Orange and banana waste can contribute to ethanol production

Study proposes productive use for the large amount of waste generated by fruit crops between harvesting and post-consumption.

By José Tadeu Arantes

Agência FAPESP – Oranges and bananas, the most common types of fruit grown in Brazil, could also become important supplementary sources of raw material for the production of the fuel bioethanol. This was the aim of the research project “Bioethanol production using banana and orange peels by co-fermentation of Zymomonas mobilis and Pichia stipitis,” supported by FAPESP.

The study was coordinated by Crispin Humberto Garcia Cruz, a full professor at the São José do Rio Preto Campus of São Paulo State University (UNESP) in Brazil, and developed by Michelle Cardoso Coimbra, a PhD student and holder of a scholarship from FAPESP.

“Of course, answers obtained in the laboratory can’t simply be extrapolated to a large-scale industrial process, but they serve as the basis for an estimate. Judging from the values obtained at the laboratory scale, if all of the waste generated by orange and banana crops were converted into ethanol, annual production could reach 658 million liters,” Garcia Cruz told Agência FAPESP.

Cardoso Coimbra described the process performed in the laboratory step by step. “First, the orange and banana waste is dried and ground into powder. The next step is acid hydrolysis with sulfuric acid at 5%. There are other types of pre-treatment, such as alkaline hydrolysis, steam explosion and so on. The pre-treated material is then blended with enzymes in solution for about 24 hours,” she said.

“After enzymatic hydrolysis, the mixture is filtered and detoxified with activated carbon to remove any inhibitory compounds formed during acid hydrolysis. The material is then used as a substrate for fermentation by Zymomonas mobilis and Pichia stipitis, producing ethanol.”

“Using combined cultures of Zymomonas mobilis and Pichia stipitis increased the yield compared with processes based on only one of the microorganisms,” Garcia Cruz explained. “This is because with two microorganisms, both pentoses and hexoses released by peel hydrolysis can be converted into ethanol.”

Brazilian production of anhydrous and hydrous sugarcane ethanol amounts to some 27 billion liters per year. Ethanol from orange and banana waste would correspond to approximately 2.5% of this volume. Considering only hydrous sugarcane ethanol, which is widely used as automotive fuel, annual production amounts to 15 billion liters. In this case, orange and banana ethanol would correspond to 4.3%.

Most of the feedstock would come from oranges.

12 million metric tons of waste

Brazil is the world’s largest producer of oranges, with an annual crop amounting to some 18 million metric tons. “Approximately 50% of an orange’s weight consists of peel and bagasse, which are the main forms of waste. Thus, we can estimate that 9 million tons of orange waste per year could be generated, which ideally could be converted into 570 million liters of ethanol,” Cardoso Coimbra said.

Annual production of bananas, the second most commonly grown fruit in Brazil, amounts to approximately 7 million tons. According to EMBRAPA, the Brazilian Agricultural Research Corporation, 46 kg out of every 100 kg harvested cannot be sold due to non-compliance with consumer quality standards. Rejects alone could generate some 3 million tons of recyclable waste per year.

“In ideal terms, this amount could be converted into 88 million liters of ethanol,” Cardoso Coimbra said.

If the 570 million liters from orange waste are added to the 88 million liters of banana waste, the total obtained is 658 million liters per year.

These numbers are purely theoretical for now. No production facilities that can produce ethanol from fruit currently exist in Brazil. These plants would preferably be built close to plantations. Alternatively, existing plants could be adapted to process the feedstock. In addition, collection and processing of fruit waste depend on several factors. One of these is that waste is generated during different stages of the orange and banana production processes, from harvesting and transportation to resale and post-consumption.

An even more important factor is that ethanol production from fruit waste depends on second-generation technology, which involves breaking down cellulose and hemicellulose chains by enzymatic hydrolysis and conversion of the resulting sugars to ethanol.

Cost of enzymes

“One of the main bottlenecks is the high cost of the enzymes required to release sugars in the cellulose and hemicellulose hydrolysis stage,” Garcia Cruz said. “Another is the use of genetically modified microorganisms or combined cultures of microorganisms capable of fermenting the hexoses and pentoses released by hydrolysis, thereby increasing yields in second-generation ethanol production.”

Brazil’s first second-generation sugarcane ethanol plant began operating commercially late last year in São Miguel dos Campos, Alagoas State. The plant is located near three mills that produce sugar and first-generation ethanol, selling part of their waste (cane straw and bagasse) to the second-generation plant. This arrangement is essential to keep transportation costs down and would not be possible in the case of fruit waste, at least in the near term.

Despite these caveats, the researchers consider fruit waste ethanol to be a commercially promising option, all the more so because there would be by-products. “The waste resulting from the various stages of ethanol production could generate electricity,” Garcia Cruz said. “Moreover, before using orange peel, it would be advisable to extract the essential oils, which consist mainly of limonene, a by-product of value to the food industry.”