Microorganisms are agents for transforming biomass into energy inputs

17.10.2011 | 21:59 (UTC -3)
Carolina Madalozzo Poletto

Embrapa Agroenergy, based on the National Agroenergy Plan – PNA 2006-2011, executes projects focusing on four Research, Development and Innovation platforms: ethanol, biodiesel, energy forests and waste utilization. Several researches covering these themes have been carried out seeking to meet the country's demand for new sources of renewable energy.

In general, the process of transforming biomass into some form of energy involves steps that involve the action of microorganisms directly or indirectly. The objective here is to highlight the importance that these microorganisms represent within each stage, in addition to reporting in a simple and objective way the principles of the biochemical processes involved.

The production of first generation ethanol can be carried out via two technological routes, using sugary or starchy raw materials. Among the sugary raw materials we have sugar cane, sugar beet, sweet sorghum and sweet cassava, where after obtaining the sugar-rich juice, ethanol is obtained through fermentation carried out by yeasts called Saccharomyces cerevisiae.

Among the starchy raw materials, we have corn, starchy cassava, sweet potatoes, etc. These materials release a broth rich in starch, which must first be converted to sugars. One of the ways of converting this starch is enzymatic hydrolysis, where enzymes called amylases break the bonds of starch molecules, obtaining glucose molecules.

Amylases can be found in vegetables, in saliva, in the pancreas, and can also be produced by microorganisms. Microbial amylases are preferred in the enzyme market due to their high selectivity for certain substrates, wide range of temperature and pH, in addition to being biodegradable. Among bacteria, species of the genus Bacillus are one of the most investigated groups regarding the production of amylase. The main biological sources of enzymes are filamentous fungi, particularly the genus Aspergillus. After enzymatic hydrolysis, the broth obtained, now composed of fermentable sugars, can be fermented and transformed into ethanol through the action of yeast.

Second generation ethanol is obtained from lignocellulosic raw materials, such as sugarcane bagasse, grass, rice straw, wood and agro-industrial waste in general. This technological route differs from that used in the production of first generation ethanol, mainly due to the complexity of the material structure.

Lignocellulosic materials are mainly composed of lignin, hemicellulose and cellulose. Cellulose is a polysaccharide formed by chains of hexoses, which are six-carbon sugars. Hemicellulose is formed by hexoses and pentoses, which are five-carbon sugars. To be able to transform these complex molecules into ethanol, it is first necessary to break them down into simpler sugars. To achieve this, three steps are necessary, the first is normally a physical-chemical treatment that aims to disrupt the rigid formation of these molecules. At this stage, hemicellulose is broken down and pentoses are released, which can be converted into ethanol through the action of specific yeasts.

The second stage of the process is enzymatic hydrolysis, carried out by enzymes called cellulases, which break down cellulose, transforming it into glucose. This glucose will finally be subjected to the third stage of the process, where ethanol is formed through fermentation, exactly as is carried out in the conventional process. This technology is still under development, and the two biggest obstacles faced are the pentose fermentation and enzymatic hydrolysis stages, both carried out directly or indirectly by microorganisms. Not all yeast is capable of fermenting pentoses and without genetic manipulation, S. cerevisiae does not assimilate or ferment pentoses. Pichia stipitis and Pachysolen tannophilus are yeasts known to have this type of metabolism, but they are not sufficiently competent in ethanol production compared to S. cerevisiae.

Regarding enzymatic hydrolysis, it is known that the high cost of producing enzymes makes the lignocellulosic ethanol process financially unviable. Embrapa Agroenergia research groups are working on the search for cellulolytic microorganisms that can be used directly in the process. Fungi are the main producers of cellulases, as they have a differentiated enzyme arsenal, among which the genera Penicillium and Trichoderma stand out.

The process for producing biodiesel is called transesterification, where a vegetable oil or animal fat (triglyceride) is converted to a fatty acid ester (biodiesel) and glycerin, through reaction with an alcohol. The conventional way to carry out this reaction is chemical, but it can also be done by pyrolysis, supercritical fluid or through an enzymatic reaction. In chemistry, the presence of an acid or base accelerates the conversion, but this process has a negative impact on the environment due to the generation of waste and high energy consumption.

In enzymatic transesterification, conversion occurs through the action of enzymes called lipases, which are commonly found in nature and can be obtained from animal, vegetable and microbial sources, with microbial lipases having greater stability, diversity and specificity.

The use of enzymes in biodiesel production reduces the environmental impact, as it replaces the chemical catalyst. Furthermore, they are biodegradable and energy consumption in the process is lower, as the temperature required for the reaction is lower. Among yeasts, Candida and Torulopsis are known for the production of lipolytic enzymes, as are the bacteria Pseudomonas and Bacillus and the filamentous fungi Rhizopus, Penicillium, Aspergillus and Mucor.

Brazil is known for having an economy heavily based on agribusiness, and as every agro-industrial process generates waste, it is essential for the country to develop technologies that aim to use these materials, in order to convert them into different secondary forms of energy. There are several studies in search of these technologies and among those that include the participation of microorganisms is the production of biogas from organic matter. Plant residues, animal waste, residential waste or industrial waste are materials suitable for biogas production. This organic matter is fermented by the action of bacteria under anaerobic conditions, that is, in the absence of oxygen, producing biogas, a mixture of carbon dioxide and methane. Biogas can be used by burning it in engines to generate electrical energy. The formation of methane gas is a biological event specific to a small group of bacteria, classified as methanogenic, mainly found in exclusively anaerobic environments.

It is interesting to carry out constant searches for microorganisms that present characteristics of industrial interest. The prospecting, identification and improvement of microorganisms play a fundamental role in the development and consolidation of new biofuel generation technologies. Embrapa Agroenergia is increasingly investing in research of this kind, with the aim of contributing to the country's socio-economic development, seeking to ensure international leadership in the agroenergy sector.

Mosaic Biosciences March 2024