Adding an electrical jolt to fermentation of industrial food waste speeds up the process and increases the yield of platform chemicals that are valuable components in a wide range of products, new research shows.
In developing the new system, researchers at The Ohio State University also discovered that combining two bacterial species in the electro-fermentation mix not only helped accelerate the process, but allowed for more targeted chemical production.
In this study, the food waste consisted of ice cream and sour cream - but the team has expanded the work with experiments using coffee grounds and lake algae.
Eventual adoption of the technology could reap many benefits: efficient, sustainable and cost-effective production of multipurpose chemicals using source materials that would otherwise end up incinerated or in a landfill, contributing to greenhouse gas emissions.
"We are creating an industry from another industry's waste," said first author Beenish Saba, a research scientist in food, agricultural and biological engineering at Ohio State.
"We're making use of waste that a contractor charges businesses to take to a landfill, where it produces methane gas. We are suggesting that industries can put up a simple bioreactor in which they can produce other important byproducts."
The study was published recently in the Journal of Environmental Chemical Engineering.
This work builds upon previous waste valorization research done by Saba and Katrina Cornish, professor emerita of horticulture and crop science and food, agricultural and biological engineering at Ohio State and a co-lead author of the current study.
The valorization work involved analysis of physical and chemical properties of 46 food waste samples to identify good candidates for conversion to chemicals and biogases through a variety of processes - including fermentation.
In the new study, Saba and colleagues compared the output and duration of conventional fermentation and electro-fermentation. Conventional practices consist of placing food waste and bacteria in a bottle, adjusting nutrient levels and incubating the materials at 98.6 degrees Fahrenheit. Electro-fermentation is accomplished at room temperature inside a bioreactor outfitted with an electrode powered by minimal external voltage.
"In conventional fermentation, the bacteria are happily growing and they will produce some solvents and gases," Saba said. "In the second step, we gave them a little tingling electricity so the bacteria can feel a little irritation, and the metabolism was fast. They were growing and happily eating, and they produced more byproducts - meaning we can increase the yield."
There was another bonus to development of this new microbial electrochemical system: production of hydrogen gas.
Experiments showed that combining two bacterial species from the Clostridium family contributed to hydrogen gas production while also reducing fermentation waste - it is known that the commonly used species C. bijerinckii generates carbon dioxide while converting food waste into alcohols, but it turns out another species, C. carboxidivorans, consumes that CO2.
"It means the waste product of one bacteria is utilized by the other bacteria," Saba said. "It was possible that there could have been an antagonistic relationship, but we tested growing them together and found there's a synergistic relationship between these two bacteria that works well."
And in addition to consuming the CO2, C. carboxidivorans produces hydrogen gas and solvents.
"Carbon dioxide is still there, but most of it is consumed, and it gives us hydrogen gas - an additional product. We now we have two valuable products and one waste product," she said.
The work dovetails with an increased focus on using food waste and agricultural residue to create biobased products, Saba said.
"We are working on improving the yield, cost efficiency and scalability," she said. "The government is asking for work in this area and industry is interested in getting value from waste and not paying for its disposal.
"So much material that is agricultural or biological in nature is just going to waste. It's much better to utilize them and make valuable products."
This research was supported by the Ohio State President's Research Excellence-Catalyst program funding and the Ohio Water Development Authority.
Additional co-authors were Stephen Akinola, Ann Christy and Thaddeus Ezeji, all of Ohio State.
