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Students try bacterial fermentation for ethanol production

Students in course 10.27 (Chemical Engineering Processes Laboratory) are learning about ethanol production through bacterial fermentation in a two-part, hands-on process developed by Jean-Francois Hamel, research engineer and lecturer in chemical engineering.

During fermentation, Zymomonas mobilis bacteria consume glucose and excrete ethanol in the bioreactor. Ethanol is used to make other chemicals, and scientists and engineers are working on ways to make it an economically feasible alternative to fossil fuels. Yeast rather than bacteria is normally used in commercial production of ethanol, the alcohol found in beer and liquor. Yeast feeds on products that are less expensive than glucose, such as molasses and corn syrup, though Zymomonas may have the potential to produce ethanol faster and in greater yields at similar concentrations.

In a refinement planned for next semester, the optimum glucose level will be maintained by a feedback mechanism in which a sensor monitors the glucose level within the fermenter and sends a signal to a supervisory computer, which triggers a pump to add more glucose when needed. (Too little glucose will cause the bacteria to starve, but too much inhibits fermentation.)

When fermentation is complete, ethanol is extracted from the 15 liters of broth (composed of bacteria, water, leftover food medium and about 100 grams of ethanol per liter). The liquid is filtered to remove the bacteria and taken to the eight-foot-high distillation column in Building 66, where it is placed in a large glass bulb at the bottom and heated in repeated cycles. The ethanol, in the form of vapor, rises through the column and is captured as it condenses. Students take samples through an opening along the column.

A version of this article appeared in MIT Tech Talk on December 6, 1995.

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