It is quite astonishing that esterifying butyric acid, an acid known for its pungent and rather unpleasant smell, can generate pleasant aromas. Perfumes, liqueurs and juices usually get their fruity flavour by adding butyrates (i.e. butyric acid esters) that have a range of aromas resembling apple, pineapple, apricot and strawberry. The reason butyrates are used for this purpose is because they occur naturally in fruits and other plant parts. Butyric acid (systematic name: butanoic acid) accumulates as an intermediary product during the fermentation of plant material in biogas plants. It is produced by certain bacteria under anaerobic conditions, i.e. in the absence of oxygen, which makes them ideal for use in the huge bioreactors (fermenters) at biogas production plants.

Researchers at the University of Hohenheim and their project partners from the Fraunhofer Institute for Chemical Technology ICT, the European Institute for Energy Research EDF-KIT EWIV (EIfER) and LIPP GmbH are working on a collaborative project called the "Development of efficient two-phase biogas plants via a coupled energetic and material use of feed stocks after the removal of hydrolysis products (Optigär)". The aim of the project is to develop a procedure to remove butyric acid and potentially other economically viable carboxylic acids during the biogas production process. As these acids are not needed for biogas production, the researchers assume that removing them will not interfere with the overall process. On the other hand, removing butyric acid could have a negative effect on the amount of biogas that normally arises from a given quantity of substrate. “Removing this organic substance from the biogas production process means that it will no longer be available for the process. We need to explore whether this will be of economic relevance,” says Dr. agr. Hans Oechnser, head of the University of Hohenheim’s Institute of Agricultural Engineering and Bioenergy. Oechnser is coordinating the collaborative project in which Oechsner’s doctoral student Jörg Steinbrenner (M. Sc.) is also involved in. The project is being financed as part of a German Federal Ministry of Food and Agriculture (BMEL) programme managed by the ministry’s project management organisation, the Agency for Renewable Resources (FNR), with funds totalling around 800,000 euros.

Silage: not shaken, not stirred

In their search for a suitable substrate, the researchers from Hohenheim did their first experiments with wilted grass. “We intend to use grass silage for the large-scale production of hydrolysis products as this material can be produced and stored easily and because it is the major substrate used for producing biogas,” explains Oechsner.

Grass silage naturally contains a broad range of different microorganisms. The researchers need to focus on promoting the growth of microbial species that produce large quantities of butyric acid in order to increase the yield. “By keeping the bioreactor at a specific temperature and pH, we are looking to promote the growth of certain microorganisms and inhibit the growth of others. But we do not stir the broth,” says Steinbrenner. Lactic acid bacteria prevail at the start of the 90-day ensiling process. As the name implies, these bacteria produce lactic acid. This acid can subsequently be converted into butyric acid by bacteria known as Clostridiae. This conversion works particularly well under conditions when the acidity (pH) resulting from lactic acid production does not fall to values that favour biomass preservation. Under such conditions, Clostridia bacteria will gradually outgrow the lactic acid bacteria.

The researchers are also focussing on parameters other than temperature and pH in order to increase the amount of butyric acid produced. One idea is to reduce the amount of dry matter by adding water or wet substrate from different stages of biogas production. Hydrolysis occurs relatively early on in the ensiling process. Depending on the biogas plant used, hydrolysis occurs along with or separately from the reactions that increasingly lead to small-molecule products. “Our goal is to develop a two-phase process involving spatially and temporally separating the two phases, thus allowing us to have greater influence on the fermentation process and the microorganisms involved,” says Oechnser. Oechnser and his team are also investigating whether it makes sense to use only the substrate arising from hydrolysis for generating butyric acid, i.e. without ensiling. Either way, identifying the processes and equipment that are needed to achieve conditions under which economically viable amounts of butyric acid can be obtained is a huge challenge. Moreover, the butyric acid also has to be as pure as possible. This is why the researchers from Hohenheim are working with external separation technology specialists.

The Fraunhofer Institute for Chemical Technology ICT in Pfinztal is bringing to the project its knowledge on how to separate butyric acid from other metabolic products using special membranes. "The idea is to squeeze the material out of the fermenter and then filter the press juice through membranes before using a specific extraction method to generate a solution with a high level of acidity,” explains Steinbrenner. The remaining solids can be returned to the biogas production process. The details of this process are being investigated as part of the current project. Another project partner in this process is a company called Lipp GmbH from Tannhausen in eastern Württemberg. "Lipp GmbH is helping us assess the conditions required to produce the highest possible amounts of butyric acid. And it is also contributing valuable experience that it has gained from being involved in biogas production and ensiling,” says Oechsner. The project partners are also focussing on the composition of the microbial population during the process, which they hope will provide them with insights into how and when a mixture needs to be changed in order to obtain a higher butyric acid yield.

The Karlsruhe-based EIfER is an important partner in all phases of the project. The institute is studying the process from a bioeconomic point of view and is also carrying out environmental assessments. It is drawing up bioeconomic and economic balances and assessing the potential of the processes, including from a commercial point of view. “The EIfER is carrying out a holistic life cycle assessment. Estimating the potential of the process under development will then help us to define appropriate utilisation segments,” says Oechsner. Although the primary goal of the project is to produce butyric acid, the partners are also studying which other substances are produced during substrate fermentation. The process is already known to give rise to succinic acid (also known as butanedioic acid), which is another carboxylic acid that serves as a platform chemical. Succinic acid is, for example, used for synthesising solvents and softeners. The researchers from Hohenheim are also considering other possible uses for the carboxylic acids generated. “The findings from the project have also raised the possibility of using the carboxylic acid solution for producing and stabilising feed mixtures, or achieving better digestibility of feed,” says Oechsner.