Modified yeast increases sugar output from energy sugarcane

Researchers at the National Bioethanol Science & Technology Laboratory, in Brazil, have developed a microorganism that reduces glucose and fructose in "supercane" and crystallizes sucrose.

By Elton Alisson  |  Agência FAPESP – Varieties of “energy sugarcane” obtained by crossbreeding the species Saccharum officinarum and Saccharum spontaneum have been grown in Brazil in recent years. 

More robust and with higher fiber content, these “supercanes” developed by companies like GranBio and Vignis, and by institutions such as the Agronomy Institute (IAC), are seen as a solution to increase yields from Brazil’s sugarcane plantations, produce second-generation (cellulosic) ethanol, and boost co-generation of electricity and steam.

Albeit far more productive than conventional sugarcane, “energy cane” has hitherto been used solely for the production of ethanol and electricity, owing to the difficulty of crystallizing sucrose from its juice to make table sugar, the most profitable end-product for mills.

Now researchers at the National Bioethanol Science & Technology Laboratory (CTBE), which belongs to the National Energy & Materials Research Center (CNPEM) in Campinas, São Paulo State, have developed a new yeast that promises to surmount this obstacle. 

The new yeast is based on Pedra 2, one of the most widely used commercial yeast strains for the production of ethanol in Brazil. A patent application was filed in June with the National Industrial Property Institute (INPI).

“The last obstacle to be overcome before energy cane could be planted on a large scale was crystallizing its sucrose to produce granulated table sugar,” said Maria Carolina de Barros Grassi, associate coordinator of CTBE’s molecular division and head of its Energy Cane Program, in an interview with Agência FAPESP.

According to Grassi, the proportions of fructose, glucose and sucrose in energy cane differ from those in conventional sugarcane. “Supercane” contains larger proportions of fructose and glucose and lower juice sucrose content than the varieties typically grown in Brazil to date.

In addition, in ordinary cane, glucose and fructose molecules are fused together to form sucrose molecules, whereas in energy cane, they are separate, hindering the crystallization of sucrose to make sugar.

The solution found by the researchers at CTBE involved developing mutations of several genes in a commercial yeast already used by the sugar and ethanol industry so that it would consume only fructose and glucose.

“By reducing the amount of fructose and glucose in fermentation, we ensured that the cane juice consisted only of pure sucrose. As a result, we were able to crystallize it,” Grassi explained.

Higher yield per hectare

The researchers estimate that it is possible to produce 35 kilos of sugar per metric ton of energy cane with the modified yeast, compared with 71 kilos per ton from conventional cane. 

This comparative disadvantage is more than offset by supercane’s higher yields, which can be three times greater than yields from conventional cane.

While a hectare of conventional cane produces 90-100 tons of the plant, with 13%-14% sucrose and total sugar, it is possible to produce 180 tons of energy cane in the same area, with a sugar content of 8.5%.

The result, according to the researchers’ calculations, is that conventional sugarcane and supercane respectively produce 11.6 tons and 15.3 tons of sugar per hectare.

“Because energy cane yields more per hectare than conventional cane, the amount of both sugar and ethanol will be greater, as will the energy from co-generation because of the larger quantity of leaves and bagasse available for burning,” Grassi said. 

And she gave an example: a hectare of conventional cane enables a mill to produce 8.2 tons of sugar, 1,700 liters of ethanol (from molasses) and 5.6 megawatt-hours (MWh) of surplus electricity.

In the case of energy cane, one hectare yields 8.1 tons of sugar, 4,600 liters of ethanol, and 20 MWh of surplus electricity.

The output of a standalone distillery (producing no sugar) from one hectare of energy cane could reach 9,200 liters of ethanol and 20 MWh, compared with 6,800 liters of ethanol and 5.6 MWh per hectare of conventional cane, the researchers estimated.

“Energy cane enables us to double or triple production nationwide without planting a single extra hectare, simply by raising yields,” said Gonçalo Pereira, director of CTBE and one of the inventors of the technology, alongside CTBE researchers Paulo Eduardo Mantelatto, Jaciane Lutz Ienczak, Leandro Vieira dos Santos, Tassia Lopes Junqueira and Charles Dayan Farias de Jesus.

Brazil currently has 25,000 hectares of energy cane, grown by Zillor, Raízen, Odebrecht, Citrosuco and Caramuru, among others. Manufacturers of farm implements such as New Holland are developing machinery to plant and harvest “supercane”, which has more leaves and bagasse, as well as other characteristics that differ from those of conventional cane.

Energy cane’s roots are denser, enabling the plant to anchor itself more firmly in the soil, absorb more nutrients, grow faster, and store more carbon.

Moreover, “supercane” has more stalks, and the stalks are thinner than those of conventional cane, allowing for more profuse tillering, densification, and hence greater resistance to drought and mechanical harvester pressure.

“Cane yields have fallen sharply in Brazil in the last five years owing to mechanization. Mechanical harvesting reduces yield because of the load pressure of the machinery, and losses are considerable at the time of both harvesting and planting,” Grassi said. 

“Energy cane would be a solution both to recoup the yield loss caused by mechanization and to produce more robust cane with more biomass for second-generation ethanol and other chemicals, as well as bioelectricity.”

BBEST 2017 

Technologies for improving production processes for ethanol and other biomass products are among the topics being discussed at the 2017 Brazilian BioEnergy Science & Technology Conference (BBEST). Organized by the FAPESP Bioenergy Research Program (BIOEN), BBEST 2017 runs through October 19 in Campos do Jordão, São Paulo State.

The scientific program covers issues relating to raw material, such as agronomy, genetic improvement and energy plant biotechnology, as well as motors and other conversion devices, sustainability, and environmental, social and economic impacts.

In parallel with the scientific program, BBEST 2017 also features other activities designed to promote more collaboration between business and academia, such as presentations on strategies for research, development and innovation (RD&I) by invited companies, and round tables involving representatives of private enterprise and the academic research community.

The conference program also includes talks by researchers from Brazil and abroad, poster sessions, and awards for the best scientific papers delivered by participants.

More information about BBEST 2017: bbest.org.br.