How Bread Waste Could Provide a Sustainable Source for Ethanol
By Ashlee Bushee
An introduction to the bioeconomy and how bread waste could be a new viable and reliable source for ethanol production.
Nine hundred thirty-one million tons of food waste was generated in 2019. We know that is a big number, but we are so used to hearing about really big numbers that they often fail to phase us. It would take 23 million fully loaded semi-trucks to haul this waste to the landfill, and if all of these trucks were on their way to the landfill at the same time that traffic jam would wrap around the earth 7 times. If each truck dumped their waste in 1 minute and you were the very last truck in line you would be waiting for about 43 years. We often lose the scale of these problems because there isn’t one annual pickup day for food waste, but every day around the world billions of people and businesses throw a little out and that little bit adds up by the end of the year.
Food waste is not a new problem, but researchers are continuing to try to find ways to utilize this waste in the bioeconomy. There isn’t one definition, but the general idea of the bioeconomy is to innovate to replace fossil fuel products with biological resources for the production of fuels, medicine and manufactured goods. During the production process an important aspect of a biorefinery is to use a microorganism or enzyme to catalyze the process. Using a microorganism can result in the process having fewer greenhouse gas emissions or toxic waste, because it is run at lower temperature and pressure than a chemical catalyst or physical process. The bioeconomy in the United States is currently valued at around $1 trillion and on September 12, 2022 the White House issued an Executive Order advancing biotechnology and biomanufacturing innovation for a sustainable, safe and secure bioeconomy.
Of the 931 million tons of food waste, 18% (about 168 million tons) is from bakery goods; bread, rolls, pastries, etc. Since ethanol can come from starch and grain it would make sense that scientists have assumed using bread waste would be a viable source for ethanol production, and using a waste product could drastically improve the sustainability of ethanol production. In the United States, it’s estimated that the land, water, and chemicals used to grow crops to produce ethanol actually increases greenhouse emissions by 24%. Brazil is also in need of a viable and reliable source for ethanol production as additional land expansion for the sugarcane industry poses a threat to biodiversity, food security, and has potential legal and policy implications. In this article we will briefly review the most widely used methods for ethanol production, and explore a recent advancement in utilizing commercial bread waste via filamentous fungi strains for ethanol production.
A Brief Overview of Current Ethanol Production
Ethanol has been manufactured primarily from two production processes: ethanol sugar fermentation and ethylene hydration. Ethanol from sugar fermentation starts with a sugar feedstock. This can come from sugar cane juice, sugar beet juice, cane or beet molasses, raw sugar, or refined sugar – commonly referred to as production from sucrose. The sugar needed can also come from barley, corn, grain sorghum, or wheat – commonly referred to as production from a starch. Ninety-seven percent of the estimated 15 billion gallons of ethanol produced each year in the United States starts with corn as the feedstock. Ethylene hydration for ethanol production was first introduced by Shell in 1947. It is a three-step process including the reaction in which ethylene and water are preheated to 300°C. They then enter into the catalytic reactor, which uses phosphoric acid, and results in a conversion between 4-25%. These products then enter a distillation column where the ethanol and water vapor are separated out and then purified. This portion can be costly because the ethanol and water vapor can be trapped in a zeolite and this drastically increases operating costs to extract. In 2010 this method only accounted for 7% of the global production of ethanol.
Why Fungi?
Research has been conducted to show that breadcrumbs can be efficiently converted into bioethanol using baker’s yeast, success with yeast has provided a foundation to try fermentation with fungi. Filamentous fungi are known for their capacity to make large amounts of hydrolytic enzymes which is what enables some species to ferment raw materials to produce edible biomass and ethanol. Looking at the structure of filamentous fungi can help show the difference between this structure and the cute mushrooms you see at the grocery store. Filamentous Fungi have been used at an industrial scale to produce specific enzymes, organic acids, and antibiotics. Four filamentous fungi strains were used in this fermentation process two from the Zygomycetes and Ascomycetes Phylum, see Table 1 for more information.
Table 1: The four strains of filamentous fungi used to measure which could produce more ethanol from bread.
| Phylum | Species | Features |
| Zygomycetes (also contains Rhizopus stolonifera commonly known as bread mold) | Rhizopus oryzae | The most studied species. Metabolic products are lactic or fumaric acid. |
| Zygomycetes | Mucor indicus | Has been used for the production of beer and food, can grow on hexose and pentose which can often inhibit fermentation. |
| Ascomycetes | Neurospora intermedia | Also known as red bread mold, this specific species cam also easily trigger allergic reactions and is used in allergy testing. |
| Ascomycetes | Aspergillus oryzae | Also known as Koji this species is commonly used in the production of soy sauce, soybean and to make liquor |
Ethanol Production from Bread Waste and Filamentous Fungi
Unsold wheat-rye bread that showed no visible signs of mold or other infection was obtained from a local bakery. It was then cut into 1-inch cubes and dried overnight in an oven at 122 Fahrenheit. The croutons (if you will) were then ground into bread crumbs. Water, heat, and a liquification enzyme were added to the breadcrumbs and it was agitated until it reached a slurry consistency. The four strains of filamentous fungi, from Table 1, were prepared on clean potato dextrose agar plates and incubated at 86 degrees Fahrenheit for 3-5 days. After this incubation period, the plates were flooded with water and the spores released onto a sterile glass spreader. Of the four filamentous fungi strains, Neurospora intermedia had the highest ethanol yield at approximately 29.2-31.6% by weight.
Did it Work?
Researchers successfully used the Neurospora intermedia in the production of ethanol from leftover bread. The only additions to this process were water, heat, and one-half of the recommended amount of liquefaction enzyme. The other process mentioned at the beginning of the article for production of ethanol from sugar or starch fermentation or ethylene hydration all need additional enzymes, catalysts, or require large amounts of land and high temperature or pressure. This new process using a waste product and just the filamentous fungi could result in a more affordable and lower-impact process to synthesize ethanol.
