Sugarcane – sugar, ethanol, and what else?

If you know any Brazilian, you probably might have heard about our “jeitinho”, i.e. our unique way of developing innovative solutions. Though “jeitinho” usually carries a negative connotation, I believe most of the Brazilian innovation may be linked to our creativity and power to solve issues with our own resources. Back in the 1970’s, when the western world was desperate for oil and gas, Brazil decided to move forward with what we had: sugarcane. I believe that if you also know a Brazilian, you probably heard she or he also mentioning about cachaça – the sugarcane-derived liquor, and basis for making delicious caipirinhas. Knowing that sugarcane is easily fermentable, Brazilian science decided to push a step further from cachaça, and we decided cars should be fueled with sugarcane-derived ethanol. Probably one of the most important marks in pushing Brazil to the international energy scenario, the sugarcane-to-ethanol program, Proálcool, was a major relief for the oil crisis in a politically unstable country going through a horrendous dictatorship at the moment.  Sugarcane is a significant part of Brazilian history. The Portuguese court decided to implement monoculture of sugarcane and become one of the top sugar suppliers to Europe for a few centuries. As Brazil conquered its independence and moved on its way to first develop its own steps, sugarcane still played a vital role in the overall agricultural development. Moving over a few years, ethanol followed the same path as sugarcane, on a sense that it posed Brazil as a key player in the energy scenario in the world.

Brazil is home to over 300 active sugarcane biorefineries, producing values close to 25 billion liters per year. Over 75% of the Brazilian distilleries operate using the Melle-Boinot process with yeast cell recycling. The well-established fermentation method is composed of a high-density cell culture, with a cycle usually within 6 to 10 hours. The nitrogen composition of sugarcane is low enough to provide low cell growth rates, ideal for processes that convert carbon to ethanol and CO2. The fermentation itself is close to an optimized process – cheap, fast, and with high yields. Many of the environmental issues in the sugarcane ethanol process are found in the downstream process, which is also relatively simple. On a recent article I wrote to Frontiers in Energy Research, I describe the fate of sugarcane vinasse. Vinasse, on a few words, stands for the liquid-rich fraction produced after the solid-liquid separation of the distillation bottoms of fermented sugarcane juice. Vinasse is characterized as a dark, odorous, and nutrient-rich effluent. Vinasse has been mostly applied in fertirrigation practices, i.e., by utilizing it as a liquid fertilizer for crops, reducing the water input for plant growth. Studies, as summarized on the article, implicate that such practices have negative effects on soil and ground waters in the long term. This is one of the major reasons sugarcane and ethanol production in Brazil are largely criticized, mostly also by posing a significant source of contaminants in soil and surface water.

 

 

As a chemical engineer, and a recent doctor in biosystems engineering, it astonishes me that better alternatives for sugarcane vinasse are not implemented in a large scale yet. Though there are technical difficulties and risks involved in upgrading well-established plants and processes, I still get surprised on how resilient this production has been over the past 30 years – on a critical sense though. Knowing what we know now about bioprocessing and biological-based methods, I wonder why fungi (other than yeast, of course) still do not make their way into sugarcane-based ethanol plants. Bacteria and the well-known and established anaerobic digestion practices are still far from producing methane. Same criticism goes for bagasse burning. Burning a fibrous material is for sure an option, and it does produce the amount of energy a company usually requires – but is that the actual way to go?

 

 

Taking in account that the main product in an ethanol plant is ethanol per se, it is challenging to think that a more complex reaction system to produce the same compound can correspond to the state-of- art production system. The sugarcane ethanol is produced through fermentation of sugarcane juice, i.e., a solution with readily available sugars to yeast, mostly composed by sucrose, while corn, the main feedstock for ethanol production in the United States, is a feedstock material with higher complexity than sugarcane. Corn is a rich substrate for most biological systems, rich in proteins, starch, lipids, and with enough minerals to convey and support microbial growth. Sugarcane, on the other hand, is a simple, yet, effective feedstock for ethanol production. Sugarcane does not have significant quantities of lipids, complex sugars in its juice, and is deficient in minerals, such as phosphorus and nitrogen. Sugarcane by itself would not be a suitable fermentation feedstock for most industrial applications without added nutrients. It is, however, extremely suitable for ethanol fermentation via yeast due to a number of factors: (i) simple fermentable sugars that do not require saccharification, and consequential addition of exogenous enzymes, e.g., alpha-amylases, (ii) nitrogen deficiency promotes cell stress that inhibit cell growth, promoting most of the carbon flux go through ethanol production instead of the undesirable cell growth, (iii) rheological properties that allow a fast nutrient transfer from the media to the yeast. Corn when processed for ethanol production, accounting as of today for over 40% the farmable numbers for its production, especially when processed through the dry-grind process, that provides a viscous mash with all the corn kernel components grinded to a saccharified and liquefied mass, as fermentation material possess opposite characteristics as those pointed for sugarcane. Yet, against the technical odds, dry-grind corn ethanol production accounts for over 90% of the corn-to- ethanol production. The dry-grind ethanol industry is living its golden age, and is growing in production and capacity over the past years, and it is still projected to continue growing. Sugarcane juice is usually fermented to its maximum potential within 6 to 10 hours in an average ethanol plant, while corn through the dry-grind process has a retention time in a fermenter from 48 to 72 hours. Sugarcane is harvested in Brazil 3-4 times a year, and present today higher plantation and harvesting yields than those in the 1980’s, while corn is still harvested once. Yet, sugarcane lies behind corn in total volume processed for ethanol production. The answer to such an intriguing question involves political, social, and technical issues. By-products are no longer a term accepted by the dry-grind corn-to- ethanol industry, and rather are named co-products. The change in upgrading secondary products, processing, and marketing them to another industrial sector, such as the feeding industry, is an underdeveloped task in the sugarcane-to-ethanol process that can, and should, be changed.

Reference:

Rodrigues Reis, C. E., & Hu, B. (2017). Vinasse from Sugarcane Ethanol Production: Better Treatment or Better Utilization?. Frontiers in Energy Research5, 7.

Reis, C. E. R., Rajendran, A., & Hu, B. (2017). New technologies in value addition to the thin stillage from corn-to-ethanol process. Reviews in Environmental Science and Bio/Technology, 1-32.

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