), 872 -880, 2007. DOI: 10.1021/bp0700408 S8756-7938(07)00040-9